Patent Publication Number: US-9411776-B2

Title: Separation of data and control in a switching device

Description:
This application is a continuation of U.S. patent application Ser. No. 13/465,566, filed May 7, 2012, which is a continuation of U.S. patent application Ser. No. 12/346,918, filed Dec. 31, 2008, now U.S. Pat. No. 8,189,605, which is a continuation of U.S. patent application Ser. No. 11/123,112, filed May 6, 2005, now U.S. Pat. No. 7,489,699, which is a continuation of U.S. patent application Ser. No. 09/466,864, filed Dec. 17, 1999, now U.S. Pat. No. 6,917,620, the disclosures of which are hereby incorporated by reference. U.S. patent application Ser. No. 09/466,864 is a continuation-in-part of U.S. patent application Ser. No. 08/901,061, filed Jul. 24, 1997, now U.S. Pat. No. 6,493,347, which is a continuation-in-part of U.S. patent application Ser. No. 08/844,171, filed Apr. 18, 1997, now U.S. Pat. No. 5,905,725, which is a continuation-in-part of U.S. patent application Ser. No. 08/767,576, filed Dec. 16, 1996, now U.S. Pat. No. 5,909,440. 
    
    
     BACKGROUND 
     The present invention relates generally to data routing systems, and more particularly to methods and apparatus for efficiently routing packets through a network. 
     In packet switched communication systems, a router is a switching device which receives packets containing data or control information on one port, and based on destination information contained within the packet, routes the packet out another port to the destination (or an intermediary destination). 
     Conventional routers perform this switching function by evaluating header information contained within a first data block in the packet in order to determine the proper output port for a particular packet. 
     Efficient switching of packets through the router is of paramount concern. Referring now to  FIG. 1A , a conventional router includes a plurality of input ports  2  each including an input buffer (memory)  4 , a switching device  6  and a plurality of output ports  8 . 
     Data packets received at an input port  2  are stored, at least temporarily, in input buffer  4  while destination information associated with each packet is decoded to determine the appropriate switching through the switching device  6 . The size of input buffer  4  is based in part on the speed with which the destination information may be decoded. If the decoding process takes too long as compared to the rate at which packets are received, large sized memory elements may be required or packets may be dropped. 
     In addition, the size of input buffer may be influenced by a condition referred to as “blocking”. Packets may be forced to remain in the input buffer after the destination information is decoded if the switching device cannot make the connection. Blocking refers to a condition in which a connection cannot be made in the switch due to the unavailability of the desired output port (the port is busy, e.g., routing another packet from a different input port). In summary, the size of input buffer  4  is dependent on a number of factors including the line input rate, the speed of the look-up process, and the blocking characteristics for the switching device. Unfortunately, conventional routers are inefficient in a number of respects. Each input port includes a dedicated input buffer and memory sharing between input ports is not provided for in the design. Each input buffer must be sized to meet the maximum throughput requirements for a given port. However, design trade-offs (cost) often necessitate smaller buffers for each port. With the smaller buffers, the possibility arises for packets to be dropped due to blocking conditions. While excess memory capacity typically exists in the router (due to the varied usage of the input ports), no means for taking advantage of the excess is afforded. 
     To minimize the occurrence of dropping packets, designers developed non head-of-line blocking routers. Referring now to  FIG. 1B , a conventional non head-of-line blocking router includes a plurality of input ports  2  each including an input buffer (memory)  4 , a switching device  6  and a plurality of output ports  8  each having an output buffer  9 . In order to provide non head-of-line blocking, each output port  8  is configured to include an output buffer  9 . Each output port could simultaneously be outputting packets as well as receiving new packets for output at a later time. As the size of the output buffer is increased, fewer packets are dropped due to head-of line blocking at input ports. 
     However, these designs are even more inefficient in terms of memory capacity and cost. Again, each output port includes a dedicated output buffer and memory sharing between output ports is not provided for in the design. Each output buffer must be sized to meet the maximum throughput requirements for a given port (in order to maintain its non head-of-line blocking characteristics). Even more excess memory capacity typically exists in the router (due to the varied usage of the input ports and output ports), yet no means for taking advantage of the excess is afforded. Twice the amount and bandwidth of memory has to be used than required to support the amount of data being moved through these types of devices. 
     What is desirable is to produce a router where the data packets can flow to a common memory, while routing decisions are made off-line. By separating the data path, the path along which the packet data traverses through the router, and the control path, a path used in evaluating the packet headers, memory can be conserved. 
     SUMMARY OF THE INVENTION 
     In general, in one aspect, the invention provides a method of looking up a key associated with a packet to determine a route through a routing device. The method includes, upon receipt of the key, forward traversing one or more nodes which make up a trie stored in a memory by evaluating at each node traversed one or more bits in the key as indicated by a bits-to-test indicator associated with each node where a value of the bits in the key determines a path traversed along the trie. An end node in the trie is located where the end node has a route. The route is compared to the key. If they match, destination information associated with the end node is outputted to guide the transfer of the packet through the routing device. If they do not match, the trie is traversed backwards to locate a best match for the key. 
     Aspects of the invention can include one or more of the following features. The step of forward traversing can include storing on a stack, for each node having one or more attached routes, the bits-to-test indicator and pointers to the attached routes. The step of traversing the trie backwards can include comparing the key with the route to determine a first dissimilar bit location, popping entries off the stack to determine when the bits-to-test indicator associated with a first node in the backward traversal is less than or equal to the first dissimilar bit location and outputting destination information associated with the first node to guide the transfer of the packet through the routing device. 
     The first node can include a route. The method can further include calculating statistical information for each packet and storing in memory the statistical information with the route associated with each of the end node and the first node. The method can further include transferring the statistical information along with the destination information to an output port in the routing device for transfer to a destination. Prior to a forward traversal of the trie, a root table can be searched for a match of a predetermined number of bits in the key. The root table can be indexed by the predetermined number of bits where each entry includes a pointer to a start node in the trie to begin the forward traversal. 
     The forward traversal of the trie can include loading node information for each node traversed in the trie until the end node is reached. The node information can include a bits-to-test indicator, a plurality of child pointers and one or more attached routes. The nodes in the tree can include T child pointers, where N is an integer greater than 1. Each pointer can include a bits-to-test indicator and an address in the memory where the child node is stored where the bits-to-test indicator indicates one or more bits to be tested in the key associated with a child node to which the child pointers indicate. The forward traversal can include testing one or more bits indicated by the bits-to-test indicator and retrieving an appropriate child pointer associated with the child node that indicates a next node in the trie to be traversed after the child node. Accesses to the memory can be minimized in the forward traversal of the trie by loading a single pointer at each node until the end node is reached. 
     In one aspect the invention provides a method of routing a packet through a switch including upon receipt of the packet, extracting a key from the packet. A trie is forward traversed by evaluating at each node one or more bits in the key as indicated by a bits-to-test indicator associated with each node where values of the bits in the key located at a position indicated by the bits-to-test indicator determine a path traversed along the trie at each node. An end node in the tree is located having a route. The route is compared to the key. If they match, destination information associated with the end node is retrieved. If they do not match, the trie is traversed backwards to locate a best match for the key having a route and destination information associated therewith. Thereafter the packet is routed through the switch according to the destination information. 
     In one aspect the invention provides a method of inserting a route in a route table where the route table is stored as a trie in a memory of a routing device. The route table defines a path by which a packet is transferred through the routing device. The method includes traversing the trie to determine an insertion point and determining if the insertion point has an associated parent node and sibling node in the trie. If so, a multi-node is created from the parent, sibling and the route including setting one or more child pointers in the multi-node to indicate a node directly beneath the insertion point. Thereafter, the multi-node is stored in the memory and the child pointer in a node directly above the parent node is updated to indicate a starting address in the memory for the multi-node. 
     In one aspect the invention provides a router for routing packets in a packet switched network including one or more input ports for receiving packets, a packet memory and an input switch coupled to each input port and the memory. The input switch includes a transfer engine for transferring packets from an input port to the packet memory and a key extraction engine for extracting a key from each packet. The router includes a controller coupled to the input switch. The controller includes a key look-up engine and a route memory. The route memory stores a route table where the route table includes a trie. The key look-up engine traverses the trie to determine a best match to the key, and upon determining the best match for the key, generates notification information. The key look-up engine includes a forward traversal engine for forward traversing the trie and is operable to evaluate at each node traversed one or more bits in the key as indicated by a bits-to-test indicator associated with each node, locate an end node having a route, compare the route to the key, if they match, outputting destination information associated with the end node to guide transfer of a packet through the routing device, and if they do not match, traversing the trie backwards to locate a best match for the key. The router includes one or more output ports and an output switch coupled to the controller, the packet memory and the output port for transferring packets from packet memory to an appropriate output port based on the notification information received from the controller. 
     Aspects of the invention can include one or more of the following features. The apparatus can include a stack. The forward traversal engine can store on the stack a bits-to-test indicator for the node and pointers to attached routes for each node having attached routes. The key look-up engine can include a backward traversal engine operable to compare the key with the route to determine a first dissimilar bit location, pop entries off the stack to determine when a bits-to-test indicator associated with a first node in the backward traversal is less than or equal to the first dissimilar bit location, and output destination information associated with the first node to guide a transfer of a packet through the router. The apparatus can include a statistical engine for calculating statistical information for each data packet and outputting the statistical information with destination information for transfer to a destination port. The route memory can be divided into a plurality of banks. Parent and children nodes in the tree can be stored in different banks. Each node in the tree can include a bits-to-test indicator, 2 n  child pointers, and n or fewer attached routes, where a pair of the 2 n  child pointers indicate a child node to be traversed in the forward traversal when a value of a bit in the key as indicated by the bits-to-test indicator is a first value and second value, respectively. The child pointers and attached routes can be stored in contiguous locations in the route memory. The trie can be a modified radix trie. The nodes in the modified radix trie can include 2 n  child pointers, each pointer including a bits-to-test indicator and an address in the memory where the child node is stored. The bits-to-test indicator indicates one or more bits to be tested in the key associated with a child node to which the child pointers indicate. The forward traversal engine is operable to test one or more bits indicated by the bits-to-test indicator and retrieve an appropriate child pointer associated with the child node that indicates a next node in the trie to be traversed after the child node. 
     In one aspect, the invention provides a route look-up engine for locating a best match for a key in a route table. The route table includes a trie stored in a memory associated with a routing device. The trie includes one or more entries defusing a path through a routing device for transferring a packet in a packet switched network from a source to a destination. The route look-up engine includes a stack for storing stack entries including a bits-to-test indicator and a pointer to the destination and a plurality of look-up engines each including a buffer, a bit comparison engine and a key comparison engine. The buffer stores node information that is retrieved from the memory. The node information includes a bits-to-test indicator. Responsive to receiving the key associated with a packet, the look-up engine forward traverses the trie. The bit comparison engine evaluates at each node traversed one or more bits in the key as indicated by a bits-to-test indicator associated with each node. A value of the bits in the key determines the path traversed along the trie. The look-up engine stores stack entries on the stack for each node traversed having an attached route. The look-up engine locates an end node having a route. The key comparison engine performs a singular key comparison for each packet routed through the routing device by comparing the key with the route. If they match, the look-up engine outputs destination information associated with the end node to guide the transfer of the packet through the routing device. If they do not match, the look-up engine traverses the trie backwards, popping entries off the stack, to locate a best match for the key and destination information associated therewith. 
     In one aspect the invention provides a router for switching a data packet between a source and destination in a network. The router includes an input port including a data handler. The input port receives the data packet from the source. The data handler divides the data packet into one or more fixed length cells. The router includes an output port for routing the data packet to the destination, a memory divided into a plurality of memory banks and an input switch for receiving the fixed length cells from the input port and routing consecutive cells of the data packet to different memory banks. A single cell is transferred in a cell slot time span to a memory bank. The input switch includes a key reading engine for extracting key information from a first cell received at the input switch associated with the data packet. The router includes a controller coupled to the input switch and receiving the key information therefrom. The controller decodes destination information from the key information received from the input switch and outputs a notification defining a routing of the data packet from the memory to the output port. The controller includes a plurality of look-up engines and a memory for storing routes. Each look-up engine is operable to compare the key information with routes stored in the memory and determine a best route through the router for a given data packet. An output switch is included for routing cells received from the memory to the output port. 
     Aspects of the invention can include one or more of the following features. The input switch can include a linking engine for linking cells in the data packet to allow retrieval of the data packet from non-contiguous locations in the memory. The router can include an indirect cell generator for generating one or more indirect cells. The linking engine can track the location in the memory where consecutive cells of the data packet are stored and provide an address in memory of each cell in the data packet for storage in indirect cells. The input switch time division multiplexes the writing of data packets to the memory such that consecutive cells from the input port are written to consecutive banks in the memory. The output port can include a result processor for receiving the notification from the controller and initiating a transfer of the data packet from the memory to the output port. 
     The input switch can include a reservation table for scheduling transfers from the memory to the output switch. The output switch can route the notification to the output port and thereafter the output port can issue a request to the input switch to transfer the data packet from memory to the output port through the output switch. The request from the output port can be stored in the reservation table. Requests to transfer cells from memory to the output switch can be time domain multiplexed so that during one cell slot time span at most a single read request is issued to each bank in the memory for servicing. The memory can be configured to output at most a single cell per bank in one cell slot time span. 
     In one aspect the invention provides a method implemented in a router for switching a data packet between a source and destination in a network. The data packet includes a header portion and a data portion. The header portion includes routing information for the data packet. The method includes defining a data path in the router comprising a path through the router along which the data portion of the data packet travels and defining a control path comprising a path through the router along which routing information from the header portion travels. The method includes separating the data path and control path in the router such that the routing information can be separated from the data portion allowing for the separate processing of each in the router. The data portion can be stored in a global memory while routing decisions are made on the routing information in the control path. 
     Other advantages and features will be apparent from the following description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are block diagrams of conventional router devices. 
         FIG. 2A  is a schematic block diagram of a data routing system according to one embodiment of the present invention. 
         FIG. 2B  is a schematic block diagram of a router according to one embodiment of the present invention. 
         FIG. 3A  is a schematic block diagram of an multi-function port according to one embodiment of the present invention. 
         FIG. 3B  is a schematic block diagram indicating data transfers between components of the router of  FIG. 2B  according to one embodiment of the present invention. 
         FIG. 3C  is a data structure for a cell transferred between a multi-function port and an input switch according to one embodiment of the present invention. 
         FIG. 4  is a schematic block diagram of a router including the timing and ordering of transfers from the input port to input switch according to one embodiment of the present invention. 
         FIG. 5A  is a schematic block diagram of an input switch according to one embodiment of the present invention. 
         FIG. 5B  is a schematic block diagram of a router including the timing and ordering of transfers from the input port to memory according to one embodiment of the present invention. 
         FIG. 6  is a data structure for a route request stored in a key buffer according to one embodiment of the present invention. 
         FIG. 7  is a data structure for an indirect cell according to one embodiment of the present invention. 
         FIG. 8  is a data structure for a cell transferred between the input switch and a memory bank according to one embodiment of the present invention. 
         FIG. 9  is a schematic block diagram of a reservation table according to one embodiment of the present invention. 
         FIG. 10  is a flow diagram of a process of loading a reservation table according to one embodiment of the present invention. 
         FIG. 11A  is a schematic block diagram of main memory according to one embodiment of the present invention. 
         FIG. 11B  is a schematic block diagram of a global data buffer according to one embodiment of the present invention. 
         FIG. 11C  is a schematic block diagram of a router including 3 memory banks according to one embodiment of the present invention. 
         FIG. 12  is a schematic block diagram of a router including the timing and ordering of transfers from the input port to memory according to one embodiment of the present invention. 
         FIG. 13A  is a schematic block diagram of a controller according to one embodiment of the present invention. 
         FIG. 13B  is a schematic block diagram of a route trie according to one embodiment of the present invention. 
         FIG. 13C  is a schematic block diagram of a data structure for an internal node with an attached route according to one embodiment of the present invention. 
         FIG. 13D  is a schematic block diagram of a data structure for an internal node having four children and with an attached route according to one embodiment of the present invention. 
         FIG. 13E  is a schematic block diagram of a data structure for a start node according to one embodiment of the present invention. 
         FIG. 13F  is a schematic block diagram of a data structure for a route node according to one embodiment of the present invention. 
         FIG. 13G  is a schematic block diagram of a data structure for a internal node according to one embodiment of the present invention. 
         FIGS. 13H-13N  collectively are a flow chart for a populating a trie according to one embodiment of the present invention. 
         FIGS. 13P and 13Q  collectively are a process for routing data through the router according to one embodiment of the present invention. 
         FIG. 13R  is a schematic block diagram of a data structure for a internal node according to an alternative embodiment of the present invention. 
         FIG. 13S  is a schematic block diagram of a data structure for a internal node with an attached route according to an alternative embodiment of the present invention. 
         FIG. 13T  is a schematic block diagram of a data structure for a route node according to an alternative embodiment of the present invention. 
         FIGS. 13U and 13V  collectively are a process for routing data through the router according to an alternative embodiment of the present invention. 
         FIG. 14  is a data structure for an output request transferred from the controller to the output switch according to one embodiment of the present invention. 
         FIG. 15A  is a schematic block diagram of an output switch according to one embodiment of the present invention. 
         FIG. 15B  is data structure for a cell output from a memory bank to output switch according to one embodiment of the present invention. 
         FIG. 16  is a data structure for a cell transferred from the output switch to an output port in a multi-function multiport according to one embodiment of the present invention. 
         FIG. 17A  is a schematic block diagram for an output section of a multi-function port according to one embodiment of the present invention. 
         FIG. 17B  is a schematic block diagram for a queue system for storing notifications according to one embodiment of the present invention. 
         FIG. 18  is a flow diagram for a process of routing a packet through a router according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 2A , in a packet switching system, a source  10  is connected to one or more routers  20  for transmitting packets to one or more destinations  30 . Each router includes a plurality of multi-function multiports that are connected to various sources and destinations. A packet from source  10  may pass through more than one router  20  prior to arriving at its destination. 
     Referring to  FIG. 2B , each router  20  includes an input switch  100 , an output switch  102 , a global data buffer  104  including one or more memory banks  105 , a controller  106  and a plurality of multi-function multiports  150  ( 150 - 0  through  150 - 3 ), respectively. Associated with the controller  106  is controller memory  109  for storing a routing table. Input switch  100  and output switch  102  are connected to each multi-function multiport  150  in router  20 . In one embodiment, router  20  includes plug-and-play multi-function multiports which allows for easy expansion capability. The present invention will be described with reference to a system including eight multi-function 1.5 multiports  150  (even though  FIG. 2B  only shows four), with each multi-function multiport including up to sixteen input ports and sixteen output ports. Other configurations may be used depending on user load conditions. Each multi-function multiport includes one or more input ports, one or more output ports and a memory. The configuration and operation of the multi-function multiports will be described in greater detail below. 
     In operation, packets are received at a multi-function multiport  150 , transferred to input switch  100  and stored temporarily in global data buffer  104 . When the packet is received by switch  100 , a key is read from the first data block in the packet and transferred to controller  106 . The key contains destination information which is derived from the header field associated with the first block of data in a packet and other information (such as source ID, priority data and flow ID). 
     A route look-up engine  110  in controller  106  performs a trie based search based on the key information and returns a result which includes the output multiport associated with the destination. The result is coupled with other information (such as source ID, flow ID and packet length) for routing the packet through router  20  and provided as a notification from controller  106  to output switch  102 . Output switch  102  transfers the notification to the identified multi-function multiport  150 . Upon receiving the notification information, the multi-function multiport  150  initiates the transfer of the packet from global data buffer  104  through output switch  102  to the appropriate multi-function multiport  150 . 
     Multi-Function Multiports 
     Referring to  FIG. 3A , each multi-function multiport  150  includes an input section  270 , an output section  280  and a memory section  290 . 
     Input section  270  includes a line input interface  300 , a data handler  302  and an input switch interface  304 . Output section  280  includes an output request processor  306 , a line output interface  308 , a storage device  310 , stream output buffers  312  (one for each output stream), output formatter  314 , an output switch interface  316  and head and tail queue buffer  318 . In addition, the output section includes a portion of input switch interface  304 . Specifically, input switch interface  304  includes read request queues  305 , one for each memory bank. The use and operation of the read request queues, stream output buffers, and head and tail queue will be discussed in greater detail below in association with  FIGS. 17A and 17B . 
     Memory section  290  includes a memory bank  105  (which represents a portion of the global data buffer  104 ) and a notification queue body  319 . The use an operation of the memory section will be discussed in greater detail below in association with  FIG. 17B . 
     The multi-function multiport is used in conjunction with the input switch, output switch and controller as is shown in  FIG. 3B . The various piece components of the input section, output section and memory section will be described in greater detail below. The combination of the devices into a single unit simplifies the interfaces between the components. 
     Referring again to  FIG. 3A , packets are received at line input interface  300 . As the packets are received, data handler  302  divides the packets received into fixed lengths cells. In one embodiment, the length of each cell is 80 bytes, with 16 bytes of internal header (control information) and 64 bytes of cell data. As the data handler divides the incoming packets into fixed length cells, it synchronously outputs the cells to input switch  100  through input switch interface  304 . 
     The format for transfers between the multi-function multiport and the input switch is shown in  FIG. 3C . A cell  350  transferred from a multi-function multiport  150  to the input switch contains a cell header  352  and cell data  354 . Cell header  352  includes a type field  356 , stream field  358 , and packet header fields  360 . In addition, cell header  352  includes an independent read request in the form of a multi-function multiport identifier  366  and address  368 . 
     The type field  356  indicates the type of cell to be transferred from the multi-function multiport. At each cell slot (20 clock cycles in one embodiment), a multi-function multiport may transfer either a data cell, an indirect cell placeholder, or a delayed indirect cell placeholder. Data cells contain data associated with an incoming packet. An indirect cell placeholder is an empty cell, and is used in conjunction with indirect addressing for the storage of the cells in the global data buffer  104 . Delayed indirect cell placeholders arise when a data stream that requires indirect addressing terminates at a time prior to the designated time for writing the last indirect addressing cell associated with the data stream to global data buffer  104 . The generation and operation of indirect placeholders and delayed indirect placeholders will be discussed in greater detail below in conjunction with  FIG. 7 . 
     Stream field  358  indicates the stream to which the cell data belongs. In one embodiment of the present invention, each multi-function multiport is capable of handling up to sixteen separate streams of data at a time, one on each of its respective  16  input ports. 
     Packet header field  360  contains header information associated with a given packet and includes start offset information, packet length and interface index information. 
     Multi-function multiport identifier  366  identifies the multi-function multiport which is sourcing the read request. Address  368  indicates the address in global data buffer  104  to be read. 
     Referring now to  FIG. 4 , a single cell  450  is transferred from a multi-function multiport  150  to input switch  100  at each cell (time) slot “T”. For a given cell slot “T”, input switch  100  receives a total of “N” cells, where “N” is equal to the number of multi-function multiports. 
     In one embodiment, cells from a given stream may be written to memory in an order that is different from the arrival order. These out of order writes are performed to make efficient use of scarce bandwidth between the multi-function multiports and the input switch. When a packet comes in to the multi-function multiport, it is broken up into cells as the bytes arrive and the cells are placed in per-bank output queues on the way to the input switch. These queues are designed to share scarce interconnect bandwidth between the streams of a multi-functional multiport in the most efficient way possible, but they have the detrimental effect of reordering cells at the interface between the multi-function multiport and the input switch. Thus the cells from a given stream may arrive at the input switch out of order. The multi-function multiport marks the data cells of a stream with one of four codes: first cell (FC); intermediate data cell (DC); last cell (LC); or first cell which happens to be also a last cell (FLC). 
     Input Switch 
     Referring to  FIGS. 2B and 5A , input switch  100  includes a round robin data handler  500 , one or more input interfaces ( 501 - 0  through  501 - 7 , one for each multi-function multiport  150 ), one or more memory interfaces  502  ( 502 - 0  through  502 - 7 , one associated with each memory bank), a like plurality of pointers  504  ( 504 - 0  through  504 - 7 ), an output processor  505 , one or more output interfaces  506  ( 506 - 0  through  506 - 7 , one for each multi-function multiport  150 ), a reservation table  508 , an indirect cell processor  510 , controller interface  512  and read controller  517 . 
     a) Transfers Through the Input Switch 
     Round robin data handler  500  receives cells from each multi-function multiport and transfers them to output processor  505  for output to an appropriate memory bank  105  in global data buffer  104 . Round robin data handler  500  services the inputs (cells) received on input interfaces  501  in a round robin, time division multiplexed manner. That is, for a given cell slot, one cell from each multi-function multiport is received at the round robin data handler  500  and subsequently transferred to output processor  505  for transfer at the next cell slot to a memory bank  105  in global data buffer  104 . At the next time cell slot, data handler  500  transfers the next cell received from the same multi-function multiport to output processor  505  for transfer to a different memory bank. In one embodiment, the next cell received is transferred to the next memory bank (next in numerical order, modulo N) in the memory array. Alternatively, another time dependent permutation may be used to control the transfer of successive cells from the same multi-function multiport. 
     Referring to  FIG. 5B , the timing and ordering of transfers from the multi-function multiport to memory is shown. For the purposes of this example, a sequence of cells is depicted on each transmission line. For the purposes of this example only, each transmission line is considered to be very long and contains data associated with two or more cells. In operation, the transmission lines are short and multiple cells are not present on a transmission line at a given time. At cell slot T4 a series of cells  450 - 0  through  450 - 7  are transferred down transmission lines  458 , one from each multi-function multiport  150  to input switch  100 . At cell slot T3 (one just prior in time to cell slot T4) a series of cells  452 - 0  through  452 - 7  are transferred down transmission lines  458 , one from each multi-function multiport  150  to input switch  100 . 
     Round robin data handler  500  and output processor  505  within the input switch  100  transfer cells out to global data buffer  104  on transmission lines  460 . As can be seen at cell slot T2, output processor  505  outputs one cell  454 -B 0  to  454 -B 7  to each memory bank in a single cell slot. The “B.” designator indicates the multi-function multiport from which the particular cell was received. One cell from each multifunction multiport is written to global data buffer  104  per cell slot. At time period T1 (one cell slot prior to cell slot T2), again one cell ( 456 -B 0  to  456 -B 7 ) is written to each memory bank. Round robin data handler  500  time division multiplexes the transfers to output processor  505  such that consecutive cells from the same multi-function multiport are written to consecutive memory banks  105  (modulo N) in global data buffer  104 . 
     Referring again to  FIG. 5A , pointer  504  indicates the location in an associated memory bank to which the next cell will be written. Output processor  505  writes a cell to a memory location in a particular memory bank based on the next available address in the bank as is indicated by the associated pointer  504 . 
     b) Key Reading and the Linking Process 
     Round robin data handler  500  includes a key reading engine  514  for determining the key information associated with a first cell in a packet and a linking engine  515  for linking cells in the same packet. 
     The process of reading key information is known in the art. After the key is determined for a given packet, it is stored temporarily in key buffer  516  in input switch  100  until the entire packet has been stored in global data buffer  104 . The data structure for entries  600  in the key buffer  516  is shown in  FIG. 6 . Each entry or “info cell”  600  includes a key  602 , full address  604 , offsets  606  and an indirect cell indicator  608 . 
     Referring now to  FIGS. 5A, 6 and 7 , linking engine  515  determines the starting address (full address) in memory for where the first cell in a given packet is to be stored in memory. The starting address includes the bank number in global data buffer  104  (the bank number which is assigned to store the cell by round robin data handler  500 ) and the first available address location in the designated bank (as is indicated by the associated pointer  504 ). The starting address (full address  604 ) is stored in key buffer  516  along with the associated key  602  for the packet. When the next cell associated with the same packet arrives at switch  100 , an offset  606  associated with the offset at which the cell is to be written (relative to the full address) is computed and stored in key buffer  516 . In one embodiment of the present invention, up to four offsets  606  are stored. Each offset address is computed based on the relative offset in memory between the location of the last cell in memory and the value of the pointer  504  associated with the current memory bank which is to be written. 
     If more than five data cells are included in a packet, then the indirect cell indicator for that packet is set, and the last offset indicates the address in memory where the first indirect cell associated with the packet is stored. Indirect cells will be described in greater detail below in reference to  FIG. 7 . After the packet has been stored in memory, the associated info cell in key buffer  516  (a route look-up request) is forwarded through the controller interface  512  to the controller  106  for processing. Alternatively, the info cell may be transferred after the first five cells have been stored in memory. 
     As described above, the data cells are stored in the global buffer upon receipt. The data path for the data packets flows directly from the input port on which a packet is received (the multi-function multiport  150 ) to the global data buffer  104 . The data packets remain in the global data buffer  104  while a routing decision is made in a separate control path using controller  106 . The separation of the data path and control path allows for the sharing of the memory resources among all of the input ports. 
     The linking or threading of cells for a packet is performed by using the offsets described above and indirect cells. Offsets are used to link cells in a packet. Offsets may be stored along with key information and routed through controller  106  ( FIG. 2B ) or may be stored in indirect cells. In one embodiment, if a cell contains 5 cells or less, no indirect cells are required to be used. Indirect cell processor  510  performs the linking of cells in memory for a given packet. Indirect cell processor  510  generates indirect cells for storage in global data buffer  104 . Indirect cells contain offset information associated with the relative offset in memory space between contiguous cells in the packet. Indirect cell processor includes indirect cell memory  520  for storing indirect cell data during the formation of indirect cells. 
     Referring now to  FIG. 7 , the data structure for an indirect cell  700  includes a linking field  702 , a plurality of offset fields  704 , and a last field  706 . Linking field  702 , when not set, indicates the current indirect cell is the last cell in the chain of indirect cells for a given packet. If set, then more indirect cells exist for the given packet. If more indirect cells exist, then last field  706  indicates the offset to the location in memory of the next indirect cell associated with the packet. In one embodiment, indirect cells contains up to 56 offset data blocks for linking 56 cells in memory. 
     As was described above, when a packet is received, the linking engine processes the first five cells and stores linking information in the form of a start address and four offsets in key buffer  516 . In the event more than five cells are contained within a packet, the indirect cell processor takes over for the linking engine and computes the offsets associated with the locations in memory where the remaining cells in the packet are stored. Round robin processor  500  passes cells to the output processor  505  for transfer to an associated memory bank in global data buffer  104 . Round robin processor  500  enables the indirect cell processor when the packet being processed contains more than 5 cells (based on header information included within the first cell). At the time for writing the fifth cell to memory, indirect cell processor  510  stores in indirect cell memory  520  the address (the “indirect cell address”) associated with the location in memory at which the fifth cell would have been written if it had been the last cell in the packet. The indirect cell address indicates the location in memory where the indirect cell is to be written when full (or when the last cell of the packet is processed). 
     When an indirect cell is full (having stored offsets in all available locations except the last field  706 ), then the indirect cell processor stores the offset associated with the location in memory where the next indirect cell is located in the last field  706 . Thereafter, the full indirect cell is written to its appropriate place in memory. The writing of the indirect cell to memory coincides with the receipt of an indirect cell placeholder by the input switch  100  from the associated multi-function multiport  150 . This process continues until the last cell in a packet is stored in memory. At that time, the last indirect cell is written to memory, and the associated entry  600  from the key buffer  516  is transferred to the controller  106  for processing. For a given packet, all indirect cells are written to the same memory bank in the global memory buffer. 
     As often will be the case, the last cell of a packet will not coincide with the timing required to write the completed indirect cell immediately into memory. This is because packet length is completely arbitrary. The end of a packet will likely not coincide with the last available entry of an indirect cell. When a packet has completed (all cells have been received by the input switch) and a last entry in the indirect cell is written, the indirect cell is free to be written to memory. However, the writing will be delayed until the proper time, hence the term delayed indirect cell. A delayed indirect cell is a indirect cell that is the last indirect cell associated with a packet. It is delayed, because it is written to memory after the rest of the packet has been written to memory. The timing of the write to memory is dictated by the address which is reserved for the indirect cell. As was described above, at the time for the creation of an indirect cell, its position in memory is reserved. The delayed indirect cell will be written to memory at the next time slot available for the particular multi-function multiport to write to the particular memory bank after the packet has been completed. The timing of the write to memory of delayed indirect cells coincides with the receipt of a delayed indirect placeholder from the appropriate multi-function multiport  150 . 
     c) Transfers to Memory 
     The data structure of a cell transferred from input switch  100  (via the output processor  505 ) to a memory bank  105  in global data buffer  104  is shown in  FIG. 8 . The unit of addressing and memory allocation is a 64-byte cell, and all accesses to memory are either cell reads or cell writes. A cell size of 64 bytes was chosen as a compromise between the conflicting requirements of bandwidth efficiency and storage efficiency. DRAM bandwidth efficiency dictates larger sizes, while storage loss caused by internal fragmentation when fitting variable size packets into fixed size cells dictates smaller sizes. 
     At each cell slot, output processor  505  generates a cell  800  which includes a read request source field  802 , read address  804 , write address  806  and data field (cell data received from multiport  150 )  808 . The read request source field  802  indicates the output port (in the particular multi-function multiport  150 ) requesting the read (destination output port). Output processor  505  receives read requests from read controller  517  and bundles the read request with any write request received from round robin data handler  500  destined for the same memory bank. At each cell slot, output processor  505  provides a cell  800  which may include a write and read request to each memory bank  105  in global data buffer  104 . 
     Read controller  517  controls the transfer of read request signals flowing from input switch  100  out memory interface  502  to the individual memory banks in global data buffer  104 . Read controller  517  receives read requests from each multi-function multiport through output interfaces  506 . The format of each request includes source identification (output port) and a full address in memory which is to be read. At each cell slot, each multifunction multiport port may generate a read request for processing by switch  100  to read a memory location in global data buffer  104 , resulting in the reading of a cell (a read reply) from a memory bank  105  (on a subsequent cell slot) to output switch  102 . 
     Read controller  517  loads a reservation table  508  as requests to transfer packets are received from the various multi-function multiports  150 . The reservation table is loaded such that at every cell slot a single read request is generated for each bank of memory  105 . Referring now to  FIG. 9 , reservation table  508  includes a plurality of columns  900 , one for each memory bank  105  in global data buffer  104 , a plurality of rows  902 , placeholders  904  and loaded entries  906 . Each row represents a set of read requests (one per memory bank) to be generated on a single cell slot. Each row includes a single entry for each multi-function multiport  150 . At each cell slot, each multi-function multiport is capable of requesting a read from a single memory bank  105  in global data buffer  104 . Associated with reservation table  508  is a read pointer  908 . The pointer points to the next row in the reservation table to be read. Rows ahead of the read pointer correspond to requests that will be queued at a later cell slot time. In one embodiment, the pointer moves at least one row in each cell slot time. 
     Loaded entries  906  reflect read requests to be performed as a result of reservation requests received from individual multi-function multiports. Placeholders  904  represent available slots in which read requests for a particular memory bank are still available (e.g., read requests which have not as of yet been received for this memory bank from a particular multi-function multiport). At each cell slot, the read controller  517  performs three functions: loading entries in the reservation table at the first available location in the table (after the read pointer), outputting the last row as read requests to the output processor  505 ; and refreshing the table, moving out the last row, incrementing the rows and creating a new row at the top of the table. The number of rows in the reservation table must be as large as the product of the latency in processing read requests multiplied by the number of banks. In one embodiment, 48 rows are included in reservation table  508  reflecting a system including six cell slots of latency and eight memory banks. 
     At initialization, reservation table  508  contains placeholders  904  in all of the rows  902 . Placeholders  904  are locations in the reservation table which have not been loaded. As read requests are processed by the read processor, certain ones of the placeholders  904  are converted to loaded entries  906  based on the read requests. Loaded entries  906  include a read request address. 
     Referring now to  FIG. 10 , the process of loading the reservation table includes receiving a read request (full address) from an multi-function multiport ( 1000 ). The read controller decodes the read request to determine the column (based on the memory bank to be read from) in the reservation table to search ( 1002 ). The read processor searches, starting at the location indicated by pointer  908  in the reservation table, for the first placeholder associated with the multi-function multiport that generated the read request ( 1004 ). The read processor transforms the placeholder  904  to a loaded entry  906  by writing the full address of the read request at the location ( 1006 ). The process repeats for each read request received by the read controller ( 1008 ). 
     Memory Architecture 
     Referring now to  FIGS. 11A-C , main memory  1050  is used as temporary buffer storage for packets flowing into the system on input streams  1052  and out of the system on output streams  1054 . Main memory is divided into two distinct parts: a global data buffer  104  that is used to store incoming packets while the lookup engine determines the outgoing stream for each packet; and packet notification queues  319  that are used to store packet pointers (notifications) after the outgoing stream has been determined. Notification queues  319  are associated with outgoing streams, whereas the global data buffer  104  forms a common pool shared amongst all the streams. 
     Global data buffer  104  includes a plurality of memory banks  105 . Associated with each memory bank is an input switch interface (an input port)  304  and output switch interface (an output port)  316 . At each cell slot, each memory bank receives at most one write and one read request via input switch interface  304 . The write requests are associated with cells received from a multi-function multiport  150 . Read requests reflect a request for cell data to be transferred from a memory bank  105  to output switch  102  for ultimate transfer to a requesting multi-function multiport  150 . 
     The memory in the multi-function multiport configuration is physically distributed across a number of banks b, one bank for each active multi-function multiport in the system.  FIG. 11C  show a system including three banks, numbered 0, 1 and 4 respectively, representative of three active multi-function multiports. Each bank is divided into two contiguous, non-overlapping regions referred to as global data area (memory bank  105 ) and the notification area (notification queue  319 ). The global data area for a bank constitutes 1/b of the memory of the global data buffer  104 . The notification area provides space for queuing notifications that will be sent out on the line output interface  308  for a given multi-function multiport. Typically, the global data area is four times larger than the notification area; this factor derives from the ratio between data size and notification size for the shortest packet. 
     In one implementation, each bank&#39;s memory bandwidth is sufficient for reading and writing packets from a full-duplex OC-48 interface as well as for queuing and dequeuing notifications for the worst-case example of single-cell packets. Thus, both the aggregate memory size and the aggregate memory bandwidth scale linearly with the number of active multi-function multiports b in the system. 
     In one embodiment, each memory bank is implemented as two sub-banks using two 72-bit wide SDRAM (static dynamic random access memory) DIMM&#39;s (dynamic in-line memory modules) cycling at 125 MHZ. The sub-banks are transparent to the input and output switch resulting in what appears to be one continuous bank from the perspective of the switches. However, the sub-bank architecture allows for better throughput. Each DIMM has a 72-bit wide ECC (error correction code) protected data path going to 9 SDRAM chips each of which is 8 bits wide. The two DIMM&#39;s have separate address busses and are addressed independently of one another. The DIMM&#39;s are interleaved on bit  0  of the 23-bit address. In one embodiment, the smallest memory bank configuration is 32 MBytes, using 16 Mbit chips and the largest is 512 MBytes, using 256 Mbit chips. 
     As was described above, a bank can receive at most one read request and one write request every cell slot. Since a cell slot is 20 clock cycles at 125 MHZ, this works out to a peak bandwidth demand of 400 MBytes/sec for reads and 400 MBytes/sec for writes. The worst case notification load occurs for single cell packets. For unicast traffic, this load is exactly ¼ the data bandwidth which works out to 100 MBytes/sec for reads and 100 MBytes/sec for writes. In this embodiment, the total peak memory bandwidth needed is therefore 1 GByte/sec. 
     In this embodiment, the peak transfer rate of each DIMM is 1 GByte/sec, but the sustained rate depends on the actual mix of reads and writes and how the addresses are distributed over the internal DIMM banks. In practice, each DIMM is expected to deliver a sustained data rate of around 650 MBytes/sec. The total of 1.3 GBytes/sec supplied by the two groups is 30% larger than the maximum sustained requirement of 1 GByte/sec/. The 30% headroom provides a way to sustain instantaneous loads where one DIMM has more references directed to it than the other. The memory controller for the two DIMM&#39;s resides in the multi-function multiport. 
     In one embodiment, all banks are made the same size and approximately ⅕th of the memory in each bank is allocated to the notification area and ⅘th to the global data area. The purpose of this allocation is to make it exceedingly unlikely for a stream to run out of memory because of space in its notification queue. With a worst case packet size of 64 bytes, notifications (sized at 16 bytes) need ¼th the amount of storage that packet data needs, which is exactly the proportion allocated. Any cell in the global data buffer may be accessed via its physical cell pointer, which identifies the physical bank number and the address of the cell within the bank. The physical cell pointer defines a system-wide physical address space. To simplify address computations, as well as to provide a mechanism to detect old packets, accesses to the global packet buffer are performed through a system-wide virtual address space that maps to the physical address space. 
     Incoming packets are broken up into as many cells as needed and the cells are written to the global packet data buffer as they arrive as described above. The global data buffer is treated as a single large circular buffer. The input switch maintains an array of write pointers, one per active bank, to keep track of where to write the next cell. The pointers start out at one end of the buffer and advance until they eventually wrap around and overwrite packets that were written a long time ago. An ageing mechanism is used to guard against reading cells that may have been overwritten by subsequent packets. The cells of packets arriving on a given stream are interleaved strictly across the active banks to spread the bandwidth load. 
     a) Transfers from the Input Switch to Memory 
     Referring now to  FIG. 12 , the transfer of cells from the input switch  100  to global data buffer  104  is performed in a time division multiplex fashion. That is, consecutive cells from a given multiport are directed to different memory destination locations. At each time period (cell slot), the input switch transfers to memory a single cell received from each multi-function multiport (as available) into memory. At a next time T+1 the input switch transfers again a single cell from each multiport into memory. Successive entries from the same input multiport are written to different memory banks  105  in global data buffer  104 . 
     Controller 
     Referring now to  FIG. 13A , controller  106  includes controller memory  109 , route look-up engine  110 , input switch interface  1300 , and output switch interface  1302 . Controller  106  receives a route look-up request from input switch  100  at the input switch interface  1300 . In one embodiment of the present invention a plurality of route look-up engines  110  are included in controller  106 , each receiving look-up requests in round-robin fashion so as to speed the routing process. In one embodiment, controller memory  109  is a four-bank static random access memory (SRAM) that requires thirty two route look-up engines  110  to service at full bandwidth. 
     The present invention is scalable with respect to performance. That is, the number of route look-up engines  110  included within the controller may be increased to provide higher performance without requiring an increase in memory size. In one embodiment, the number of route look-up engines is eight times as great as the number of memory banks in controller memory  109 . Alternatively, lesser cost and performance units may use lesser numbers of route look-up engines  110 . 
     a) Controller Operation 
     Referring to  FIGS. 2B and 13   a , in operation, packets are received at an input port  150 , transferred to input switch  100  and stored temporarily in memory  104 . When the packet is received by switch  100 , a key extraction engine reads the key from the first data block in the packet and transfers the key to controller  106 . The input switch also includes a transfer engine for transferring packets received from an input port  150  to memory  104 . 
     The key includes at least destination information and may also include source information, a flow identifier and physical source information (input port ID). The key is located in the header field associated with the first block of data in a packet. The header may contain other information (ISO layer 2 and layer 3 headers), such information is passed to memory for storage. The process of reading key information from a packet is known in the art. The present invention accommodates keys of various types. For example, keys for various protocols may be designated (IPV4, IPV6, etc.). The length of the key is user definable. In general, the key is derived from the header, but portions may also be derived from the payload (data field associated with the packet). 
     When the controller receives the key information, it must determine a key type. In one implementation, a plurality of key types are defined. The user may define up to 4 types of keys, each having variable length. The key type can be defined by a two bit field in the header. A look-up of the two bit field is used to determine an appropriate trie to search. 
     Thereafter, an assigned route look-up engine  110  performs a trie based search for the best variable length match of the key, with each key type defining a particular trie for searching. A trie is a data structure that is used to locate the best (longest) matching route for a given key. At the completion of the search, the route look-up engine returns a result which includes the output port associated with the destination. The result and other information (source ID, flow ID, packet length, quality of service and statistical information) for routing the packet through the router combine to form a notification. The notification is transferred from the controller  106  to the output switch  102 . Upon receiving the notification, the output switch  102  initiates the transfer of the packet from memory  104  to the respective output port  150  associated with the result. 
     Referring now to  FIG. 14 , the data structure associated with the notification outputted by the controller  106  to the output switch  102  is shown. The data structure  1400  for the notification includes a mask  1402 , a next hop index pointer  1404 , full address  1406 , offsets  1408  and packet length  1410 . 
     The mask field  1402  is used to indicate which multi-function multiport connected to output switch  102  is to transfer the packet. In one embodiment, the notification may be sent to more than one multi-function multiport resulting in the broadcast of the associated packet. Associated with each multi-function multiport  150  is a storage  310 . The next hop index pointer points to a location in storage  310 . Storage  310  is used to store media header information associated with a particular type of packet transfer. Next hop addresses, media headers and storage  310  will be described in greater detail below in association with the output section of multi-function multiport  150 . 
     The full address  1406  indicates the starting address in the global data buffer where the first cell in the packet is stored. As was described above, offsets  1408  provide linking information for retrieving cells or an indirect cell associated with the packet. Packet length filed  1410  indicates the length of the associated packet and may be used to determine if indirect cells will have to be retrieved. 
     1) Route Look-Up 
     Route look-up engine  110  performs the best match variable length look-up based on a modified radix trie search routine. Specifically, in one embodiment of the invention, a route table is stored in memory  109  in the form of one or more tries. Each trie is comprised of one or more nodes including a start node  2300 , and may include internal nodes (without an attached route)  2302 , internal nodes with an attached route  2304  and route nodes  2306  as is shown in  FIG. 13B . In one embodiment, the starting node includes an attached route that is guaranteed to match any key having a type associated with this particular trie. In this way, the attached route provides a default path in the event no other match is located in the trie for a particular key. 
     Nodes are the decision points in the look-up process. A key associated with a packet has various of its bits tested at nodes along the trie, and depending on the value of the indicated bit(s) in the key being tested, will result in the traversal down a particular branch of the trie. The bits of a key are numbered left to right. Associated with each node is a bits-to-test indicator. The value of the bits-to-test indicator in a trie branch increases until a leaf (route node) is reached. Internal nodes may be of two types, those with and those without attached routes. An internal node with an attached route  2304 , may be a match for a key. Internal nodes without attached routes are never a match for a key. Route nodes  2306  are nodes which have no children and, accordingly, only contain route information. 
     The data structure for each internal node in a trie according to one embodiment of the present invention is shown in  FIG. 13C . Associated with each node is a bits-to-test indicator field  2400 , a left child pointer  2402 , a right child pointer  2404  and an attached route  2406 . The bits-to-test indicator field indicates the bit(s) in the key which is (are) to be tested at this node in order to determine which branch (left child or right child for a single bit implementation) in the trie should be traversed in the look-up process. The number of bits included in the bits-to-test indicator field  2400  may be variable. The number of children that branch from a given node is determined by the number of bits in the bits-to-test indicator field  2400 , e.g., one bit yields two children, two bits yields four children and so on. 
     In a one bit implementation, a left child pointer  2402  and right child pointer  2404  are included. The left child pointer points to a left child in the trie structure while the right child pointer  2404  points to a right child in the trie structure. A left child is the next node in the trie to be traversed when the bit tested for the present node has a value of 0. Conversely the right child is the next node in the trie to traverse when the bit tested associated with the present node has a value of 1. 
     In a two-bit implementation, a left child pointer  2402 , left-center child pointer  2403 , right child pointer  2404  and right-center child pointer  2405  are included as shown in  FIG. 13 d   . A left child is the next node in the trie to be traversed when the bits tested for the present node have a value of 00. Conversely the right child is the next node in the trie to traverse when the bits tested associated with the present node have a value of 11. The left-center child is the next node in the trie to traverse when the bits tested associated with the present node have a value of 01. Finally, the right-center child is the next node in the trie to traverse when the bits tested associated with the present node have a value of 10. 
     The attached route  2406  indicates a result which is associated with a particular node. In one embodiment, the attached route is two words and includes both output port and prefix information. Alternatively, the attached route may also include other data such as quality of service and statistical information. In one embodiment, the attached route includes fields associated with the route (destination) which are updated each time a packet is processed for the given route. The information is stored along with the attached route and may be downloaded in response to a query from a user or the destination. Alternatively, a portion or all of the information may be downloaded with each packet transferred to a destination as part of the notification. Often it is desirable to attach information to a route that indicates statistical information associated with a given route (destination). Examples of statistical information that may be transferred to a destination include number of packets (in a given time period), frequency of packets, average size of packets, and time since the last packet. Other statistical information including accounting information may also be stored with the attached route and may be incorporated into the notification. For example, billing information for a particular transfer or cumulative billing information may also be incorporated into the notification. In one embodiment of the present invention, controller  106  ( FIG. 2 ) includes statistical modules for calculating statistical information related to each packet transferred through the router. As each packet is processed by the controller, statistical modules update the statistical information stored with the destination route determined in the look-up process. 
     In addition, quality of service information may be stored with an attached route to speed the routing of certain packets through the switch structure. A quality of service value may be set for each route to allow for the prioritization of packets transferred out a particular output port. 
     In one embodiment, the data structure of an attached route is comprised of a prefix length, a prefix (the portion that is left justified and matched against the key), a result field, a statistics field and a quality of service field. Prefix length indicates the length of the prefix in bits. The prefix is a pattern (portion of a key) that is compared against a key to determine a best match. The result field stores the output port (destination port) associated with the particular route (prefix). In the event that the prefix associated with a particular node is itself the best match for a given key, then the attached route indicates the output port to which the packet is to be routed. In one embodiment of the present invention, the attached route is a pointer which points to the location in memory where the attached route is stored. Alternatively, the attached route may be stored in consecutive memory locations with the rest of the node information as described above. 
     The data structure for a start node according to one embodiment of the present invention (a one-bit bits-to-test indicator field) is shown in  FIG. 13E . Associated with the start node are a left child pointer  2402 , a right child pointer  2404  and an attached route  2406 . In one embodiment of the present invention, no bits-to-test indicator field is stored in memory for the start node. This is because, typically the bits-to-test indicator for the start node is set to bit zero. Alternatively, another bit(s) in the key may be the first bits-to-test. If another bit(s) is the first bits-to-test, then the associated bit identifier is stored in a bits-to-test indicator field for the start node. 
     The data structure for each route node according to one embodiment of the present invention is shown in  FIG. 13F . Each route node has only an attached route  2406 . 
     Internal nodes without routes attached have a data structure as shown in  FIG. 13G . Specifically, no space is set aside for an attached route. Otherwise, the data structure is identical to that shown in  FIG. 13C . 
     In a typical router implementation, a route table is not fully populated. That is, not all of the possible key values are mapped to a particular output port. Accordingly, some bits in the key are “don&#39;t care” values, and are not required to be tested during the route look-up process, because they do not affect the determination of the output port. In some prior art systems, the look-up of keys in the route table required the comparison of each bit in the key with entries in the route table. Such systems were inefficient. With a trie structure, not every bit in a key needs to be tested in order to determine the best match associated with a particular packet. By testing only the smallest number of bits that allow the discrimination between a key and existing routes, the speed of determining a match is improved. 
     ii) Trie Creation 
     Accordingly, for each key type, a route table is created in trie format. Referring now to  FIG. 13H , in a method of creating a trie, a start node is created for the trie ( 2500 ). The start node is assigned an attached route ( 2502 ). The left and right child pointers are assigned a null value (a binary 511 in one embodiment of the present invention) indicating that no left or right child exists ( 2504 ). Finally, the four word start node including left child pointer ( 1 ), right child pointer ( 1 ) and attached route ( 2 ) is stored in memory  108  ( 2506 ). 
     A check is made to determine if any other routes are needed to be placed in the trie ( 2508 ). If not then, the process ends ( 2510 ). Else, the next route is loaded ( 2512 ). The first route in the trie is added as a route node ( 2514 ). An associated route is assigned based on the destination for the particular prefix ( 2516 ). The attached route and prefix information is stored in memory ( 2518 ). Thereafter, the appropriate pointer in the start node (left child pointer or right child pointer) is updated based on the value of the zero bit of the first prefix ( 2520 ). If the value of the zero bit in the first prefix (route) loaded is a “0”, then the left child pointer is updated to reflect the starting address in memory where the route node for this prefix is stored. Alternatively, if the value of the zero bit in the first prefix loaded is a “1”, then the right child pointer is updated to reflect the starting address in memory where the route node for this prefix is stored. 
     A check is made to determine if any other prefixes are needed to be placed in the trie ( 2522 ). If so, the next prefix is loaded ( 2523 ). The next prefix will be placed in the trie as either a route node, an internal node, an internal node with an attached route, or as an attached route to an existing internal node. A prefix search is performed in the existing trie ( 2524 ). The search entails traversing the by testing the new prefix based on the bit indicated by the bits-to-test indicator for each node. At a given node, the bit in the new prefix designated by the bits-to-test indicator for the node is checked in order to determine the next node in the search. The prefix search continues until an end route is determined ( 2526 ). An end route will be found either at a route node or at an internal node having an attached route. An end route is either the route associated with a route node or the first route located on the after the bits-to-test indicator associated with the current node on the trie exceeds the number of bits in the prefix being searched. The first route located may be an attached route to an internal node, or a route node. The search must terminate at a node which has an associated route in order to determine where (and how) the new prefix will be inserted into trie. 
     That is, at the start node, the value of the zero bit is checked in the new prefix to determine if the search should proceed down a left child or the right child. At each subsequent node in the trie, the bit indicated by the bits-to-test indicator associated with the node is checked to determine a next node in the traversal. Eventually, the search will come to an end as described above. In our initial two node trie (start node and a route node), the search is deminimus, and may end at either the start node (if the new prefix being searched has a value in the zero bit different from the prefix associated with the route node) or at the route node. 
     At the end of the search, a comparison is made between the prefix being searched and the end route to determine the first dissimilar bit (BIT Dis ) in the new prefix ( 2528 ). In one embodiment of the present invention, an EXCLUSIVE OR operation on the two prefixes is performed to reveal the first dissimilar bit. If a dissimilar bit is found, then branch A in the process is performed ( 2529 ). If no dissimilar bits are found, then all of the bits of the new prefix match the end route and branch B in the process is performed. 
     Referring to  FIG. 13 i   , in branch A, a dissimilar bit has been determined, and the new prefix will be inserted into the trie as route node pointed to by a new internal node which is to be inserted into the existing trie. Specifically, a new route node is created and stored in memory indicating the destination associated with the new prefix ( 2530 ). Thereafter, a new internal node is created ( 2532 ). The bits-to-test indicator for the new internal node is assigned to be the first dissimilar bit (BIT Dis ) discovered above ( 2534 ). One of the child pointers (either the left child or the right child, in a one-bit implementation) of the new internal node is assigned the address of the new route node depending on the value of the bits-to-test indicator in the new prefix ( 2536 ). Starting at the node associated with the end route identified above, the trie is traversed in reverse to locate the first node (the parent node) having a bits-to-test indicator which is less than the first dissimilar bit (BIT Dis ) ( 2538 ). The node immediately beneath the parent node in the original trie is referred to as the grand-child node. The new internal node is inserted into the trie between the parent and the grand-child nodes by assigning the remaining child pointer in the new internal node the starting address associated with the grand-child node ( 2540 ). Finally, the address of the pointer in the parent node which previously pointed to the grand-child node is updated to indicate the starting address in memory of the new internal node ( 2542 ). Branch A is complete. 
     Referring now to  FIG. 13 j   , in branch B, no dissimilar bits were determined, necessitating a comparison of the length of the new prefix with the length of the end route ( 2545 ). If the length of the new prefix is longer than the length of end route then branch B1 is traversed ( 2546 ). If the length of the new prefix is shorter than the length of end route, then branch B2 will be traversed ( 2547 ). Finally, if the length of the new prefix is the same as the length of end route, branch B3 will be traversed. 
     Referring now to  FIG. 13K , in branch B1 (the new prefix is longer than the length of end route), the new prefix will be inserted into the trie below the node associated with the end route as a new route node. Specifically, a new route node is created and stored in memory indicating the destination associated with the new prefix ( 2550 ). Thereafter, a determination is made of the type of node associated with the end route ( 2552 ). If the node associated with the end route is an internal node with an associated route, then the appropriate child pointer (based on the value of the bit in the new prefix of the bits-to-test indicator associated with the internal node) is assigned the starting address of the new route node ( 2554 ). Else, a new internal node is created ( 2556 ). The bits-to-test indicator for the new internal node is assigned to be the last bit in the end route ( 2558 ). One of the child pointers (either the left child or the right child, in a one-bit implementation) of the new internal node is assigned the address of the new route node depending on the value of the bit in the new prefix of the bits-to-test indicator associated with the new internal node ( 2560 ). Finally, the address of the pointer in the parent node (the node immediately preceding the route node associated with the end route and whose pointer previously indicated the starting address of the route node associated with the end route) is updated to indicate the starting address in memory of the new internal node ( 2562 ). Branch B1 is complete. 
     Referring now to  FIG. 13L , in branch B2 the new prefix is shorter than the end route. The new prefix is inserted into the trie above the node associated with the end route, as either a new route or as an internal node with a route attached. Specifically, starting at the node associated with the end route, the trie is traversed backwards until a node (the parent node) is found having a bits-to-test indicator less than or equal to the index (length) of the new prefix ( 2570 ). The node immediately beneath the parent node in the original the is referred to as the grand-child node. 
     If the bits-to-test indicator is less than the index of the new prefix ( 2571 ), then a new internal node is inserted below the parent node ( 2572 ). The bits-to-test indicator for the new internal node is assigned the value of the index (length) of the new prefix ( 2574 ). Accordingly, the new internal node tests the first bit after the end of the new prefix. The new internal node is inserted into the trie between the parent and the grand-child nodes by assigning one of the child pointers (as indicated by the value of the bits-to-test indicator of the new internal node) the starting address associated with the grand-child node ( 2576 ). The other child pointer (either the left child or the right child in a one-bit implementation) of the new internal node is assigned a null value (binary 511 in one embodiment) ( 2578 ). Finally, the address of the pointer in the parent node which previously pointed to the grand-child node is updated to indicate the starting address in memory of the new internal node ( 2580 ). 
     If the bits-to-test indicator of the node discovered in the backwards search is equal to the index of the new prefix, then the new prefix is inserted as an attached route for the node ( 2590 ). Specifically, the attached route associated with the node determined in the search is updated to reflect the destination of the new prefix. Branch B2 is complete. 
     Referring now to  FIG. 13M , in branch B3 the new prefix is the same length as the end route. The new prefix is inserted into the trie as an update to the end route ( 2592 ). Specifically, the route associated with the end node is updated to reflect the destination of the new prefix. 
     Referring now to  FIG. 13N , at the completion of branches A and B (and the various sub-branches of branch B including B1, B2 and B3), a check is performed to determine if any more prefixes need to be inserted into the trie ( 2598 ). If so, then the process continues at step ( 2523 ). Else, the process ends ( 2510 ). 
     In one embodiment of the present invention, the process of constructing a trie is performed for each type of key based on an initial set of prefixes and associated destination information. In addition, each trie may be updated dynamically by a routing protocol associated with the controller  106  ( FIG. 2 ). The routing protocol services the tries, and may insert new prefixes as necessary. The process for inserting a new prefix is identical to the process described above for inserting a next prefix after the first prefix in the trie has been installed as a route node (steps  2522 - 2592 ). 
     The updating process advantageously does not require the shutting down of the route look-up process. As was described above, the insertion of a route is accomplished off line, and only the final step in the process affects the trie. The final step in any insertion process requires writing in memory the address for a new node to the new node&#39;s parent. Accordingly, searches in a trie may be performed the entire time that an insertion process is operated to insert new entries or make changes to existing entries. The trie is never invalid, thus never requiring the shutting down of the look-up process for trie maintenance. 
     iii) Route Look-Up 
     Having created a route table including one or more tries which are stored in memory as described above, the router is ready to process packets. A key is copied from an inbound packet by input switch  100  ( FIG. 2 ) and transferred to a selected route look-up engine  110  ( FIG. 2 ) in the controller  106  ( FIG. 2 ). Referring to  FIGS. 2 and 13   p - q , in a best variable length match look-up method, a key type is determined for a new key to identify the particular trie that must be traversed in the look-up process ( 2600 ). The look-up process includes a forward and backward traversal of the trie to determine the best match associated with a given key. The forward search begins at the start node. Specifically, the four word data block associated with the first node in the appropriate trie type is loaded into a buffer in the route look-up engine  110  from memory  108  ( 2602 ). A bit test engine in route look-up engine  110  determines the value of the bit(s) in the new key indicated by the bits-to-test indicator (as defined by the bits-to-test indicator field retrieved in step ( 2602 )) ( 2604 ). Thereafter, the route look-up engine determines if the pointer indicated by the bit(s) value (either the left child pointer or right child pointer in a one-bit implementation) indicates a child node ( 2606 ). If no child is indicated, then route look-up engine  110  performs a key comparison as is shown in branch C. 
     If the pointer indicates a child, then a check is made to determine if the start node has an attached route ( 2608 ). If an attached route is associated with the start node, the route look-up engine stores the starting address in memory for the attached route (or the starting address associated with the contiguous memory locations associated with the start node) and the bits-to-test indicator associated therewith on a forward search stack in the route look-up engine ( 2610 ). If no attached route exists, the process continues at branch G. The route look-up engine thereafter loads the data words associated with the next node indicated by the respective child pointer ( 2612 ). For example in a one-bit implementation, where the start node bits-to-test indicator is set to the “zero&#39;th” bit, and if the zero&#39;th bit in the new key location has a value of “0”, then the route look-up engine will load the node indicated by the left child pointer. Alternatively, if the zero&#39;th bit has a value of “1”, the route look-up engine will load the node indicated by the right child pointer. In one embodiment of the present invention, the loading process of a node entails loading all of the data blocks associated with a given node (5 for internal nodes or 2 for route nodes). Alternatively, a lesser number of data words may be loaded as will be described below. 
     Thereafter the route look-up engine determines if the new node is a route node ( 2614 ). If so, then a route has been determined and the process continues at branch C. If not, then the route look-up engine (bit comparison engine) determines the bit value in the new key of the bit indicated by the bits-to-test indicator associated with the new node (node loaded in step  2612 ), where the bits-to-test is defined by the bits-to-test indicator field retrieved as part of the load in step  2612  ( 2616 ). Thereafter, the route look-up engine determines if the pointer indicated by the bit value (either the left child pointer or right child pointer) indicates a child node ( 2618 ). If no child is indicated then the route look-up engine performs a key comparison as is shown in branch C. 
     Alternatively, the route look-up engine checks to determine if the new node has an attached route ( 2620 ). If so, then the process continues at branch F resulting in the storage of the address associated with the attached route onto the stack (or the pointer indicating the starting address in memory for the attached route) prior to the loading of a next route. Alternatively, the process continues at branch G and the next node is loaded for processing. The route look up engine repeats this process for each node (steps  2610 - 2620 ) until an end node has been located. 
     An end node may be a route node or an internal node having an attached route. An internal node with an attached route will only be an end node if the value of the bit(s) indicated by the bits-to-test indicator in the key being tested indicates a child pointer associated with the internal node that points nowhere (null). When an end node is reached, the route look-up engine performs branch C. Specifically, if the pointer indicated by a bits-to-test value in the new key points to a route, then the forward search portion of the process ends when the route is retrieved. If however, the pointer indicates a null, the search ends with the internal node. 
     Branch C can be characterized as follows: the forward search has terminated at an internal node (the end node) having a pointer to a child which has no data, and, this “no-data” pointer is indicated by the bit(s) value(s) in the new key of the bits-to-test bit(s) of the end node; or, the search has terminated at a route node. The backward portion of the search begins by comparing the route associated with the current node (either an attached route for an internal node or a route associated with a route node) and the new key. 
     At this point, the best match for the new key will be at the end node or is guaranteed to be somewhere back-up stream (along the path from the end node back to the start node). Accordingly, in order to determine the best match, a key comparison engine in route look-up engine  110  compares the key being searched and the end route (the route associated with the end node) to determine the first dissimilar bit (BIT Dis ) in the new key ( 2630 ). In one embodiment of the present invention, an EXCLUSIVE OR operation on the new key and prefix (route) is performed to reveal the first dissimilar bit. If a dissimilar bit is found, then branch D in the process is performed ( 2632 ). If no dissimilar bits are found, then all of the bits of the new key match the end route and branch E in the process is performed. 
     In branch D, a dissimilar bit has been determined, and the end route is not a match for the new key. Starting at the end node, the trie is traversed in reverse to locate the first node (the best match node) having a bits-to-test indicator which is less than or equal to the bit number of the first dissimilar bit (BIT Dis ) ( 2640 ). In one embodiment of the present invention, this is performed by popping off entries from the forward search stack in the route look-up engine (each entry associated with nodes that have attached routes and have been traversed as part of the forward search), until an entry is located which has a bits-to-test indicator value that is less than or equal to the bit number of the first dissimilar bit. When the best match node has been located, then the process continues at branch E. 
     In branch E, a best match route has been determined either by discovering no dissimilar bits when comparing the new key to the end route, or by traversing the trie and locating the route associated with the best match node. The route look-up engine loads the best match route from memory ( 2650 ). In one embodiment, if the end route associated with the end node matches the new key, no load is required, such load having already been accomplished as part of the loading of the end node in the forward search of the trie. Alternatively, if the trie is required to be traversed backwards, the route information is loaded based on the pointer stored on the stack. The route look-up engine transfers the best match route information to the output switch  102  ( FIG. 2 ) ( 2652 ). Thereafter the process ends ( 2654 ). 
     One of the advantages of the present invention is that the trie structure guarantees a best match for each key transmitted through the switch. As is described above, each route stored in the trie has a prefix associated therewith. A prefix is a left justified subset of a key. Accordingly, a key may match many prefixes in a trie. The best match is the matching prefix having the longest length. The system of the present invention provides for a best (longest) match for a key. In addition, only a single key comparison is required in the entire matching process. The key is compared to a single prefix (at the end node) and thereafter the best match may be determined without performing any additional comparisons of the key to prefixes stored in the route look-up table. 
     iv) Minimized Data Word Loads 
     As was described above in conjunction with the forward traversal of a trie, a load operation is performed of the data words associated with each node in the forward search. This requires the loading of up to five words per node (bits-to-test indicator, left child pointer, right child pointer, and attached route (2 words)). Alternatively, lesser number of words may be loaded. 
     Specifically, in one embodiment of the invention, only the bits-to-test indicator and child pointers are loaded for a given node. In the event that the node is determined to be the end node in a forward search, then the attached route information can be loaded as required. 
     In another embodiment of the present invention, only the bits-to-test indicator is loaded initially for a given node. After the bits-to-test indicator is loaded for a given node, the bit(s) value of the new key is used to determine which child should be loaded for this node, if any. For example, if the bit value of the bit in the new key indicated by the bits-to-test indicator is a “0” in a one-bit implementation, the left child only is loaded. If the bit value in the bit in the new key is a “1”, then only the right child is loaded. In this way, even where plural bits are to be tested, only a single load is required to retrieve the correct next child node in the trie. 
     v) Data Structure 
     In another embodiment of the present invention, an alternative data structure associated with the nodes is used to minimize memory accesses. Referring to  FIG. 13R , a data structure for an internal node without an attached route is shown in a one-bit implementation. Each internal node without an attached route includes a left child pointer  2700 , and a right child pointer  2702 . The left and right child pointers include two status bits  2704  and  2706  (little “r” and big “R”, respectively), a pointer field  2708  which points to the starting address of an associated child, and a bits-to-test indicator field  2710  which indicates the bits-to-test in the child node indicated by the pointer field  2708 . The first status bit (little “r”)  2704  indicates whether the pointer stored in the pointer field  2708  points to a route node or an internal node. The second status bit (big “R”)  2706  indicates whether an attached route exists for this node. 
     The data structure for an internal node with an attached route for a one-bit implementation is shown in  FIG. 13S . Each internal node with an attached route includes a left child pointer  2700 , a right child pointer  702  and an attached route  2720 . The left and right child pointers include two status bits  2704  and  2706  (little “r” and big “R”, respectively), a pointer field  2708  which points to the starting address of an associated child, and a bits-to-test indicator field  2710  which indicates the bits-to-test in the child node indicated by the pointer field  2708 . The attached route  2720  contains an index  2722 , a prefix  2724 , a result  2726  (destination port), a statistics field  2728  and quality of service field  2730 , the functions of which have been described above. 
     The data structure for a route node for a one-bit implementation is shown in  FIG. 13T . Each route node includes an attached route  2720 . 
     With this data structure, a single word per node is required to be loaded in order to traverse the trie. Only minor variations in the creation of the trie and in the traversal of the trie are required, yet the memory accesses required have been reduced substantially. 
     During the look-up operation, the benefits of the data structure proposed above are readily apparent. Referring to  FIG. 13U , the process of performing a best match variable length look-up of a route for a one-bit implementation begins with determining the type of key for a new key received at the controller ( 2800 ). The starting address of an appropriate start node is determined based on the trie type ( 2802 ). Thereafter, a check is made of the value of the bit in the new key indicated by the bits-to-test indicator associated with the start node (typically bit zero) ( 2804 ). If the value of bit in the new key is a “0”, then the route look-up engine retrieves the left child pointer associated with the start node which will be located at the starting address determined above in step  2802  ( 2806 ). If the value of bit in the new key is a “1”, the route look-up engine will increment by one the starting address received in step  2802  ( 2807 ) and retrieve the right child pointer associated with the start node ( 2808 ). 
     A check is made to determine if the pointer retrieved points to a route (node) or to another internal node ( 2810 ). If the pointer points to a route node (as indicated by the status of the little “r” status bit  2704 ) then the route indicated by the pointer (the first left or right child pointer) is retrieved ( 2812 ). If the pointer points to an internal node, then the route look-up engine will store the pointer retrieved on a stack if the “R” status bit is set ( 2814 ). Specifically, the route look-up engine stores the bits-to-test indicator associated with the given node and the pointer retrieved in step  2812 . Thereafter, the route look-up engine (bit comparison engine) will test the bit of the new key indicated by the bits-to-test indicator field in the pointer retrieved in steps  2806  or  2808 . 
     If the bit(s) indicated by the bits-to-test indicator has a value of “0” in the new key, then the route look-up engine will retrieve the left child associated with the address indicated by the pointer ( 2816 ). Specifically, the route look-up engine will retrieve the data word which is indicated by the address indicated by the left or right child pointer. 
     If the bit (in step  2812 ) has a value of “1” in the new key, then the route look-up engine will retrieve the right child associated with the address indicated by the pointer ( 2818 ). Specifically, the route look-up engine will retrieve the data word which is indicated by the address indicated by the child pointer incremented by one. 
     This process repeats (steps  2814 - 2822 ) until an end route is located. Specifically, if the pointer indicated by a bits-to-test value in the new key points to a route ( 2820 ), then the forward search portion of the process ends when the route is retrieved ( 2812 ). If however, the pointer retrieved indicates a null value ( 2822 ), a check is made to determine if the node has an attached route ( 2824 ). If so, then the forward search portion of the process ends and the associated internal route is retrieved in step  2812 . If no attached route exists, a check is made to determine if the stack is empty ( 2825 ). If so, an error condition occurs and the route look-up terminates ( 2826 ). If the stack is not empty, then the last entry on the stack is retrieved ( 2828 ) and the process continues at step  2830 . The route retrieved from the route node, the internal node with attached route, or the route indicated by the stack entry will become the end route for the purposes of performing the backward portion of the look-up process. 
     Referring to  FIG. 13V , in order to determine the best match, the route look-up engine (key comparison engine) compares the key being searched and the end route to determine the first dissimilar bit (BIT Dis ) in the new key ( 2830 ). If a dissimilar bit is found ( 2832 ), then starting at the end node, the trie is traversed in reverse to locate the first node (the best match node) having a bits-to-test indicator stored on the stack which is less than or equal to the bit number of the first dissimilar bit (BIT Dis ) ( 2840 ). In one embodiment of the present invention, this is performed by popping off entries from the forward search stack in the route look-up engine, each entry associated with nodes that have been traversed as part of the forward search, until a entry is located which has a bits-to-test indicator value that is less than or equal to the bit number of the first dissimilar bit. If no dissimilar bits are found or if the best match node has been determined in step  2840 , then the route look-up engine loads the best match route, either the route associated with the best match node or the end route from memory ( 2850 ). If the best match route is the end route, no memory load is required, such load having been accomplished as part of the comparison above. Alternatively, the route information is loaded by incrementing the memory address retrieved from the stack node by two (if the starting address for the node is stored on the stack as opposed to the address for the attached route), and loading the appropriate route information. Thereafter, the route look-up engine transfers the best match route to the output switch  102  ( 2852 ). 
     vi) Multiple Memory Banks 
     Referring again to  FIG. 2 , in one embodiment of the invention, the memory  108  attached to controller  106  is a bank of four static RAMs (SRAMs). In the construction of the route table, parents and children are distributed among the SRAM banks such that no parent and child are stored in the same bank of static RAM. Accordingly, when a route look-up engine accesses a particular memory location in memory  108 , it is guaranteed to not to have to access the same memory bank in order to traverse to the next node in a given path. The use of multiple memory banks is useful to increase the number of simultaneous read operations which can be performed in the look-up process. Distribution of parent and children nodes across the memory banks helps to avoid bank conflicts. 
     Another memory saving technique included in one embodiment of the present invention is the attachment of routes sequentially in memory for a given node. As was described above, if an internal node has an attached route, then the route is placed by design immediately following the associated child pointer in memory (after the right child). Accordingly, when the best match is located for a given node, the destination information associated therewith is known precisely, and may be retrieved directly from memory (as opposed to indirect retrieval if a pointer system is used). This organization technique saves on memory accesses. 
     For example, upon a match condition, the route look-up engine must load the result which includes the output port associated with the particular switch. With the memory structure described above, the result may be loaded directly by incrementing the pointer information popped off the stack during the backward traversal of the trie (by adding 2 or 3 address locations to the stack pointers depending on the type of node for a one-bit implementation, or by adding 2, 3, 4 or 5 address locations for a two-bit implementation having four children). 
     In one embodiment of the present invention, a root table is used as a preliminary screen for matching the most significant bits in a key. A root table may be used to perform a quick match of the most significant bits in a key so as to avoid having to perform many memory accesses in the trie search. The root table is stored in controller  106  ( FIG. 2 ). Each entry in the route table includes an address to the start node in memory at which the look-up is to begin and a stack entry. The stack entry includes a bits-to-test indicator and a pointer to an attached route. The table is indexed according to the first N bits of the key. In one embodiment of the present invention, the first eight bits of the key are indexed. 
     Output Switch 
     Referring now to  FIG. 15A , output switch  102  includes a controller interface  1500 , one or more memory inputs  1502  ( 1502 - 0  through  1502 - 7 , one for each memory bank), one or more outputs  1504  ( 1504 - 0  through  1504 - 7 , one for each multi-function multiport), a result processor  1506  and an output processor  1508 . Output switch  102  performs four functions: receive output results, process output results, receive cells from memory and output cells to output ports. 
     a) Transfers from Memory to the Output Switch 
     Cells from memory are received at memory inputs  1502  and transferred to output processor  1508 . Cells are transferred based on read requests received at the input switch from multi-function multiports. The data structure associated with the cells transferred from global data buffer  104  to output switch  102  is shown in  FIG. 15B . Each cell  1510  includes an output port identifier  1512  and cell data  1514 . Output processor  1508  decodes the destination multi-function multiport from the cell information received from memory and transfers the cell data to the appropriate outputs  1502 . At each cell slot, output switch  102  may receive a cell for processing from each bank in global data buffer  104 . 
     b) Transfers from the Output Switch to the Multi-Function Multiports 
     Output switch  102  receives notification from controller  106  on controller interface  1500 . Result processor  1506  decodes the result (route) and determines which multi-function multiport(s)  150  is (are) to receive the route data. Based on mask  1402  in the notification, result processor  1506  transfers the notification to output processor  1508  for transfer to each multi-function multiport  150  indicated. At each cell slot, output processor  1508  provides (via outputs  1504 ) a route to each multi-function multiport  150 . 
     The data structure associated with the data transferred from output processor  1508  to multi-function multiports  150  is shown in  FIG. 16 . A cell  1600  includes a header  1602  and data field  1604 . The header  1602  includes memory bank source information  1606  and route information  1608 . The memory bank source information includes a source identifier for indicating which memory bank provided the cell in data field  1604 . Route information  1608  contains data from the notification including a next hop index, packet length, full address and offsets. 
     Output Section of a Multi-Function Multiport 
     Referring now to  FIGS. 17A and 17B  each multi-function multiport  150  includes an output switch interface  316 , an input switch interface  304  including read request queues  305 , head and tail queue buffer  318 , an output request processor  306 , an line output interface  308 , storage device (memory)  310 , stream output buffers  312  and output formatter  314 . 
     a) Notification Queues 
     A multi-function multiport  150  receives notification that a packet is to be processed in the form of a notification cell  1600  ( FIG. 16 ) received at the output switch interface  316 . 
     Output request processor  306  processes notifications, storing each in an appropriate location in head and tail queue buffer  318  and servicing notification requests as they make their way through the various priority queues in head and tail queue buffer  318 . The servicing of requests results in the generation of a read request to input switch  100  associated with the first address in memory where the packet (associated with the particular notification) is stored. 
     Referring now to  FIG. 17B , head and tail queue buffer  319  includes a plurality of notification queues Q 1700, where Q=4*s, and where s is the number of active streams in the multi-function multiport. Unlike the global data buffer, the queues Q are implemented on a per-port basis. The queues in a respective multi-function multiport store only those notifications associated with streams to be outputted from the respective port of the multi-function multiport. Each queue is itself divided into a head region  1702 , a tail region  1704  and a body region  1706 . The head and tail region for a queue are stored in the head and tail queue buffer  318 . 
     The size of the portion of the head and tail queue buffer dedicated to each stream is fixed at initialization time and is proportional to the peak bandwidth of its stream. The partitions between portions are “hard” in the sense that a stream cannot use more than the memory allocated to it. The partitions between queues associated with the same stream are “soft”. The size of an individual queue is proportional to the nominal bandwidth allocated to its queue. The body region of the notification queue is stored in the notification area  319  ( FIG. 3A ) of the memory section  290  of the multi-function multiport  150 . Each stream is assigned 4 queues (the body portions of the priority queues) in the notification area  319  ( FIG. 3A ). The body region is sized to be ⅕ of the overall memory section. 
     Each queue associated with a given stream is serviced according to a priority scheme. Notifications that are received by the output request processor  306  are loaded into an appropriate queue associated with a stream based on the priority of the notification. Priority for notifications can be set by an external source and may be included in the packet received by the router. Alternatively, controller  106  ( FIG. 3A ) may set the priority depending on the amount of time required to perform the route look-up or other criteria. 
     Once a queue  1700  has been identified based on the priority information and stream ID, the output request processor  306  loads the notification into the appropriate tail queue  1704 . Notifications are transferred between the respective head, tail and body portions of a queue based on available space by a queue manager (not shown). In one embodiment, each notification is 16 bytes, and the notification area  319  is sized to hold 64 bytes. Accordingly, for reasons of bandwidth efficiency, all reads and writes to the notification area are done using 64-byte cells containing four 16-byte notifications each. 
     The head and tail of each queue is sized to store only a small number of notifications, the bulk of queue storage being provided by the notification area in the multi-function multiport memory bank. As long as space is available on-chip (on the multiport) to hold the notifications for a queue, the notification area is completely bypassed. When on-chip space runs out, the notification area acts as the large “middle” of the queue, with a few notifications at the head and tail being held on-chip. 
     While the size of the notification area will tend to limit the numbers of dropped packets, occasionally a queue will become full. Output request processor includes a drop engine (not shown) for determining which entries in a particular queue are to be dropped based on a predefined algorithm. In one embodiment, the drop engine institutes a programmable random early drop routine. The routine is programmable in that the user can define one or more parameters, random in that a random number generator is used to determine whether a entry will be dropped. Early refers dropping from the head of the queue. 
     The programmable random early drop routine may be implemented in software and when executed performs the following sequence of operations. The process begins by calculating the amount of data stored in a particular queue. This information is stored in the form of a fraction (or percentage) of fullness. Thereafter, a drop criterion is determined based on the fraction of fullness. In one embodiment, a table of drop criterion values ranging from zero to one is mapped against fullness fractional values. The drop engine then derives a random number from zero to one. The random number may be generated by a random number generator or other means as is known in the art. A comparison is made between the random number generated and the drop criterion value. Thereafter, the entry at the head of the particular queue is dropped if the random number generated is larger than the drop criterion. Alternatively, the drop engine could avoid the drop if the random number generated is less than the drop criterion. The drop engine operates on each queue at a regular interval to assure that the queues do not overflow and a orderly method of dropping packets is achieved if required. This process is extremely helpful when transmitting packets across the Internet. 
     b) Per Bank Notification Queues 
     Each stream includes four queues  1700  that are serviced using a weighted round robin discipline. The weighting is used to reflect the priority associated with a given queue. For example, the four queues for a given stream may be serviced in the following ratios: Q1 at 50%, Q2 at 25%, Q3 at 15% and Q4 at 10%. 
     The multi-function multiport maintains four cell pointers for each queue: start, end, head, and tail. In one embodiment, each pointer is 23 bits long and can address the entire memory associated with the multi-function multiport. The start and end pointers mark the boundaries of the queue&#39;s region, while the head and tail pointers point to the next cell (notification) to read and next cell to write respectively. The head and tail pointers are restricted to align within the region defined by the start and end pointers, and standard wraparound arithmetic is performed when incrementing these pointers. 
     Given the description above, it should be clear that the region for a queue can be as small as one cell and as large as the entire memory bank. It is up to the software to configure the pointers at initialization time to define the sizes of the regions, and to ensure that regions are non-overlapping with each other and with the memory allocated to the global packet buffer. 
     Typically, the software is used to allocate memory to a stream proportional to the stream&#39;s bandwidth. 
     C) Read Request Generation 
     Output request processor  306  services the queues to extract notifications from the head regions of queues  1700 . Output request processor generates a first read request based on the full address received from output switch  102 . Thereafter subsequent read requests are generated for transmission to the input switch based on the offset information provided in the request (from cell  1600 ) or indirect cells (as will be described below). Read requests include a stream identifier and a full address. Read requests are sent by the output request processor to an appropriate read request queue  305 . One read request queue  305  is provided for each bank of memory. 
     If the packet length, as determined from the route information provided with the cell  1600 , is greater than five (5) cells, then the output request processor first requests the transfer (read from memory) of the first indirect cell associated with the packet. This is accomplished by computing the address of the indirect cell based on the full address and the offsets provided in cell  1600 . After the indirect cell request is generated, the output request processor generates read requests for the remaining cells in the packet based on the full address and the offsets provided in cell  1600 . Upon receipt of a indirect cell from the output switch  102 , output request processor continues to generate read requests for the remaining cells in the packet based on the offset information contained within the indirect cell. 
     Subsequent indirect cells are retrieved in a similar fashion. That is, at the time for reading the next indirect cell, the address of the next indirect cell is computed based on the last offset stored in the previous indirect cell. The timing of retrieving the indirect cells is accomplished such that no delays in the output stream are incurred. Each subsequent indirect cell is retrieved prior to the end of the processing of the prior indirect cell. In this way, once the output stream is initialized, no buffering of data is required and no interruptions due to the latency associated with the retrieval process are experienced. 
     Output requests to an individual memory bank are processed strictly in order. That is, the multi-function multiport may track each request issued to a memory bank (through the read request queues) and is assured that the data received in response to a series of requests to the same memory bank will be strictly delivered according to the sequence or pattern in which they were issued. Output request processor  306  keeps track of requests generated for each memory bank through the use of reply queues (not shown). The request queue contains a stream number and a read address. When a request is issued to memory, the entry is removed from the request queue and the stream number portion is placed in an associated reply queue. When a reply is received, the entry at the head of the reply queue is removed and the reply is sent to the stream number (in stream output buffer  312 ) indicated by the stream number retrieved from the reply queue. 
     As cells are received back at the multi-function multiport  150  (responsive to the read requests), they are stored in an associated stream output buffer  312 . Stream output buffer  312  includes a plurality of FIFOs, one for each stream. Each cell received for a stream is placed in the streams associated FIFO. For given packet, the multi-function multiport stores a fixed number of cells (in the FIFO) required to provide a streamed output prior to initializing the output of the stream to line output interface  308 . In one embodiment of the present invention, twelve cells are stored prior to beginning output (stream data) from the output port. The selection of the number of cells for storage in output buffer  312  is based on the latency in the read process (number of clock cycles between a read request from an multi-function multiport and the arrival of the cell associated with the read request to the output port). 
     Output formatter  314  receives the cells from output buffer  312  and couples the data with media header information stored in memory  310 . Each request (notification) received from output switch  102  includes a next hop index. The next hop index indicates the starting address in memory  310  of the media header information associated with a given type of transmission (derived from the destination of the packet). Media header information stored in memory  310  may be loaded upon initialization of the router and updated by the controller as required. Output formatter  314  couples the cell data returned from memory with the appropriate media header to generate a proper packet for transfer out of router  20  on the line output interface  308 . 
     Packet Routing Overview 
     Referring now to  FIG. 18 , in a method of routing packets through a switch a packet is received at a multi-function multiport ( 1800 ). The multi-function multiport divides the packet into fixed length cells and transfers the cells to an input switch ( 1802 ). Input switch removes the key information from the first cell in a packet and stores it temporarily in a key buffer ( 1804 ). Thereafter the input switch routes the cells to memory banks resident in the multi-function multiports in a time division multiplexed manner ( 1806 ). The input switch stores the first address in memory where the first cell is stored and computes offsets for each additional cell associated with the offset in memory for the next contiguous memory bank into which the next cell is written ( 1808 ). The input switch creates indirect cells to store linking information for the packet if the packet length exceeds five cells ( 1810 ). If the number of cells exceeds the number of available offsets in an indirect cell, then the old indirect cell is stored in memory and a new indirect cell is created and loaded based on the offsets calculated for each new cell received at the input switch. 
     When the packet (and its indirect cells if any) have been stored in memory, then the key, full address of the first cell and offset information is transferred as a look-up request to a controller ( 1814 ). The controller performs a best match look-up and generates a result of the look-up. The result includes the destination port (multi-function multiport), address, offset information and next hop index ( 1816 ). A notification including the result is transferred to the output switch for transfer to the appropriate multi-function multiport ( 1818 ). 
     Upon receipt of a notification, the multi-function multiport generates read requests a cell at a time to the input switch for the data associated with the packet ( 1820 ). The input switch issues the read requests in a time division multiplexed fashion generating a single request to each memory bank per cell slot ( 1822 ). When the memory bank receives the request from the input switch, cell data and a multi-function multiport identifier associated with the request are transferred to the output switch ( 1824 ). Again, at each cell slot, the output switch transfers a single cell to each of the multi-function multiports. Upon receipt, the multi-function multiport couples the cell data with media header information and streams the data to the destination ( 1826 ). 
     Alternative Implementations 
     i) Multifunction Multiports 
     In an alternative implementation, multi-function multiport  150  may be divided into separate functional blocks and embodied in separate physical structures. For example, each multi-function multiport may be separated into an input port, an output port and a memory. Memory may include both the global data buffer and the notification queues. Alternatively, memory may be divided further with the notification queues physically manifested as a part of an output port. 
     ii) Multi-Nodes 
     As described above, a node in a trie can have more than two children. For example, a node can include a bits-to-test indicator that points to two bits in the key to examine at the given node. The two bits can be consecutive bits, or can be separated by one or more bits. In one implementation, the bits to test can be indicated directly by the bits-to-test indicator, e.g. test bits  7  and  8  of the key. Alternatively, the bits-to-test indicator can point to the first bit of a series of consecutive bits to test in the key (i.e., the bits-to-test indicator points to a first bit-to-test where plural bits can be tested). In one implementation, a code is used to signal a bits-to-test indicator that points to plural bits in the key, such as configuring the “r” and “R” bits stored with the bit-to test indicator with a value of binary 11. 
     A multi-node refers to a node having more than two children. Multi-nodes can be configured including four or more (2 to the n-th children where n is greater than 1) children. Multi-nodes are configured using a data structure, as described above, so that a minimum of data reads from memory are required. More specifically, the pointers to each child are sequentially stored in memory so that depending on the value of the bits in the key, an appropriate child node can be loaded directly. As described above, the sequential ordering minimizes the number of reads required from memory when performing a trie search. 
     When forward searching a trie and a multi-node is tested, the bits-to-test indicator that is stored on the stack includes plural bits (reflecting the plural bits that have been matched in the search process). When traversing the trie in reverse, entries are popped off the stack in a conventional fashion until the first dissimilar bit is discovered as described above. 
     Associated with each multi-node can be n or fewer attached routes. A route can be attached for each “bit” in the multi-node. The attached route defines the route to be traversed if a key match arises at the particular bit of the multi-node (the bit is a don&#39;t care bit). 
     iii) Combining Nodes 
     In an alternative implementation, the process for combining or deleting a node in a trie can be expanded to include creating or disassembling multi-nodes (nodes having more than two children). When adding a node to a trie, a test can be made to determine if the node that is proposed to be added can be combined with other existing nodes in the trie to form a multi-node. For example, a check can be made to determine if the point of insertion for the new node also includes a parent node that has an existing child node (a “sibling node”) that points to the same bit as the new node. If so, then a multi-node can be created from the three nodes (the parent node, the new node and the sibling node). If a multi-node can be formed, then the multi-node is constructed and added to the trie as opposed to just adding a single new node. In the deletion process, the deletion of a node can also present the opportunity to create new multi-nodes. More specifically, a check can be made when deleting a node for a combination of a parent node having two children nodes that both point to the same bit-to test bit (without any intervening nodes between the parent and children, e.g. not a combination of “grandparent” and “grandchild” nodes). 
     The present invention has been described in terms of specific embodiments, which are illustrative of the invention and not to be construed as limiting. Other embodiments are within the scope of the following claims.