Abstract:
A loop network hub including a hub port with a loop initialization insertion mechanism. The loop initialization insertion mechanism causes a hub port which detects a new node port connection to automatically begin generating loop initialization data. A hub port continues to generate loop initialization data until that hub port receives a loop initialization sequence. The loop initialization data propagates around the loop of the hub, halting ordinary processing. In this way, the entire loop is cleared. Upon receiving a loop initialization sequence, the hub port originating the loop initialization data inserts the new node port into the loop. At this point, loop initialization begins and each node port in the loop network obtains a unique loop network address.

Description:
CLAIM OF PRIORITY 
   This application claims priority to co-assigned U.S. patent application Ser. No. 09/071,275, filed on May 1, 1998 now U.S. Pat. No. 6,560,205, entitled “Loop Network Hub Using Loop Initialization Insertion,” which is incorporated by reference herein by reference. 

   TECHNICAL FIELD 
   The present invention relates to electronic network systems, and more specifically to a loop network hub designed such that loop address conflicts are reduced by forcing initialization of the loop upon insertion of a new node port into the loop. 
   BACKGROUND INFORMATION 
   Electronic data systems are frequently interconnected using network communication systems. Area-wide networks and channels are two approaches that have been developed for computer network architectures. Traditional networks (e.g., LAN&#39;s and WAN&#39;s) offer a great deal of flexibility and relatively large distance capabilities. Channels, such as the Enterprise System Connection (ESCON) and the Small Computer System Interface (SCSI), have been developed for high performance and reliability. Channels typically use dedicated short-distance connections between computers or between computers and peripherals. 
   Features of both channels and networks have been incorporated into a new network standard known as “Fibre Channel.” Fibre Channel systems combine the speed and reliability of channels with the flexibility and connectivity of networks. Fibre Channel products currently can run at very high data rates, such as 266 Mbps or 1062 Mbps. These speeds are sufficient to handle quite demanding applications, such as uncompressed, full motion, high-quality video. ANSI specifications, such as X3.230-1994, define the Fibre Channel network. This specification distributes Fibre Channel functions among five layers. The five functional layers of the Fibre Channel are: FC-0—the physical media layer; FC-1—the coding and encoding layer; FC-2—the actual transport mechanism, including the framing protocol and flow control between nodes; FC-3—the common services layer; and FC-4—the upper layer protocol. 
   There are generally three ways to deploy a Fibre Channel network: simple point-to-point connections; arbitrated loops; and switched fabrics. The simplest topology is the point-to-point configuration, which simply connects any two Fibre Channel systems directly. Arbitrated loops are Fibre Channel ring connections that provide shared access to bandwidth via arbitration. Switched Fibre Channel networks, called “fabrics”, are a form of cross-point switching. 
   Conventional Fibre Channel Arbitrated Loop (“FC-AL”) protocols provide for loop functionality in the interconnection of devices or loop segments through node ports. However, direct interconnection of node ports is problematic in that a failure at one node port in a loop typically causes the failure of the entire loop. This difficulty is overcome in conventional Fibre Channel technology through the use of hubs. Hubs include a number of hub ports interconnected in a loop topology. Node ports are connected to hub ports forming a star topology with the hub at the center. Hub ports which are not connected to node ports or which are connected to failed node ports are bypassed. In this way, the loop is maintained despite removal or failure of node ports. 
   More particularly,  FIG. 1A  illustrates a conventional loop configuration  100 . Four node ports  101 ,  102 ,  104 ,  106  are shown joined together node port to node port. Each node port represents a connection to a device or to another loop. Node port  101  is connected to node port  102  such that data is transmitted from node port  101  to node port  102 . Node port  102  is in turn connected to node port  104  which is in turn connected to node port  106 . Node port  106  is connected to the first node port, node port  101 . In this manner, a loop datapath is established; from node port  101  to node port  102  to node port  104  to node port  106  back to node port  101 . 
     FIG. 1B  illustrates a loop  107  where node ports  108 ,  110 ,  112 ,  114  are organized in a physical star topology with a hub  116  in the center. Node port  108  is connected to a hub port  118  in hub  116  as are node ports  110 ,  112  and  114  to their own respective hub ports  120 ,  122 , and  124 . Internal to hub  116  is a loop, where hub ports  118 - 124  of hub  116  form a loop datapath similar to the conventional loop configuration shown in  FIG. 1A . 
   The use of a hub as a central component to a loop network allows for operation when one or more hub ports are not connected to node ports, or one or more hub ports are connected to node ports which have failed, by bypassing such hub ports. Each hub port typically contains circuitry which provides a bypass mode for the hub port. When a hub port is in bypass mode, data received by the hub port from the previous hub port in the loop is passed directly to the next hub port in the loop. 
   An additional advantage of the use of hubs is that node ports may be hot insertable. Hot insertable functionality allows the insertion and removal of node ports from a loop without powering down the entire loop or the hub and then restarting again. However, as a result of this hot insertability, the addresses of node ports attached to a loop are not always properly maintained. 
   Under FC-AL protocols, a loop initialization process is used to provide each node port attached to the loop with a unique address, referred to as an Arbitrated Loop Physical Address (“AL_PA”). Loop initialization is invoked under FC-AL protocols by generating a sequence of Loop Initialization Primitive (“LIP”) ordered sets. In a loop which is not hot insertable, after insertion or removal of a node port the entire loop is restarted and re-initialized. In a hot insertable loop, the loop is not always restarted and so is not necessarily re-initialized upon each insert or removal. As a result, when a new node port is inserted into the loop a unique address may not necessarily be generated if the loop is not re-initialized. 
   In addition, a hub port may be connected to a hub port on another hub. When hubs are linked one hub to another through hub ports, sometimes hubs do not properly initiate an initialization routine upon insertion, especially in the case of quiescent hubs (i.e., no loop traffic at the time of insertion). At this point there is a possibility of address conflicts between the node ports on the first hub and the node ports on the second hub. 
   Such an address conflict problem is illustrated in  FIGS. 2A and 2B . As shown in  FIG. 2A , four node ports A 1 , B 1 , C 1 , D 1 , are linked to a hub  200 . Three node ports A 2 , B 2 , C 2 , are connected to a hub  202 . The numbers  1  and  2  are illustrative only and in fact the addresses for each node port are still represented by the letter A, B, C, or D. At this point, each node port has a unique address within its own loop. However, when hubs  200  and  202  are joined, as shown in  FIG. 2B , the addresses for the node ports are no longer necessarily unique. In the single loop shown in  FIG. 2B , two node ports have address A, two node ports have address B, and two node ports have address C. Upon detecting an address conflict, an error is generated which starts an initialization sequence, ultimately resulting in unique addresses for each node port. However, before that conflict is detected, messages may still continue to pass which are received by incorrect node ports resulting in possible data corruption. 
   For example, in the situation shown in  FIG. 2A , when node port B 1  sends data to node port A 1 , the hub ports are adjacent and node port A 1  receives the data from node port B 1  possibly without an error. As shown in  FIG. 2B , the connection from node port B 1  to node port A 1  may begin without generating an address conflict because messages from B 1  successfully pass along the loop to node port A 1 , the intended destination, as long as node port B 2  was not arbitrating. 
   However, when node port A 1  attempts to send data to node port B 1 , data corruption may result. In the situation shown in  FIG. 2A , the data is sent from node port A 1 , past node port C 1 , past node port D 1 , and then to node port B 1 , the intended destination. However, in the situation shown in  FIG. 2B , data passes from node port A 1 , past node port C 1 , past node port D 1 , through the hub ports connecting hub  200  and hub  202 , past node port C 2  and is received by node port B 2 . As noted above, the numerals indicate only the difference between node ports from hub  200  and node ports from hub  202 . From node port A 1 &#39;s perspective, node port B 2  is indistinguishable from node port B 1 . Node port A 1  sends data addressed to node port B. Similarly, node port B 2  accepts data which is addressed to node port B. Accordingly, node port B 2  receives data addressed to node port B, though node port A 1  intended the data to be received by node port B 1 . Thus, “B” is not a unique address. Neither node port A 1  nor node port B 2  is aware of the existence of either node port B 2  or node port A 1 . As a result, depending on the nature of the transaction entered into, data corruption may result. At some point, a proper error may be generated resulting in the initialization sequence. That may be too late, however, to prevent or recover from unwanted data corruption. 
   The inventors have determined that it would be desirable to provide a loop network hub which can provide unique addresses upon insertion of a new node port or a new hub into a loop by forcing the loop to initialize before data corruption occurs. 
   SUMMARY 
   A loop network hub of the preferred embodiment includes a hub port with a loop initialization insertion mechanism. The loop initialization insertion mechanism causes a hub port which detects a new connection to automatically begin generating loop initialization data. A hub port continues to generate loop initialization data until that hub port receives a loop initialization sequence. The loop initialization data propagates around the loop of the hub, halting ordinary processing. In this way, the entire loop is cleared. Upon receiving a loop initialization sequence, the hub port originating the loop initialization data stops sending the loop initialization data and inserts the new node port into the loop. At this point, loop initialization begins and each node port in the loop network obtains a unique loop network address. 
   In an FC-AL implementation, a hub of the preferred embodiment includes a hub port with a LIP insertion mechanism. The loop initialization insertion mechanism causes a hub port which detects a new connection to automatically begin generating LIP (F 7 , F 7 ) ordered sets. The hub port continues to generate LIP (F 7 , F 7 ) ordered sets until that hub port receives a LIP primitive sequence, where a LIP primitive sequence includes three consecutive identical LIP ordered sets. The LIP (F 7 , F 7 ) ordered sets propagate around the loop of the hub, halting ordinary processing. In this way, the entire loop is cleared. Upon receiving a LIP primitive sequence, the hub port originating the LIP (F 7 , F 7 ) ordered sets stops inserting LIP (F 7 , F 7 ) ordered sets and inserts the new node port into the loop. At this point loop initialization begins and each node port obtains, according to known FC-AL protocols, a unique physical address (an Arbitrated Loop Physical Address, “AL_PA”). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  shows a prior art node port to node port loop. 
       FIG. 1B  shows a prior art loop including a hub. 
       FIG. 2A  shows two separate prior art loops. 
       FIG. 2B  shows two prior art loops connected to form a single loop. 
       FIG. 3  shows a loop including a hub. 
       FIG. 4  shows a block diagram of a hub port according to the preferred embodiment. 
       FIG. 5A  shows a hub with two node ports. 
       FIG. 5B  shows a hub with three node ports. 
       FIG. 6A  shows two separate loops including hubs. 
       FIG. 6B  shows two loops including hubs connected by hub ports. 
   

   DETAILED DESCRIPTION 
   The preferred embodiment provides a mechanism to force loop initialization upon insertion of a node port into a loop network. The invention is explained below in the context of a Fibre Channel Arbitrated Loop (“FC-AL”) as an illustration of the preferred embodiment. However, the invention may have applicability to networks with similar characteristics as FC-AL networks. 
   An overview of loop operation in a loop network is described below with reference to a configuration illustrated in  FIG. 3 .  FIG. 3  shows a hub  300  with six hub ports  302 ,  304 ,  306 ,  308 ,  310 , and  312 . Each hub port is connected to another hub port with a unidirectional internal hub link forming an internal hub loop. In  FIG. 3 , data flows from hub port  302  to hub port  304  and so on in a counter clockwise manner. Alternatively hub ports may be connected such that data flows in a clockwise direction so long as the loop topology is maintained. 
   Attached to three hub ports  302 ,  310 ,  312 , are three node ports  314 ,  316 ,  318 . Node port  314  is attached to hub port  302 , node port  316  is attached to hub port  312 , and node port  318  is attached to hub port  310 . Each node port is preferably attached to a hub port by two data channels: one data channel sends data from the hub port to the node port, one data channel sends data from the node port to the hub port. Thus, a data channel carries data from hub port  302  to node port  314  and another data channel carries data from node port  314  to hub port  302 . Data from node port  314  to be received by node port  316  passes from node port  314  through a data channel to hub port  302 , then from hub port  302  to hub port  306 , then to hub port  306 , to hub port  308 , to hub port  310 . If node port  318  is operating in the loop, the data passes through a data channel to node port  318  and back through a data channel to hub port  310 , and then passes to hub port  312 . The data passes through a data channel from hub port  312  and is received at node port  316 . 
   In the preferred embodiment, incoming data entering a hub port from the previous hub port in the loop is sent to the node port connected to the hub port, if present. If the hub port is in bypass mode, the incoming data is sent directly from the hub port to the next hub port in the loop without including any data from the node port in response to the incoming data. The preferred embodiment uses a switching device such as a multiplexer to accomplish this bypass, as described below with reference to  FIG. 4 . In addition, the attached node port recognizes whether the data received from the hub port is addressed to that node port or not and responds appropriately. The bypass is accomplished in the hub port, however, not in the node port. Thus the loop is protected from node port failures. A hub port which has no attached node port, such as hub ports  304 ,  306  or  308  shown in  FIG. 3 , is always in bypass mode and passes any data directly to the next hub port. In this way, a signal from hub port  302  received by hub port  304  is passed directly to hub port  306 . When a hub port with an attached node port, such as hub port  310 ,  312 , or  302  as shown in  FIG. 3 , receives data from the previous hub port on the loop, the hub port passes the data to the attached node port. The node port responds appropriately and passes the data back to the hub port. 
   For example, data which is addressed from node port  318  to node port  314  flows from node port  318  to hub port  310  then to hub port  312 . Hub port  312  passes the data to node port  316 , if node port  316  is not bypassed. Node port  316  recognizes that the data is not addressed to node port  316  and so passes the data back to hub port  312 . Hub port  312  passes the data to hub port  302 . Hub port  302  passes the data to node port  314 , if node port  314  is not bypassed. Node port  314  recognizes the data is addressed to node port  314  and responds appropriately. 
     FIG. 4  illustrates internal components of a hub port according to the preferred embodiment. A hub port  400  as shown in  FIG. 4  is equivalent to hub ports  302 ,  304 ,  306 .  308 ,  310 , and  312  shown in  FIG. 3 . An incoming internal hub link  402  enters hub port  400  from a previous hub port in the loop (not shown). Incoming internal hub link  402  is connected to a hub port transmit circuit  404 . Thus, data from a previous hub port passes along internal hub link  402  into hub port  400  and then into hub port transmit circuit  404 . Hub port transmit circuit  404  sends the data received through a data channel  406  out to a node port  408  after converting the data into a form usable by node port  408 . Alternatively, data channel  406  may be connected to a hub port in a different hub, allowing interconnection hub to hub. 
   Node port  408  outputs data to hub port  400  via a data channel  410 . Data channel  410  is connected to a hub port receive circuit  412 . Hub port receive circuit  412  converts data received from node port  408  into a form usable inside the hub. In one implementation, hub port receive circuit  412  converts data from serial to parallel and decodes the data. Hub port receive circuit  412  also includes a loop initialization data detect circuit  414  and a hub port output control circuit  416 . In an FC-AL implementation, the loop initialization data detect circuit  414  is a LIP detect circuit. Hub port receive circuit  412  outputs data via a hub port output line  418 . Hub port output control circuit  416  outputs control signals via a hub port output control line  420 . Hub port output line  418  is connected to a first input A of a switching device  422 , such as a multiplexer. Incoming internal hub link  402  is connected to a second input B of switching device  422 . A loop initialization data generator  424  generates loop initialization data and outputs those ordered sets to a loop initialization data line  426 . In an FC-AL implementation, loop initialization data generator  424  is a LIP generator and generates LIP (F 7 , F 7 ) ordered sets. Loop initialization data line  426  is connected to a third input C of switching device  422 . Hub port output control line  420  is connected to a control input of switching device  422 . In this way, switching device  422  selects a single input A, B, or C to be output depending upon the control signal generated by hub port output control circuit  416 . The output of switching device  422  is sent to outgoing internal hub link  428 . Outgoing internal hub link  428  passes data to the next hub port in the hub in the same manner that internal hub link  402  passes into hub port  400 , forming a loop as shown in  FIG. 3 . 
   When no device is attached to hub port  400 , hub port output control circuit  416  holds hub port  400  in bypass mode. By selecting input B of switching device  422  data received from the previous hub port on incoming internal hub link  402  is output to outgoing internal hub link  428 . In bypass mode, data on incoming internal hub link  402  enters input B of switching device  422  and is output unchanged onto outgoing internal hub link  428  to be passed to the next hub port in the loop (not shown). 
   If, however, an operational device, such as an FC-AL NL_Port or loop segment, is attached to hub port  400 , represented by node port  408 , data received from node port  408  by hub port receive circuit  412  is sent to the next hub port along outgoing internal hub link  498 . In order to pass data from hub port receive circuit  412  to outgoing internal hub link  428 , hub port output control circuit  416  selects input A of switching device  422  via hub port output control line  420 . 
   In a conventional FC-AL hub port, typically upon initial attachment of an operational device at node port  408 , hub port receive circuit  412  detects the reception of data from node port  408  and ends bypass mode (where input B of switching device  422  is selected). Data received from node port  408  is inserted onto the loop by selecting input A of switching device  422 . The data received by hub port receive circuit  412  from node port  408  is immediately passed along to the next hub port via outgoing internal hub link  428 . However, as discussed above, this immediate insertion into the loop of a new device or hub may generate address conflicts and lead to undesirable data corruption. 
   In order to overcome this difficulty, the preferred embodiment provides a loop initialization insertion mechanism. When an operational device or hub is attached to hub port  400 , hub port receive circuit  412  detects that new device or hub by detecting the reception of formatted data along data channel  410  where previously there was no data. Rather than immediately passing along data from node port  408  through hub port output line  418  onto outgoing internal hub link  428 , hub port output control circuit  416  selects input C of switching device  422 . Loop initialization data generator  424  generates a constant stream of loop initialization data which indicates to other hub ports in the loop that a new device or hub has been attached. Other hub ports in the loop upon receiving a loop initialization sequence pass the sequence along. A loop initialization sequence is a specified combination of loop initialization data. In an FC-AL implementation, a LIP primitive sequence consists of three consecutive identical LIP ordered sets of the same type. In this way, the processing of transactions on the loop stops and each hub port begins to pass along or generate loop initialization data. Loop initialization data generator  424  repeatedly generates loop initialization data, preferably in coordination with the frame sequence appropriate to the loop network. 
   Hub port output control circuit  416  continues to select input C of the hub port switching device  422  until loop initialization data detect circuit  414  detects a loop initialization sequence received from node port  408 . Node port  408 , as described above receives signals from incoming internal hub link  402  via hub port transmit circuit  404 . The selection of inputs on switching device  422  does not affect the reception of data by node port  408  because switching device  422  controls the output of hub port  400  onto the loop, not the input from the loop. 
   In an FC-AL implementation, the loop initialization data is LIP (F 7 , F 7 ) ordered sets. These LIP (F 7 , F 7 ) ordered sets are preferably in the form (K28.5 D21.0 D23.7 D23.7), compliant with FC-AL protocols. 
   In this way, a loop initialization sequence generated from a previous port in the loop (possibly this same port) enters hub port  400  on incoming internal hub link  402  and is sent to node port  408  through hub port transmit circuit  404 . Node port  408  sends the loop initialization sequence to hub port receive circuit  412 . Loop initialization data detect circuit  414  detects the loop initialization sequence. Upon detecting such a loop initialization sequence, hub port output control circuit  416  switches from selecting input C of switching device  422  to selecting input A of switching device  422 . At this point, a loop initialization procedure begins according to appropriate network protocols. 
   A LIP detect circuit  414  generates an affirmative detection signal upon detecting any LIP primitive sequence, not necessarily the same LIP (F 7 , F 7 ) primitive sequence. The detected LIP primitive sequence does not need to be from the same hub port as originally began the LIP (F 7 , F 7 ) ordered set generation from detecting a new device or hub. 
   At hub ports other than the hub port originating the loop initialization data, when a node port receives loop initialization data from a hub port, the node port passes some of the loop initialization data back to the hub port. In the preferred embodiment the hub port passes along data from the node port (by selecting input A of the switching device as shown in  FIG. 4 ) 
   Thus, loop initialization is forced upon attachment of a new operational device or a new hub to an existing hub. In the preferred embodiment, the generation and propagation of loop initialization sequences halts ordinary loop operation and begins loop initialization. As described above, loop initialization is desirable upon connection of a new device or upon connection of a second loop to a first loop because the loop initialization process is an assured way under network protocols such as FC-AL protocols to assign each device on the newly established loop a unique physical address. 
     FIGS. 5A and 5B  illustrate an example of inserting an operational device in a loop according to a preferred embodiment.  FIG. 5A  illustrates a loop and components before the new device is inserted. A hub  500  has four hub ports  502 ,  504 ,  506 ,  508 . As shown in  FIG. 5A , hub  500  has only four hub ports, however, hubs may have more or less hub ports. The number of hub ports shown in  FIG. 5A  is for illustrative purposes only. Hub ports  502 ,  504 ,  506 ,  508 , are connected to one another by internal hub links to form a loop. Two node ports  510 ,  512  are attached to hub ports  502 ,  508 , respectively. Data from node port  510  to node port  512  flows through a data channel into hub port  502 . Hub port  502  outputs the data along the internal hub link to hub port  504 . Hub port  504  does not have an attached operational device and so is in bypass mode. Thus hub port  504  passes the data from hub port  502  along the internal hub link to hub port  506 . Hub port  506  is also in bypass mode and so passes the data along the internal hub link to hub port  508 . Hub port  508  has an operational device attached at node port  512  and so is not in bypass mode. Similarly, data from node port  512  to be sent to node port  510  passes through a data channel to hub port  508  and passes along the internal hub link to hub port  502 . Hub port  502  sends the data along a data channel to node port  510 . In this way, hub ports  502 - 508  and hub  500  operate to maintain a loop topology. 
   Upon insertion of a new device attached to a node port  514 , the process described above with respect to  FIG. 4  proceeds. Node port  514  is attached to hub port  504 . Hub port  504  detects the new node port  514  from the presence of data incoming to hub port  504  in a particular formation of data. Upon detecting node port  514 , hub port  504  does not immediately pass along data from node port  514 . Hub port  504  synchronizes timing and frames with data from node port  514  and validates the proper operation of the node port  514 . As described above, hub port  504  begins to send loop initialization data (e.g., LIP (F 7 , F 7 ) ordered sets) along the internal hub link by selecting an input of a switching device inside of hub port  504  which corresponds to a loop initialization data generator. The loop initialization data passes along the internal hub link to hub port  506 . 
   Hub port  506  is in bypass mode because no node port is attached to hub port  506 . Hence, the loop initialization data passes along the internal hub link to hub port  508 . 
   Hub port  508  passes the loop initialization data to node port  512 , if node port  512  is not already bypassed. The operational device attached to node port  512  preferably responds to the loop initialization data and node port  512  passes the loop initialization data back to hub port  508 . Because the operational device attached to node port  512  generates a proper response to the loop initialization data, in the preferred embodiment hub port  508  selects the signal received from node port  512  to pass along the internal hub link of hub  500 . A hub port such as hub port  508 , which is attached to an operational device through a node port, passes along the loop initialization data received from the node port by selecting input A of the hub port switching device as shown in  FIG. 4 . Thus, the loop initialization data is preferably passed along the internal hub link to the next hub port. 
   As shown in  FIG. 5B , hub port  508  passes the loop initialization data to hub port  502 . Hub port  502  follows a similar process as hub port  508  because hub port  502  also has an operational device attached, represented by node port  510 . Accordingly, the loop initialization data passes from hub port  502  to hub port  504 . 
   Hub port  504  receives the loop initialization data and transmits the loop initialization data to node port  514 , if node port  514  is not already bypassed. Node port  514  passes the loop initialization data back to hub port  504 , similar to node ports  512  and  510 . The loop initialization data detect circuit ( 414  as shown in  FIG. 4 ) in the hub port receive circuit of hub port  504  detects the loop initialization data. Hub port  504  stops outputting loop initialization data when a loop initialization sequence has been received. In this case, hub port  504  may have received the loop initialization sequence which originated at hub port  504 . However, as described above, hub port  504  ceases outputting loop initialization data upon detecting a loop initialization sequence from any source. In an FC-AL implementation, a hub port stops outputting LIP (F 7 , F 7 ) ordered sets upon detecting a LIP primitive sequence of any type. In an alternative embodiment, the hub port transmit logic detects a loop initialization sequence received along the internal hub link and does not necessarily wait for a response from the connected node port. In either case, hub port  504  switches from outputting loop initialization data (through selecting input C of the switching device as shown in  FIG. 4 ) to a loop initialization procedure defined by the appropriate network protocols. 
     FIGS. 6A and 6B  illustrate the connection of one hub loop to a second hub loop. In general, the process is similar to that illustrated in  FIGS. 5A and 5B  for the insertion of a new operational device to a single hub loop. 
     FIG. 6A  shows a first hub  600  with six hub ports  602 ,  604 ,  606 ,  608 ,  610 ,  612 . Three node ports  614 ,  616 , and  618  are connected to hub ports  602 ,  604 , and  606 , respectively. A second hub  620  also has six hub ports  622 ,  624 ,  626 ,  628 ,  630 ,  632 . Three node ports  634 ,  636 , and  638  are connected to three hub ports  622 ,  624 , and  626 , respectively. The hub ports of each hub are connected in a loop. 
     FIG. 6B  illustrates the connection of hub  600  to hub  620 . A pair of data channels connect hub port  608  to hub port  632 . One data channel carries data from hub port  608  to hub port  632 . One data channel carries data from hub port  632  to hub port  608 . In this way, the two loops contained in two separate hubs are joined together to form a single loop. The new circular datapath among hub ports has the following pattern: hub port  608  to  610  to  612  to  602  to  604  to  606  back to  608 , then to hub port  632  to  622  to  624  to  626  to  628  to  630  back to  632 , then back to hub port  608 , completing the circle. When data enters hub port  608  from hub port  606 , the data passes through a transmit circuit of hub port  608  (recall  FIG. 4 ) and then out through the data channel to hub port  632 . The data has not yet entered a receive circuit of hub port  608 , and does not until the data returns from hub port  632 . In this way, data flows in a circular pattern through two hubs and the two previously physically distinct loops operate as one virtual loop. 
   Upon connection of one hub to another, however, the potential for address conflicts and undesirable data corruption exists, as described above with respect to  FIGS. 2A and 2B . The loop initialization insertion mechanism provided by the preferred embodiment overcomes this problem and forces loop initialization. Hub port  608  detects the connection to hub port  632  of hub  620  through the new reception of properly formatted data. Upon detection of hub port  632 , hub port  608  follows the procedure as defined above for detection of a new device. Hub port  608  selects a loop initialization data generator internal to hub port  608  and outputs loop initialization data along the hub loop. Accordingly, loop initialization data passes from hub port  608  to hub port  610 . Hub port  610  is in bypass mode because there is no node port attached to hub port  610 . Hub port  610  passes the loop initialization data along to the next hub port, and the process continues as described above with respect to  FIG. 5B . Similarly, hub port  632  detects the connection to hub port  608  of hub  600 . Thus, hub port  632  selects a loop initialization data generator internal to hub port  632  and outputs loop initialization data onto the hub loop of hub  620 . 
   Accordingly, each of hub ports  608  and  632  are generating loop initialization data which is being passed along the loop. The loop initialization data from hub port  608  passes from hub port  608 , to  610 , to  612 , to  602 , to node port  614  (if node port  614  is not bypassed), to hub port  602 , to  604 , to node port  616  (if node port  616  is not bypassed), to hub port  604 , to  606 , to node port  618  (if node port  618  is not bypassed), to hub port  606 , and back to  608 . However, in the preferred embodiment, at this point hub port  608  does not detect the loop initialization data because the loop initialization data detection circuit of hub port  608  is in the hub port receiving circuit of hub port  608 . The loop initialization data received along the internal hub link from hub port  606  is in the hub port transmit circuit of hub port  608 . Accordingly, the loop initialization data passes to hub port  632 . Hub port  632  receives the loop initialization data in its hub port receiving circuit and detects the loop initialization data using its loop initialization data detection circuit. When hub port  632  has detected a loop initialization sequence, in this case from the loop initialization data generated by hub port  608 , hub port  632  changes the selection of input on the internal switching device of hub port  632  so that loop initialization proceeds. The bypass accomplished internal to hub port  632  by selecting the loop initialization data generator ends and loop initialization commences. 
   Similarly, hub port  608  receives the loop initialization data generated by hub port  632  which passed along the internal hub link of hub  620  and eventually from hub port  632  to hub port  608 . The loop initialization data detect circuit in the hub port receiving circuit of hub port  608  detects the loop initialization sequence, ends the bypass, and begins loop initialization processing according to standard FC-AL protocols. Thus, both hub ports  608 .  632  begin loop initialization processing. The handling of loop initialization is conventionally understood and defined according to network protocols, such as FC-AL protocols. In addition, the technique is still effective if one of the interconnected hubs is a conventional hub, so long as at least one hub in the loop operates according to the present invention. 
   Various embodiments of the invention have been described with reference to the figures, however, the scope of the invention is not to be limited by the description provided herein but rather only by the scope of the following claims. Alternative embodiments which fall within the scope of the claims will also be apparent to those of ordinary skill in the art.