Patent Publication Number: US-8995444-B2

Title: Method and system for extending routing domain to non-routing end stations

Description:
RELATED APPLICATION 
     This application is a continuation application of application Ser. No. 12/730,749, entitled “METHOD AND SYSTEM FOR EXTENDING ROUTING DOMAIN TO NON-ROUTING END STATIONS,” by inventors Pankaj K. Jha and Mitri Halabi, filed 24 Mar. 2010, the disclosure of which is incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Field 
     The present disclosure relates to network management. More specifically, the present disclosure relates to a method and system for extending the routing domain in a network to a non-routing node. 
     2. Related Art 
     Currently, end stations in layer-2 networks have not been able to take advantage of the routing functionalities available in such networks. End stations can only operate as leaf nodes and are constrained to an interface with only one of the routing nodes. Consequently, an end station is required to exchange all the data with that node, and there is no node redundancy (e.g., for protection against routing node failures) available to end stations. Further more, other routing nodes in the network can send data to that end station only via one routing switch to which the end station is connected. 
     Meanwhile, layer-2 networking technologies continue to evolve. More routing functionalities, which have traditionally been the characteristics of layer-3 (e.g., IP) networks, are migrating into layer-2. Notably, the recent development of the Transparent Interconnection of Lots of Links (TRILL) protocol allows Ethernet switches to function more like routing nodes. TRILL overcomes the inherent inefficiency of the conventional spanning tree protocol, which forces layer-2 switches to be coupled in a logical spanning-tree topology to avoid looping. TRILL allows routing bridges (RBridges) to be coupled in an arbitrary topology without the risk of looping by implementing routing functions in switches and including a hop count in the TRILL header. 
     However, a switch or end host not running the TRILL protocol cannot benefit from the rich set of functionalities provided in TRILL networks. 
     SUMMARY 
     One embodiment of the present invention provides a system for facilitating assignment of a virtual routing node identifier to a non-routing node. During operation, the system assigns to a non-routing node coupled to a switch a virtual routing node identifier unique to the non-routing node. In addition, the system communicates reachability information corresponding to the virtual routing node identifier to other switches in the network. 
     In a variation on this embodiment, the switch is a routing bridge capable of routing data frames without requiring the network topology to be a spanning tree topology. 
     In a variation on this embodiment, the virtual routing node identifier is a routing bridge identifier in accordance with the Transparent Interconnection of Lots of Links (TRILL) protocol. 
     In a variation on this embodiment, the system performs a routing function based on an intermediate system to intermediate system (IS-IS) routing protocol. The system further advertises reachability associated with the virtual routing node identifier in IS-IS link state protocol data units. 
     In a variation on this embodiment, the system marks an ingress-switch field of a frame received from the non-routing node with the virtual routing node identifier. 
     In a variation on this embodiment, the non-routing node is also coupled to at least a second routing switch capable of performing a routing function. The first routing switch communicates to the second routing switch and optionally other routing switches the media access control (MAC) address(es) learned from a port to which the none-routing node is coupled. 
     In a variation on this embodiment, in response to detecting a failure of a link between the non-routing node and the switch, the system discontinues communicating the reachability information to other switches and/or provides an explicit message to inform other switch(es) about the non-reachability to the non-routing node. 
     In a variation on this embodiment, the system is precluded from forwarding a multi-destination frame to the non-routing node when an ingress-switch field of the frame matches the virtual routing node identifier assigned to the non-routing node. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1A  illustrates an exemplary network where virtual RBridge identifiers are assigned to non-TRILL nodes, in accordance with an embodiment of the present invention. 
         FIG. 1B  illustrates an exemplary network that facilitates virtual RBridge identifier assignment to a non-TRILL node coupled to multiple TRILL RBridges via link aggregation, in accordance with an embodiment of the present invention. 
         FIG. 2  presents a flowchart illustrating the process of configuring the TRILL header of an ingress frame from a non-TRILL node at an ingress physical RBridge, in accordance with an embodiment of the present invention. 
         FIG. 3  illustrates an exemplary header configuration of an ingress TRILL frame which contains a virtual RBridge nickname in its ingress RBridge nickname field, in accordance with an embodiment of the present invention. 
         FIG. 4  presents a flowchart illustrating the process of forwarding a TRILL frame at an ingress RBridge which participates in the link aggregation in conjunction with a non-TRILL node which has a virtual RBridge ID, in accordance with an embodiment of the present invention. 
         FIG. 5  illustrates a scenario where one of the physical links of a link aggregation coupled to a non-TRILL node experiences a failure, in accordance with an embodiment of the present invention. 
         FIG. 6  presents a flowchart illustrating the process of handling a link failure that affects an end station which is assigned a virtual RBridge ID, in accordance with an embodiment of the present invention. 
         FIG. 7  illustrates an exemplary architecture of a switch that facilitates assignment of a virtual RBridge ID, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the claims. 
     Overview 
     In embodiments of the present invention, the problem of extending a layer-2 routing domain to a non-routing node is solved by assigning a virtual routing node identifier to the non-routing node. The neighbor routing node(s) of the non-routing node can then advertise the connectivity to the non-routing node as if the non-routing node were part of the routed network. 
     For example, in a layer-2 network running the TRILL protocol, when an end station which does not participate in the TRILL network is coupled to one or more routing bridges (RBridges), a virtual TRILL RBridge identifier is assigned to this end station. The end station is then considered to be a virtual RBridge capable of running the TRILL protocol. An incoming frame from the end station is marked by the ingress physical RBridge with a virtual RBridge nickname as its ingress RBridge nickname and routed through the rest of the TRILL network. An end station can be coupled to multiple physical RBridges and have the same virtual RBridge ID. Consequently, other RBridges in the TRILL network can use multiple paths to reach this end station using its virtual RBridge ID, because to the rest of the TRILL network, this end station appears to be just like any other RBridge in the network. The assignment of a virtual RBridge identifier allows a non-TRILL-capable end station to participate in the routing domain of a TRILL network, and to be coupled to multiple RBridges in an arbitrary topology. Such configuration provides tremendous flexibility and facilitates redundancy against both link and node failures. For instance, an end station with a virtual RBridge identifier can be coupled to two or more physical RBridges using link aggregation. The physical RBridges can advertise connectivity to the virtual RBridge to their neighbor RBridges. When one of the aggregated links fails, the affected end station can continue operating via the remaining link(s). For the rest of the TRILL network, the virtual RBridge remains reachable. 
     Although this disclosure is presented using examples based on the TRILL protocol, embodiments of the present invention are not limited to TRILL networks, or networks defined in a particular Open System Interconnection Reference Model (OSI reference model) layer. In particular, although the term “layer-2” is mentioned several times in the examples, embodiments of the present invention are not limited to application to layer-2 networks. Other networking environment, either defined in OSI layers or not defined with a particular layering model, can also use the disclosed embodiments. For instance, these embodiments can apply to Multiprotocol Label Switching (MPLS) networks as well as Storage Area Networks (e.g., Fibre Channel networks). 
     Furthermore, although intermediate-system-to-intermediate-system (IS-IS) routing protocol is used in the examples, embodiments of the present invention are not limited to a particular routing protocol. Other routing protocols, such as Open Shortest Path First (OSPF), Routing Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP), Enhanced IGRP (EIGRP), Border Gateway Protocol (BGP), or other open or proprietary protocols can also be used. In addition, embodiments of the present invention are not limited to the TRILL frame encapsulation format. Other open or proprietary encapsulation format and methods can also be used. 
     The term “RBridge” refers to routing bridges, which are bridges implementing the TRILL protocol as described in IETF draft “RBridges: Base Protocol Specification,” available at http://tools.ietf.org/html/draft-ietf-trill-rbridge-protocol-14, which is incorporated by reference herein. Embodiments of the present invention are not limited to the application among RBridges. Other types of switches, routers, and forwarders can also be used. 
     The term “physical RBridge” refers to an RBridge running TRILL protocol, as opposed to a “virtual RBridge” which refers to a non-TRILL end station with a virtual RBridge ID. 
     The term “virtual RBridge” refers to a non-TRILL end station with a virtual RBridge ID. The physical RBridge(s) to which the non-TRILL end station is coupled can advertise the connectivity to this end station as if it were a regular RBridge. 
     The term “end station” refers to a network device that is not TRILL-capable. “End station” is a relative term with respect to the TRILL network. “End station” does not necessarily mean that the network device is an end host. An end station can be a host, a conventional layer-2 switch, or any other type of network device. Additionally, an end station can be coupled to other switches, routers, or hosts further away from the TRILL network. In other words, an end station can be an aggregation point for a number of network devices to enter the TRILL network. 
     The term “multi-homed end station” refers to an end station that has an aggregate link to two or more TRILL RBridges, where the aggregate link includes multiple physical links to the different RBridges. The aggregate link functions as one logical link to the end station. “Multi-homed end station” may also refer to an end station that connects to TRILL RBridges which do not form a logical link aggregation and do not form an association with each other. This could be the case where an end station has multiple logical networking entities (an example is a virtualized server where different servers may be coupled to different networks through different network ports in the system). A single end-station can have multiple virtual RBridge identifier assignments. 
     The term “frame” refers to a group of bits that can be transported together across a network. “Frame” should not be interpreted as limiting embodiments of the present invention to layer-2 networks. “Frame” can be replaced by other terminologies referring to a group of bits, such as “packet,” “cell,” or “datagram.” 
     The term “RBridge identifier” refers to a group of bits that can be used to identify an RBridge. Note that the TRILL standard uses “RBridge ID” to denote the 48-bit intermediate-system-to-intermediate-system (IS-IS) System ID assigned to an RBridge, and “RBridge nickname” to denote the 16-bit value that serves as an abbreviations for the “RBridge ID.” The “RBridge identifier” used in this disclosure is not limited to any bit format, and can refer to “RBridge ID” or “RBridge nickname” or any other format that can identify an RBridge. 
     Network Architecture 
       FIG. 1A  illustrates an exemplary network where virtual RBridge identifiers are assigned to non-TRILL nodes, in accordance with an embodiment of the present invention. In this example, the network is divided into three layers: a core layer, an aggregation layer, and an access layer. The core layer includes core switches  122  and  124 . These switches can be director-class high-throughput switches interconnecting multiple networks. The aggregation layer includes a number of RBridges  101 ,  102 ,  103 , and  104 . These RBridges facilitate traffic aggregation from the access layer and feed to the core layer. The access layer also includes a number of RBridges  105 ,  106 , and  107 . The RBridges in the access layer interface with end stations, such as end stations  111 ,  112 ,  113 ,  114 ,  115 , and  116 . RBridges  101 ,  102 ,  103 ,  104 ,  105 ,  106 , and  107  form the TRILL network. An end station can be a host, a switch, or any type of network device. For example, end stations  112  and  113  are both servers, and end station  114  is a switch. Furthermore, the TRILL network can be configured in any arbitrary topology, which can be a mesh and not required to be a spanning tree topology. 
     In embodiments of the present invention, an end station is assigned a virtual RBridge ID. This configuration allows the end station to be part of the routed TRILL network, and hence take advantage of the topology flexibility. For example, as illustrated in  FIG. 1A , servers  112  and  113 , and switch  114  are all multi-homed with two physical RBridges. Server  112  is coupled to RBridges  105  and  106 ; server  113  is coupled to RBridges  105  and  107 ; and switch  114  is coupled to RBridges  106  and  107 . Furthermore, switch  114  is also an aggregation point for hosts  120 . Servers  112  and  113 , as well as switch  114 , each can be assigned a virtual RBridge ID. 
     Assume that end stations  112 ,  113 , and  114  are assigned respective virtual RBridge IDs. Their neighbor RBridges broadcast their connectivity as virtual RBridges  144 ,  142 , and  140 , respectively. As a result, the rest of the TRILL network can view these end stations just like any other RBridge and route traffic toward them via any available path. 
     Take switch  114  for example. Without virtual RBridge identifier assignment, switch  114  would be “transparent” to the rest of the TRILL network. The frames forwarded by switch  114  to the TRILL network are native Ethernet frames from hosts  120 . An RBridge in the TRILL network would associate the Media Access Control (MAC) addresses for hosts  120  with an ingress RBridge (i.e., the first RBridge in the TRILL network that receives these Ethernet frames). 
     In addition, without virtual RBridge identifier assignment, the multi-homing-style connectivity would not provide the desired result, because the TRILL protocol depends on MAC address learning to determine the location of end stations (i.e., to which ingress RBridge an end station is coupled) based on a frame&#39;s ingress TRILL RBridge ID. As such, an end station can only appear to be reachable via a single physical RBridge. For example, assume that end station  111  is in communication with one of the hosts  120 . Since switch  114  is coupled to RBridges  106  and  107 , the ingress RBridge would be RBridges  106  and  107 , and the egress RBridge would be RBridge  105 . The incoming frames forwarded by switch  114  would have either RBridge  106  or RBridge  107  marked as their ingress RBridge ID. When RBridge  105  receives these frames and performs MAC address learning, RBridge  105  would assume that the host is moving and is either coupled to RBridge  106  or RBridge  107  (but not both). RBridge  105  would send the frames from end station  111  to either RBridge  106  or RBridge  107 . Consequently, only one of the physical links leading to switch  114  is used, which defeats the purpose of having redundant links between switch  114  and RBridges  106  and  107 . 
     The multi-homing configuration of an end station is explained in more detail in the example illustrated in  FIG. 1B . In this example, the TRILL network includes five physical RBridges:  161 ,  162 ,  163 ,  164 , and  165 . A non-TRILL switch  168  is coupled to RBridges  162 ,  164 , and  165 , and end stations  172  and  174 . During operation, a virtual RBridge identifier is assigned to non-TRILL switch  168 , either manually or automatically by one of the coupled RBridges using Link Layer Discovery Protocol (LLDP) or any other configuration/discovery protocol. 
     Non-TRILL switch  168  has its links to RBridges  162 ,  164 , and  165  configured as a link aggregation (LAG)  170 . In other words, switch  168  can distribute ingress traffic (i.e., traffic from end stations  172  and  174  to the TRILL network) among the three links using link aggregation techniques. Such techniques can include any multi-chassis trunking techniques. In addition, RBridges  162 ,  164 , and  165  are configured to process ingress frames from switch  168  such that these frames will have the virtual RBridge nickname in their TRILL header as the ingress RBridge. When these frames are forwarded to the rest of the TRILL network with their respective TRILL headers, other RBridges in the network treat them as being originated from the virtual RBridge. 
     During operation, each physical RBridge sends TRILL HELLO messages to its neighbor to confirm its health. Each RBridge also sends link state protocol data units (LSPs) to its neighbor, so that link state information can be exchanged and propagated throughout the TRILL network. As illustrated in  FIG. 1B , RBridge  162  regularly transmits TRILL HELLO messages to its neighboring RBridges. Since switch  168  is not TRILL enabled, RBridge  162  does not send TRILL HELLO messages to switch  168 . In addition, RBridge  162  has a static link state entry for the virtual RBridge associated with switch  168 , and periodically announces the reachability to this virtual RBridge in its LSPs to other RBridges. Similarly, RBridges  164  and  165  also maintain static link state entries for the virtual RBridge and announce its reachability in their respective LSPs. 
     In a TRILL network, the reachability information associated with a MAC address is learned by an RBridge based on the source MAC address in the internal Ethernet header of a received TRILL frame. It is possible that the three RBridges coupled to switch  168  may learn different MAC reachability information about the end stations behind switch  168 . To avoid unnecessary flooding (which is the default behavior of a layer-2 switch when it encounters a frame with a destination MAC address that it does not know how to forward), in one embodiment, the MAC address reachability for end stations  172  and  174  via switch  168  is shared among all the RBridges coupled to switch  168 . That is, RBridges  162 ,  164 , and  165  share the MAC reachability information of the devices behind switch  168 . For example, assume that RBridge  162  receives all the frames originated from end station  172 , while RBridge  164  receives all the frames originated from end station  174 . RBridge  162  then shares its knowledge about the MAC of end station  172  (i.e., the knowledge that the MAC address for end station  172  is associated with the virtual RBridge and that end station  172  is behind switch  168 ) with RBridges  164  and  165 . Similarly, RBridge  164  shares its knowledge about end station  174 &#39;s MAC address with RBridges  162  and  165 . This way, when a frame destined for, say, end station  172  reaches RBridge  165 , RBridge  165  would know that it can forward the frame to switch  168 , instead of flooding it to multiple ports. 
     It should be noted that a non-TRILL switch (such as switch  168 ) is not required to change the way it is configured for handling link aggregation. A multi-homed end station only needs to be configured as would be the case with a regular link aggregation scheme. Hence, the multi-homed end station does not need to be aware that a virtual RBridge nickname is assigned to all the frames it forwards to the physical RBridge(s). Furthermore, the rest of the TRILL network (apart from RBridges  163 ,  164 , and  165 ) is also not required to be aware that the virtual RBridge is actually not a physical RBridge. In other words, to the rest of the TRILL network, the virtual RBridge associated with switch  168  is indistinguishable from any of the physical RBridges. Therefore, the present invention does not require extra configuration to the rest of the TRILL network. 
     Frame Processing 
       FIG. 2  presents a flowchart illustrating the process of configuring the TRILL header of an ingress frame from a non-TRILL node at an ingress physical RBridge, in accordance with an embodiment of the present invention. During operation, a physical RBridge participating in link aggregation receives an ingress Ethernet frame from a non-TRILL switch which has been assigned a virtual RBridge identifier (operation  202 ). The RBridge then identifies the destination MAC address of the received frame (operation  204 ). Based on the destination MAC address, the RBridge performs a lookup on the egress TRILL RBridge nickname (operation  206 ). Next, the RBridge determines the next-hop TRILL RBridge based on the egress TRILL RBridge nickname (operation  208 ). (It is assumed that the routing function in the TRILL protocol or other routing protocol is responsible for populating the forwarding information base at each RBridge.) 
     Subsequently, the RBridge sets the TRILL header of the frame (operation  210 ). In doing so, the RBridge sets the virtual RBridge as the ingress RBridge for the frame. The egress RBridge of the TRILL header is set based on the result of operation  206 . 
     The RBridge then sets the outer Ethernet header of the frame (operation  212 ). In doing so, the RBridge sets the MAC address of the next-hop RBridge (the result of operation  208 ) as the destination MAC address in the outer Ethernet header. The RBridge further sets the MAC address of the local transmitting RBridge as the source MAC address in the outer Ethernet header. After setting the outer Ethernet header, the RBridge transmits the TRILL-encapsulated frame to the next-hop RBridge (operation  214 ). 
       FIG. 3  illustrates an exemplary header configuration of an ingress TRILL frame which contains a virtual RBridge nickname in its ingress RBridge nickname field, in accordance with an embodiment of the present invention. In this example, a TRILL-encapsulated frame includes an outer Ethernet header  302 , a TRILL header  303 , an inner Ethernet header  308 , an Ethernet payload  310 , and an Ethernet frame check sequence (FCS)  312 . 
     TRILL header  303  includes a version field (denoted as “V”), a reserved field (denoted as “R”), a multi-destination indication field (denoted as “M”), an option-field-length indication field (denoted as “OP-LEN”), and a hop-count field (denoted as “HOP CT”). Also included are an egress RBridge nickname field  304  and an ingress RBridge nickname field  306 . 
     After an ingress physical RBridge encapsulates an ingress frame with the proper TRILL and outer Ethernet headers and transmits the frame to its destination, it is expected to receive frames in the reverse direction from the destination in response to the transmission.  FIG. 4  presents a flowchart illustrating the process of receiving and forwarding a TRILL frame at a physical RBridge which participates in the link aggregation in conjunction with a non-TRILL node which has a virtual RBridge ID, in accordance with an embodiment of the present invention. 
     During operation, a physical RBridge receives a TRILL frame (operation  402 ). The RBridge then determines whether the frame&#39;s egress RBridge nickname corresponds to the local RBridge or a virtual RBridge associated with the local RBridge (operation  403 ). If the frame&#39;s egress RBridge nickname matches neither the local RBridge nor a virtual RBridge identifier associated with a non-TRILL switch coupled to the local RBridge (i.e., the frame is not destined to the local RBridge), the RBridge transmits the frame to the next-hop RBridge based on the frame&#39;s egress RBridge nickname (operation  405 ). 
     On the other hand, if the condition in operation  403  is met, the RBridge then performs a lookup in its MAC-address table to identify an output port corresponding to the frame&#39;s destination MAC address in its inner Ethernet header (operation  404 ). In the case where the frame&#39;s destination MAC address corresponds to an end station coupled to the local RBridge, the frame is processed locally. If the frame&#39;s destination MAC address corresponds to an output port leading to the non-TRILL switch coupled to the local RBridge, the local RBridge removes the outer Ethernet header and TRILL header from the frame, and forwards the frame to the identified output port leading to the non-TRILL switch. Note that the MAC reachability information is shared between the RBridges participating in the link aggregation. Hence, even if the local RBridge has not received an ingress frame with the same source MAC address (i.e., the RBridge has not learned the MAC address locally), the RBridge can still determine that the destination MAC address is reachable via a local link based on the MAC reachability information shared from the other RBridge(s). Subsequently, the RBridge transmits the frame to the output port corresponding to the frame&#39;s destination MAC address in its inner Ethernet header (operation  408 ). 
     Failure Handling 
     One advantage of assigning a virtual RBridge identifier to a non-TRILL switch is to facilitate link aggregation across multiple physical RBridges, which in turn provides protection against both link and node failures.  FIG. 5  illustrates a scenario where one of the physical links of a link aggregation coupled to a non-TRILL node experiences a failure, in accordance with an embodiment of the present invention. In this example, a non-TRILL switch  568  is coupled to three physical RBridges  562 ,  564 , and  566  via a link aggregation  570 . Non-TRILL switch  568  is assigned a virtual RBridge ID. Suppose the link between switch  568  and RBridge  566  fails. As a result, RBridge  566  will notify its neighbor RBridges about the non-reachability of switch  568 . Meanwhile, LAG  570  remains effective with RBridges  562  and  564 . Switch  568 &#39;s virtual RBridge nickname can still be used as the ingress RBridge nickname in the TRILL headers of its ingress frames. 
     RBridge  566  may still receive some frames destined to switch  568  before the TRILL network topology converges. Since RBridges  562  and  564  can both be used to reach switch  568 , RBridge  566  can forward these frames to RBridge  562  or  564 . Thus, minimum service interruption can be achieved during link failure. Similarly, in the case of node failure (e.g., when RBridge  566  fails), switch  568  can continue operation with LAG  570 . 
       FIG. 6  presents a flowchart illustrating the process of handling a link failure that affects an end station which is assigned a virtual RBridge ID, in accordance with an embodiment of the present invention. During operation, a physical RBridge detects a failure to a physical link to an end station associated with the virtual RBridge (operation  602 ). The physical RBridge then updates its TRILL forwarding information base to reflect this topology change ( 604 ). Subsequently, the RBridge sends link state PDUs to its neighbor RBridges to update the link state (operation  606 ). Note that the non-TRILL end station corresponding to the virtual RBridge identifier does not need to be re-configured. It only needs to re-distribute the outgoing frames to the remaining links within the LAG coupling to other physical RBridges. 
     Multi-Destination Scenario 
     In conventional TRILL networks, a multi-destination frame is delivered using one or more distribution trees. Each tree has a root RBridge. Typically, an ingress RBridge marks a multi-destination TRILL frame&#39;s egress RBridge nickname field with the nickname of the root RBridge for the distribution tree, and sets the M bit of the frame to “1.” As a result, the frame is distributed through the TRILL network based on a TREE topology associated with the root RBridge. 
     It is possible that a multi-destination frame is generated by a non-TRILL switch which is coupled to a number of physical RBridges and is assigned a virtual RBridge identifier. Since the non-TRILL switch is regarded as an RBridge by the rest of the TRILL network, its virtual RBridge identifier will be included in the distribution trees. Take the network illustrated in  FIG. 1B  for example. A distribution tree can have RBridge  162  as its root. The tree can include the following links: ( 162 ,  164 ), ( 162 ,  161 ), ( 162 ,  163 ), ( 163 ,  165 ), ( 165 ,  168 ). In this case, since non-TRILL switch  168  does not follow the TRILL multi-destination distribution protocol, when it forwards a multi-destination frame (which may be generated by end station  172  or  174 ), its default behavior is to send the frame onto one of the links within LAG  170 . The link used by non-TRILL switch  168  may or may not be part of the desired distribution tree. This default behavior, if not mitigated, could cause unwanted loops in the network. For instance, suppose non-TRILL switch  168  sends a multi-destination frame onto the link toward RBridge  164 . Since RBridge  165  is adjacent to RBridge  164  in the distribution tree, RBridge  164  would forward this frame to RBridge  165 . Upon receiving this frame, RBridge  165  would by default forward it non-TRILL switch  168 , which is part of the distribution tree. As a result, a loop occurs. 
     One way to solve this looping problem is to provide proper encapsulation at the ingress physical RBridge (such as RBridges  162 ,  164 , and  165 ). In one embodiment, at the ingress physical RBridge, the multi-destination frame is encapsulated with a multi-destination TRILL header whose M bit is set to “1.” In this TRILL header, the ingress RBridge nickname field contains the virtual RBridge nickname assigned to non-TRILL switch  168 . The egress RBridge nickname field contains the nickname of RBridge  162 , which is the root of the distribution tree. The ingress physical RBridge can then forward this frame on behalf of non-TRILL switch  168 . In addition, when a physical RBridge receives a multi-destination frame with an ingress RBridge nickname matching the virtual RBridge nickname assigned to a non-TRILL switch connected to the physical RBridge, the physical RBridge is precluded from forwarding this frame to the non-TRILL network. For example, in the scenario described above, RBridge  165  would be precluded from forwarding the multi-destination frame to non-TRILL switch  168 , because the frame&#39;s ingress RBridge nickname is the same as the virtual RBridge nickname assigned to non-TRILL switch  168 . This constraint ensures that no loop can occur from these multi-destination frames. 
     Exemplary Switch System 
       FIG. 7  illustrates an exemplary architecture of a switch that facilitates assignment of a virtual RBridge ID, in accordance with an embodiment of the present invention. In this example, a RBridge  700  includes a number of communication ports  701 , a packet processor  702 , a virtual RBridge management module  704 , a virtual RBridge configuration module  705 , a storage device  706 , and a TRILL header generation module  708 . During operation, communication ports  701  receive frames from (and transmit frames to) the end stations. Packet processor  702  extracts and processes the header information from the received frames. Note that communication ports  701  include at least one inter-switch port for communication with one or more RBridges participating in a link aggregation. Virtual RBridge management module  704  manages the communication with the partner RBridges and handles various inter-switch communication, such as MAC address information sharing and link failure notification. 
     Virtual RBridge configuration module  705  allows a user to configure and assign the identifier for the virtual RBridges. It is also responsible for communicating with the partner RBridge(s) to share each other&#39;s MAC address reachability information, which is stored in storage  706 . Furthermore, TRILL header generation module  708  generates the TRILL header for ingress frames corresponding to the virtual RBridge. 
     In summary, embodiments of the present invention provide a method and system for facilitating link aggregation or diverse connections to different networks across different switches in a routed network. In one embodiment, a virtual RBridge is formed to accommodate an aggregate link from an end station to multiple physical RBridges. The virtual RBridge is used as the ingress RBridge for ingress frames from the end station. Such configuration provides a scalable and flexible solution to link aggregation across multiple switches. 
     The methods and processes described herein can be embodied as code and/or data, which can be stored in a computer-readable nontransitory storage medium. When a computer system reads and executes the code and/or data stored on the computer-readable non-transitory storage medium, the computer system performs the methods and processes embodied as data structures and code and stored within the medium. 
     The methods and processes described herein can be executed by and/or included in hardware modules or apparatus. These modules or apparatus may include, but are not limited to, an application-specific integrated circuit (ASIC) chip, a field-programmable gate array (FPGA), a dedicated or shared processor that executes a particular software module or a piece of code at a particular time, and/or other programmable-logic devices now known or later developed. When the hardware modules or apparatus are activated, they perform the methods and processes included within them. 
     The foregoing descriptions of embodiments of the present invention have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit this disclosure. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. The scope of the present invention is defined by the appended claims.