Abstract:
In certain embodiments of the present invention, each of a plurality of switches in a network can independently develop an equivalent set of network attributes without referring to the network&#39;s topology. An equivalent set of network attributes can define the entire network&#39;s topology. Each switch can also independently maintain its respective equivalent set of network attributes to reflect changes in the network&#39;s topology. As with developing a set of attributes, each switch can maintain its set of attributes without actually referring to the network&#39;s topology.

Description:
RELATED APPLICATIONS  
       [0001]     This is a divisional of co-pending U.S. patent application Ser. No. 09/607,611, filed Jun. 30, 2000, entitled “Method and Apparatus for Providing a Distributed Dictionary in a Network having an Unknown Topology.” 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention pertains to the field of networking. More particularly, this invention relates to managing the topology of a network.  
       BACKGROUND  
       [0003]     Networks allow individual devices to take advantage of one another to share information and resources, provide redundancy, increase accessibility, and so on. Networks are used in every day life at home, at work, on vacation, and just about everywhere else. A typical user does not want to know how a network operates. A typical user just wants the networks he or she encounters in daily life to work and work well. Unfortunately, network technology is exceptionally complex. All too often, highly skilled, and very expensive, technicians are needed to set up and maintain networks. The power and versatility of networks, however, virtually guarantee a continued and growing demand for better, more reliable, faster, and more user friendly networks and network equipment.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]     Examples of the present invention are illustrated in the accompanying drawings. The accompanying drawings, however, do not limit the scope of the present invention. Similar references in the drawings indicate similar elements.  
         [0005]      FIG. 1  illustrates one embodiment of the present inventions.  
         [0006]      FIG. 2  illustrates another embodiment of the present inventions.  
         [0007]      FIG. 3  demonstrates one embodiment of a distributed dictionary.  
         [0008]      FIG. 4  demonstrates another embodiment of a distributed dictionary.  
         [0009]      FIG. 5  demonstrates one embodiment of testing for loops.  
         [0010]      FIG. 6  demonstrates one embodiment of adjacency monitoring.  
         [0011]      FIG. 7  demonstrates one embodiment of topology determination.  
         [0012]      FIGS. 8-10  demonstrate embodiments of topology reporting and transient loop avoidance.  
         [0013]      FIG. 11  illustrates one embodiment of a hardware system.  
         [0014]      FIG. 12  illustrates one embodiment of a machine readable storage medium.  
     
    
     DETAILED DESCRIPTION  
       [0015]     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, those skilled in the art will understand that the present invention may be practiced without these specific details, that the present invention is not limited to the depicted embodiments, and that the present invention may be practiced in a variety of alternate embodiments. In other instances, well known methods, procedures, components, and circuits have not been described in detail.  
         [0016]     Parts of the description will be presented using terminology commonly employed by those skilled in the art to convey the substance of their work to others skilled in the art. Also, parts of the description will be presented in terms of operations performed through the execution of programming instructions. As well understood by those skilled in the art, these operations often take the form of electrical, magnetic, or optical signals capable of being stored, transferred, combined, and otherwise manipulated through, for instance, electrical components.  
         [0017]     Various operations will be described as multiple discrete steps performed in turn in a manner that is helpful in understanding the present invention. However, the order of description should not be construed as to imply that these operations are necessarily performed in the order they are presented, or even order dependent. Lastly, repeated usage of the phrase “in one embodiment” does not necessarily refer to the same embodiment, although it may.  
         [0018]     Four related inventions are described herein. Embodiments of each invention improve aspects of communication among network equipment to improve, for instance, network configuration and management. All four inventions are described below in the context of a network of data switches. The inventions, however, are not limited to the illustrated embodiments, and are generally applicable to a wide variety of networks including, for instance, a local area network (LAN).  
         [0019]      FIG. 1  illustrates four data switches,  110 ,  120 ,  130 , and  140 . Each switch includes eight data ports  150 . Other embodiments may include a different number of ports per switch and other embodiments may not have the same number of ports on each switch. The switches can be used to connect various devices, such as telephones, computers, facsimile machines, printers, networks, etc. When a data packet is received at a particular data port, the switch determines where the packet is supposed to go, and sends the packet to the appropriate data port.  
         [0020]     Each data switch can be used alone. For instance, a switch having eight data ports can connect eight different devices. If a user wants to connect more devices than one switch can handle, the user can use multiple switches. For instance, if a user wanted to couple  14  devices using switches with eight data ports, the user could couple seven devices to one switch and seven devices to another switch, and then couple the two switches together with the eighth port on each switch.  
         [0021]     Combining switches using data ports, however, can quickly become complicated. A user needs to map several ports from one switch into one port on another switch. The more switches that are combined, the more difficult it becomes to configure and manage the switches.  
         [0022]     In the embodiment illustrated in  FIG. 1 , each switch includes a number of dedicated ports, intra-stack ports  160 . The intra-stack ports  160  are designed specifically for stacking switches together. The four switches stacked in  FIG. 1  each have eight data ports, so the combined stack can connect  32  devices. Other embodiments may include larger or smaller numbers of switches, having any number of ports per switch.  
         [0023]     If a packet of data arrives at a data port on switch  140 , and the data packet is intended to be sent to a device coupled to a data port on switch  110 , switch  140  will forward the data packet to switch  120  through data cable  170 . Switch  120  will in turn forward the data packet through data cable  180  to switch  110 . Each switch needs to know the topology of the stack in order for the stack to work properly as one big switch. That is, each switch needs to know which switches are coupled to which of its intra-stack ports  160 .  
         [0024]     Various embodiments of the present inventions can be used to automatically manage the topology of the stack so that, for instance, configuring the stack topology to operate like a single large switch can be as simple as plugging in a few data cables, switches can be added or removed from the stack as needed with little interruption in service, etc. The advantages of a self-managing network of switches are numerous. For example, a user can simply couple the switches together in any random order, let the switches configure themselves, and begin using the stack.  
         [0025]     Specifically, embodiments of the present inventions include:  
         [0026]     1) A distributed dictionary to provide a unified view of network attributes to each node in a network having an unknown topology,  
         [0027]     2) An adjacency monitor to recognize adjacencies between nodes, designate a master node, and report the adjacencies to the distributed dictionary,  
         [0028]     3) A master node to generate a topology based on adjacencies reported to the distributed dictionary, and  
         [0029]     4) The master node to coordinate adoption of the generated topology among slave nodes.  
         [0030]      FIG. 2  illustrates various embodiments of the four inventions. Each of the switches,  110 ,  120 ,  130 , and  140 , includes a generic attribute registration protocol (GARP)  210 , a distributed dictionary application  220 , an adjacency monitor  230 , and a topology update unit  240 .  
         [0031]     Standard GARP (IEEE std. 802.1) is a known networking protocol that connects applications, such as distributed dictionary application  220 , to physical ports, such as intra-stack ports  160 , within network nodes. GARP is designed to operate in a network having a known topology. GARP uses the topology to know which ports lead to which nodes. For instance, when GARP receives a packet of data, GARP can identify a destination for the packet, consult the known topology to determine which port leads to that destination, and forward the packet accordingly.  
         [0032]     GARP can receive packets either directly from an application within the same node or from a separate node on a physical port. For instance, if there is no direct connection between two nodes, in order to send a packet of data from the first node to the second node, the data must be forwarded through one or more intervening nodes. GARP in the first node will receive the packet from an application at a virtual port and forward the packet to a physical port. GARP in the intervening node(s) will receive the packet at a physical port and forward the packet to a physical port. GARP in the destination node will receive the packet at a physical port and forward the packet to a virtual port for an application.  
         [0033]     GARP&#39;s ability to forward packets can be used to multicast information. For instance, if an application wants to multicast a packet of data, the application can send the packet to GARP. GARP can forward the packet to each “enabled” port. For standard GARP, an “enabled” port includes every port that has a cable connecting to another node in the network topology. When GARP receives a multicast packet from a port, GARP can report the packet to an application in the same node, and forward the packet on to all enabled ports other than the port from which the packet was received. If each GARP in each node in the network similarly forwards a multicast packet, the packet will eventually be distributed throughout the network.  
         [0034]     Standard GARP, as described above, is designed to rely on a known topology. The illustrated embodiments of the present inventions, however, use a modified version of GARP which does not rely on a known topology. Further references to GARP will be to GARP as modified according to the present inventions. Other embodiments may use any number of other distribution tools to perform the functions of GARP described below.  
         [0000]     Distributed Dictionary in a Network Having an Unknown Topology  
         [0035]     Distributed dictionary  250  is discussed below with respect to topology management, but it has much broader applicability. It can be used, for instance, to distribute virtually any information throughout a network, and is especially suited to distribute information in a network having an unknown topology.  
         [0036]     As shown in  FIG. 2 , GARP  210  and distributed dictionary application  220  in each of the four switches collectively provides the inventive stack-wide virtual distributed dictionary  250 . Anything that is stored to dictionary  250  by one switch can be seen by all of the switches. Virtual dictionary  250  is merely a conceptual convenience. In reality, information that is “stored” in virtual dictionary  250  is actually distributed to the various nodes using GARP and stored locally at each node by each of the separate distributed dictionary applications  220 .  
         [0037]     Storing information to the dictionary is called “registering” the information. GARP multicasts distribution of the data so that the data is forwarded throughout the network. If a switch “deregisters” information, the information is removed from each node. Information can be removed in more than one way. For instance, information registered to distributed dictionary  250  may have a limited life span. If the information is not updated regularly, the information may simply disappear over time. Alternately, GARP may provide for more direct deregistration by, for instance, multicasting an instruction to delete certain information.  
         [0038]     Registering and deregistering information could be accomplished in a fairly straight forward manner in a network having a known topology. The information can be distributed along known paths to known destinations. As discussed below however, registering and deregistering information becomes more challenging in a network having an unknown topology.  
         [0039]      FIG. 3  illustrates one embodiment of the first invention. The illustrated process is used by an individual switch to monitor and update its own state, as well as report information to the distributed dictionary for all of the other switches to see.  
         [0040]     In block  310 , the switch obtains a current value associated with a key. A key is used as an index in the distributed dictionary. In the context of topology management, a key may be an identifier for a particular switch and an identifier for a particular intra-stack port on the switch. That is, a network topology can be defined in terms of switches and ports. The distributed dictionary may include an entry for every switch and port in the network that has a cable coupled to it. In which case, the value associated with each key may be an identifier for a neighbor switch and its port connected to the switch and port indexed by the key.  
         [0041]     Each switch has a unique media access control (MAC) address and each port has a port number. So, MAC addresses and port numbers can work well for switch identifiers. For instance, a key/value pair may look like, key=(own MAC address, own port number), and value=(neighbor&#39;s MAC address, neighbor&#39;s port number).  
         [0042]     In block  320 , the switch determines an incarnation identifier for the value obtained in block  310 . An initial incarnation is likely to be one. If, for instance, the topology has changed more than once, the incarnation will be incremented, or advanced, for each change. The incarnation identifier makes it possible to identify the most resent value associated with a key. Together, the incarnation identifier and the key/value pair can be referred to as an “attribute.” 
         [0043]     Blocks  330  and  340  comprise one embodiment of registering an attribute. In block  330 , the key/value pair is stored locally in the switch&#39;s distributed dictionary application, along with the incarnation identifier for the value. In block  340 , the switch multicasts the attribute to the rest of the switches to be stored by the respective distributed dictionaries.  
         [0044]     GARP can be used to perform the multicasting. Of course, the topology is unknown, so GARP has no idea which ports, if any, to which the attribute should be distributed. Instead of relying on a known topology, the modified GARP treats all of the intra-stack ports as enabled ports and distributes the attributes to each port.  
         [0045]     If any switches are coupled to any of those ports, their respective GARPs will receive the attribute along with a registration command. In response to the registration command, the GARPs report the attribute to their local distributed dictionary applications and forward the attributes on to each of their respective intra-stack ports with the exception of the port from which the attribute was received. This process is discussed in more detail below with respect to  FIG. 4 . With each switch reporting and forwarding the attribute, the attribute will eventually be distributed throughout the network.  
         [0046]     Distributing attributes as discussed above works fine unless there is a loop in the network. A loop is where at least two nodes are connected by more than one path so that data can travel around the path. For instance, referring briefly to  FIG. 2 , if an additional cable  260  connected switch  110  directly to switch  130 , then the three switches,  110 ,  120 , and  130 , would be nodes in a loop. Since the topology is unknown, GARP cannot detect a loop based on the topology. If a multicast attribute enters a loop in the stack, the attribute may continuously circulate as each node in the loop forwards it on to the next node. As discussed below with respect to  FIG. 5 , GARP is modified to handle loops.  
         [0047]     Returning to  FIG. 3 , in block  350 , the switch monitors the value associated with the key to see if it changes. In the illustrated embodiment, if the value does not change, the switch delays for a time in block  360  and then multicasts the attribute again in block  340 . The switch will continue to loop until the value changes. This looping provides a refresh of the attribute on a regular basis to protect against lost data. Also, in one embodiment, attributes are stored only for a limited time to prevent old data from accumulating in the dictionary. Periodically refreshing helps maintain current data.  
         [0048]     In block  350 , when and if the value changes, the switch deregisters the attribute in block  370  and returns to loop through the process again from block  310 . As discussed above, GARP can deregister attributes in a number of ways.  
         [0049]     By performing the embodiment illustrated in  FIG. 3  at every switch in the stack, and for every intra-stack port on every switch for which a current value exists, each switch maintains current data with respect to its ports and provides that current data to the rest of the switches through the distributed dictionary.  
         [0050]      FIG. 4  illustrates another embodiment of the first invention. The illustrated process is used by an individual switch to monitor and update its distributed dictionary with respect to information reported from other switches.  
         [0051]     In block  410 , the switch receives an attribute for registration as part of a multicast from another switch. Again, GARP can be used for this purpose. In block  420 , a key from the attribute is used as an index into the switch&#39;s locally stored version of the distributed dictionary to see if the key from the attribute matches a previously stored key. If there is no matching key, then the attribute is new and it is stored by the distributed dictionary application in block  430 . For instance, when the stack is first configured, the attributes will need to be stored as they are distributed.  
         [0052]     In block  420 , if the key matches a previously stored key, the switch checks in block  440  to see if the attribute has a different incarnation value. If it does not have a different incarnation value, then the attribute is likely just a refresh of an earlier received attribute. In the illustrated embodiment, the switch just returns to block  410  to wait for the next attribute to arrive. In an alternate embodiment in which attributes have limited life spans, the repeated attribute may replace the currently stored attribute or the switch may simply reset a time on the attribute&#39;s life span.  
         [0053]     In block  440 , if the incarnation value is different, the value associated with the key has changed since it was previously stored. In block  450 , the newer incarnation is maintained, and in block  460  the older incarnation is deregistered. As discussed above, attributes can be deregistered in any number of ways. In block  470 , the attributed is forwarded to each intra-stack port on the switch with the exception of the port on which the attribute was received.  
         [0054]     If the process of  FIG. 4  is performed by each switch in the stack for each multicast that is received, the switches will maintain current attributes in the distributed dictionary. In which case, when attributes are presented to an application in a switch for various kinds of processing, the switch should receive the most recent attributes.  
         [0055]      FIG. 5  illustrates one embodiment of how GARP can be modified to protect against network loops when multicasting data in a network having an unknown topology. Basically, GARP determines if the same attribute has arrived at a node from more than one path. If it has, then the attribute is likely caught in a loop. In which case, GARP breaks the loop by not forwarding the attribute.  
         [0056]     In block  505 , GARP receives a multicast attribute and recognizes the port from which the attribute was received. The port can be a physical port, such as the intra-stack ports, or a virtual port, such as a port between GARP and an application within the node.  
         [0057]     In block  510 , GARP checks to see if the key for the attribute is new. If it is new, then the attribute is not likely caught in a loop. In which case, in block  540 , if the attribute is accompanied by a registration command, the port number that the attribute arrived at is recorded in block  545 . If the attribute is not for registration in block  540 , the attribute is forwarded in block  555 .  
         [0058]     If the key is not new, in block  515 , GARP checks to see if the incarnation for the key is new. If the incarnation is new, then the value of the attribute is new, suggesting that the attribute is not likely caught in a loop. In which case, the attribute gets the same treatment in blocks  540  and  545  as discussed above.  
         [0059]     If the incarnation is not new, in block  520 , GARP checks to see if a port number has been recorded for the key. If a port number has not been previously recorded for an attribute that has an old key and an old incarnation, the attribute is unlikely to be caught in a loop, and gets the same treatment in blocks  540  and  545  as discussed above.  
         [0060]     If a port number has been recorded, in block  525 , GARP checks to see if the port number of the current attribute matches the previously recorded port number. If it does not match, then the same attribute was received from two different ports. In which case, GARP ignores the attribute in block  530 . If the port does match, then the attribute is likely a retransmission, and not caught in a loop.  
         [0061]     In block  535 , if the attributed is accompanied by a “deregistration” command, the port number recorded for the attribute is erased in block  550 . If there is no “deregistration” command, GARP forwards the attribute in block  555 .  
         [0000]     Adjacency Monitoring  
         [0062]      FIG. 6  illustrates one embodiment of the second invention. In the illustrated embodiment, a switch determines to which of its neighbors it is coupled and through which ports. These relationships are referred to as adjacencies. This process is likely to be the first step in automatically managing a network topology. As part of this initial process, the illustrated embodiment also selects a particular switch to coordinate topology management for the entire stack, and selects an identifier for the entire stack based on the selected switch. Even after all of the adjacencies for the stack have been initially determined, the process can continue to monitor adjacencies for any topology changes.  
         [0063]     In block  605 , a switch starts the process by using its media access control (MAC) address for its stack identifier. All of the switches in the stack will eventually adopt the same stack identifier in order to identify the stack to which they belong. In which case, the switch is likely to change its stack identifier later on.  
         [0064]     When a switch is first coupled to a stack, the switch has no idea whether or not it has any neighbors. The switch starts out by assuming that it is alone. That is, the switch assumes that it is a stack of one, and it is the master of its stack. For this reason, the switch uses its own MAC address as the stack identifier.  
         [0065]     In block  610 , the switch locally stores attributes defining its own state. For instance, in the illustrated embodiment, the attributes are key/value pairs. As discussed above, the key is an index and the value is associated with the key. In one embodiment, the key is the switch&#39;s MAC address and the value is an intra-stack port number for a port on the switch and the stack identifier. In which case, the switch may store one attribute for each intra-stack port.  
         [0066]     In block  615 , the switch broadcasts each of the stored attributes on a corresponding port. The broadcast is point-to-point, as opposed to multicast, and is intended to go no further than an immediate neighbor. That is, if the attribute reaches a neighbor, the neighbor need not forward the attribute.  
         [0067]     In block  620 , the neighbor switches are doing the same thing, so the switch receives an attributed from a neighbor if the neighbor is coupled to the port. In block  625 , the switch detects the presence of an attribute at one or more ports that does not match the attribute(s) broadcast from the respective port(s). That is, the switch detects that the neighbor&#39;s attribute is different from its own.  
         [0068]     In block  630 , the switch checks the stack identifiers to seen if they are the same. In the illustrated embodiment, if the switches do not agree on a stack identifier, the switches first negotiate for the stack identifier. The switches will not identify an adjacency among switches until they have agreed on a stack identifier.  
         [0069]     Switches cannot be adjacent to one another for networking purposes if they are not part of the same stack. So, they agree on the stack identifier before determining adjacencies for a variety of reasons. As discussed below, in one embodiment, the stack identifier corresponds to a master switch. The adjacencies are used by the master switch to determine a topology for the stack. If the master switch is yet to be designated, then there is no need to record an adjacency.  
         [0070]     If the stack identifiers do not match, in block  635 , the switch will adopt the stack identifier of the neighbor if the neighbor&#39;s stack identifier is larger than the switch&#39;s. In block  640 , the switch ignores the attribute if the neighbor&#39;s stack identifier is smaller than the switches. Then the process returns to block  610  to loop through again.  
         [0071]     If all of the switches process stack identifiers in the same way, the switch having the largest MAC address will ignore all of its neighbors&#39; attributes until the neighbors adopt its MAC address as the stack identifier, and then the neighbors will ignore all of the attributes from their neighbors until their neighbors adopt the largest MAC address, and so on. In this fashion, the highest MAC address will propagate out to all of the switches until all of the switches agree on the highest MAC address for the stack identifier.  
         [0072]     In alternate embodiments, rather than using the highest MAC address, the stack could adopt the lowest MAC address. In another embodiment, a user defined MAC address could be used. For instance, a user could set a stack identifier for a particular switch that is guaranteed to be higher than any MAC address so that the user-selected value gets adopted.  
         [0073]     In one embodiment, the switch whose original stack identifier gets adopted by the stack is designated the master switch for the stack. So, by manipulating the original stack identifiers, a user can pre-select a particular switch to be the master switch if, for instance, one switch has more processing power than another. The importance of the master switch will be discussed below.  
         [0074]     If in block  630  the stack identifiers do match, for instance, upon a second iteration through block  630 , the switch obtains the neighbor&#39;s MAC address and port number from the attributes the neighbor sent and combines the neighbor&#39;s MAC address and port number with its own MAC address and port number to create an adjacency. In one embodiment, an adjacency comprises, key=(own MAC address, own Port number), and value=(neighbor&#39;s MAC address, and port number).  
         [0075]     In block  650 , the switch determines an incarnation identifier for the adjacency. If the switch was just powered up in the stack, the incarnation is likely to be an initial incarnation, such as one.  
         [0076]     In block  655 , the switch registers the adjacency and the incarnation in the distributed dictionary. As discussed above, since the topology is not yet known, GARP can be used to register the adjacency. Once the adjacency is registered, the process returns to block  610  to monitor the adjacencies for changes. If changes are detected, they are registered with a new incarnation number and the old incarnation is deregistered.  
         [0000]     Topology Calculations  
         [0077]      FIG. 7  illustrates one embodiment of the third invention. The illustrated process uses a set of adjacencies provided, for instance, by the second invention to obtain a network topology, also called a spanning tree.  
         [0078]     In block  710 , the master switch accesses the set of adjacencies in the distributed dictionary as reported by all of the switches. In block  720 , the master switch provides the set of adjacencies to a graph-theory algorithm. In one embodiment, a known shortest path first (SPF) algorithm is used. The algorithm operates on a set of nodes and links between nodes to determine the shortest path between any two nodes. In one embodiment, SPF operates on the basis of propagation delay through the respective network paths. In addition to determining shortest paths, SPF also ensures that paths do not loop back on themselves, which could potentially cause lost data or other problems.  
         [0079]     In block  730 , when the master switch receives the spanning tree back from SPF, the master switch runs a reporting task, which is the subject matter of the fourth invention discussed below.  
         [0080]     In block  740 , the switch continues to monitor the reported adjacencies for changes. For instance, as discussed above, the adjacency monitor continues to update the set of adjacencies. If an adjacency changes, the process returns to block  710  to obtain a new topology.  
         [0000]     Reporting Topology Calculations  
         [0081]      FIGS. 8 through 10  illustrate three embodiments of the fourth invention. Together, the illustrated embodiments report a new topology to the stack. The topology is used by the switches in the stack to direct data packets among the switches through the intra-stack ports. A new topology is reported in such a way so as to avoid transient loops.  
         [0082]     As discussed above, SPF generates a topology without any loops. However, as is the case with autonomous routers, while transitioning from one topology to another, it is possible to experience transient loops. For instance, if a topology changes in the vicinity surrounding a set of routers, the routers will update to the new topology at their own discretion. In which case, since the routers are unlikely to all individually adopt the new topology at the same time, for at least a brief period, not all of the routers will be routing data using the same topology.  
         [0083]     For instance, referring briefly to  FIG. 2 , if the topology where to change to include a cable  260  directly from switch  110  to switch  130 , a transient loop may occur. That is, if the new link through the new cable were adopted before the old link was disabled, for a brief period of time, switches  110 ,  120 , and  130  would be in a loop.  
         [0084]     For routers, these transient loops are not fatal because routers have a higher tolerance for lost or misguided data. For instance, the data packets may have a limited life span, so the looping data will eventually time out and dissipate. Switches, however, have very low tolerance for even transient loops. Therefore, the fourth invention rolls out a new topology in a coordinated manner between the master switch and the rest of the switches to intentionally avoid, or at least reduce, the potential for problems caused by transient loops.  
         [0085]     Basically, the fourth invention insures that all old links are disabled before new links are enabled. For instance, in the example above for the loop in  FIG. 2 , if the old link between switches  120  and  130  were disabled before the new link was formed between  110  and  130 , no transient loop would occur. In one embodiment, a port is disabled only if it cannot send and cannot receive data, and it has no packets buffered and waiting for transmission. By including sending and receiving as requirements for disablement, a link can be disabled by disabling a port on just one end of the link.  
         [0086]     In block  805 , the reporting process is initiated when a new spanning tree is obtained as discussed above. In block  810 , the master switch compares the new spanning tree to the old spanning tree. If the trees are the same, the process ends, and will be re-initiated when and if a new spanning tree is obtained. If the spanning tree is not the same, the process continues.  
         [0087]     In block  815 , the master switch determines the set of links to be disabled based on a comparison between the old spanning tree and the new spanning tree. In block  820 , the master switch determines the set of links to be enabled based on a comparison of the two spanning trees.  
         [0088]     In block  825 , the master switch checks to see if there are any links to disable. As discussed above, if there are any links to be disabled, they should be disabled before any links are enabled to reduce the likelihood of transient loops.  
         [0089]     In order to disable the links, the master switch removes the links to be disabled from the old spanning tree in block  840 . Then, the master switch advances the incarnation identifier for the old spanning tree in block  845 . And, in block  850 , the spanning tree is registered to the distributed dictionary. In one embodiment, registering to the distributed dictionary is accomplished without relying on the topology as discussed above.  
         [0090]     By registering the modified old spanning tree with a new incarnation identifier, the switches will recognize the change in the spanning tree and take appropriate action. Skipping to  FIG. 10 ,  FIG. 10  illustrates one embodiment of a process performed by each switch in response to registering the modified spanning tree.  
         [0091]     In block  1010 , the switch reads the modified spanning tree from the distributed dictionary. In block  1020 , the switch extracts its own ports from the spanning tree. In block  1030 , the switch enables all of the ports extracted from the spanning tree and disables all of the others. When the switch is done applying the modified spanning tree, the switch acknowledges completion in block  1040  by registering an acknowledgement to the distributed dictionary including the incarnation number of the modified spanning tree.  
         [0092]     Meanwhile, as illustrated in  FIG. 9 , one embodiment of the master switch is monitoring the distributed dictionary for acknowledgements. Each time a new acknowledgement is received in block  910 , the master checks to see if all of the switches have acknowledged with the most current incarnation number for the modified spanning tree in block  920 . For instance, in one embodiment, the master identifies all of the switches based on the current topology and checks an incarnation number from a response from each. In other words, the master waits until all of the switches are synchronized. Once the switches are synchronized, the master runs a reporting process again, such as the one illustrated in  FIG. 8 .  
         [0093]     Returning to  FIG. 8 , in block  805 , the master performs a second iteration of the process when it determines that the slaves are all synchronized and have disabled the links identified in the previous iteration. In block  810 , the master compares the old spanning tree, which includes the modifications from the first iteration through the reporting process, to the new spanning tree that was previously obtained as discussed above. If the changes from the old to the new only required that links be disabled, the spanning trees will be the same and the process will stop. If, however, the new spanning tree needs to have new links enabled, the process continues.  
         [0094]     It should be noted that under certain circumstances, there may be cases in which disabling links in the first iteration may necessitate enabling and/or disabling additional links in the second iteration. In which case, in the illustrated embodiment, the process determines the links to disable and the links to enable again in blocks  815  and  820 . In block  825 , if additional links need to be disabled, the process will proceed as above to report the modified spanning tree and wait for the switches to synchronize. The reporting process may go through a number of iterations until the set of links to disable is finally empty.  
         [0095]     In block  830 , the process checks to see if any prior changes remain unacknowledged. The process is preparing to enable new links which, if the switches are not synchronized, could create transient links. So for instance, if in the first iteration through the reporting process, no links needed to be disabled, the process would verify that the switches are synchronized. If they are not, the reporting process ends until it is called again.  
         [0096]     If the switches are synchronized in block  830 , the master adds the new links to the old spanning tree (which may have been modified one or more times in previous iterations) in block  835 , advances the incarnation identifier in block  845 , and registers the spanning tree to the distributed dictionary.  
         [0097]     Again, the switches will adopt the new spanning tree, for instance, as illustrated in  FIG. 10 , and when the master determines they are all synchronized, for instance, as illustrated in  FIG. 9 , the reporting process will be called again. In this iteration, in block  810 , the spanning trees should match, ending the process.  
         [0098]     In all of the embodiments of all the inventions described herein, alternate embodiments may not require all of the elements shown, may include additional elements, and may perform one or more elements in a different order. Furthermore, even though the embodiments were illustrated in the context of a switch stack, the inventions are applicable to a wide variety of alternate network environments as well.  
         [0099]      FIG. 11  illustrates one embodiment of a hardware system intended to represent a broad category of network devices such as personal computers, workstations, switches, routers, and/or embedded systems. In the illustrated embodiment, the hardware system includes processor  1110  coupled to high speed bus  1105 , which is coupled to input/output (I/O) bus  1115  through bus bridge  1130 . Temporary memory  1120  is coupled to bus  1105 . Permanent memory  1140  is coupled to bus  1115 . I/O device(s)  1150  is also coupled to bus  1115 . I/O device(s)  1150  may include a display device, a keyboard, one or more external network interfaces, etc.  
         [0100]     Certain embodiments may include additional components, may not require all of the above components, or may combine one or more components. For instance, temporary memory  1120  may be on-chip with processor  1110 . Alternately, permanent memory  1140  may be eliminated and temporary memory  1120  may be replaced with an electrically erasable programmable read only memory (EEPROM), wherein software routines are executed in place from the EEPROM. Some implementations may employ a single bus, to which all of the components are coupled, or one or more additional buses and bus bridges to which various additional components can be coupled. Those skilled in the art will be familiar with a variety of alternate internal networks including, for instance, an internal network based on a high speed system bus with a memory controller hub and an I/O controller hub. Additional components may include additional processors, a CD ROM drive, additional memories, and other peripheral components known in the art.  
         [0101]     In one embodiment, the present invention, as described above, is implemented using one or more computers such as the hardware system of  FIG. 11 . Where more than one computer is used, the systems can be coupled to communicate over an external network, such as a local area network (LAN), an IP network, etc. In one embodiment, the present invention is implemented as software routines executed by one or more execution units within the computer(s). For a given computer, the software routines can be stored on a storage device, such as permanent memory  1140 .  
         [0102]     Alternately, as shown in  FIG. 12 , the software routines can be machine executable instructions  1210  stored using any machine readable storage medium  1220 , such as a diskette, CD-ROM, magnetic tape, digital video or versatile disk (DVD), laser disk, ROM, Flash memory, etc. The series of instructions need not be stored locally, and could be received from a remote storage device, such as a server on a network, a CD ROM device, a floppy disk, etc., through, for instance, I/O device  1150  of  FIG. 11 .  
         [0103]     From whatever source, the instructions may be copied from the storage device into temporary memory  1120  and then accessed and executed by processor  1110 . In one implementation, these software routines are written in the C programming language. It is to be appreciated, however, that these routines may be implemented in any of a wide variety of programming languages.  
         [0104]     In alternate embodiments, the present invention is implemented in discrete hardware or firmware. For example, one or more application specific integrated circuits (ASICs) could be programmed with one or more of the above described functions of the present invention. In another example, one or more functions of the present invention could be implemented in one or more ASICs on additional circuit boards and the circuit boards could be inserted into the computer(s) described above. In another example, field programmable gate arrays (FPGAs) or static programmable gate arrays (SPGA) could be used to implement one or more functions of the present invention. In yet another example, a combination of hardware and software could be used to implement one or more functions of the present invention.  
         [0105]     Thus, a suite of network-related inventions is described. Whereas many alterations and modifications of the present invention will be comprehended by a person skilled in the art after having read the foregoing description, it is to be understood that the particular embodiments shown and described by way of illustration are in no way intended to be considered limiting. Therefore, references to details of particular embodiments are not intended to limit the scope of the claims.