Abstract:
A transport network employs dual homing to an access network to provide connectivity from multiple network switches. Dual homing is a mechanism by which an access network employs pair of switches in the transport network as if it were connecting to a single device. Conventional arrangements for defining multiple paths from a transport network to an access network suffer from the shortcomings of potential routing loops, increased hops to the access network, and inability or inconsistency with forwarding to different types of access networks, and may involve redirecting traffic absent faults in the access network. The dual homed network switches identify the type of access network and perform switching logic corresponding to the access network type to provide comprehensive dual-homed support to the access network independently of the type of transport employed in the access network, and employ redirection only if there is a fault in the access network.

Description:
RELATED APPLICATIONS 
     This application claims priority to and the benefit of U.S. Provisional Patent Application No. 61/381,678, filed Sep. 10, 2010, entitled “ACCESS NETWORK DUAL PATH CONNECTIVITY,” the contents of which is hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     In a computer network, network switching devices (switches) interconnect to form a path for transmitting information between an originator and a recipient. A routing mechanism, or protocol, defines switching logic that forwards the transmitted information in the form of packets between the switches as a series of “hops” along a path. At each switch, the switching logic identifies the next switch, or hop, in the path using an identifier such as a MAC address. Shortest Path Bridging (SPB) is a routing mechanism having switching logic such that each switch advertises the nodes it knows about to all the other switches, and eventually all the switches in the network have the same picture of the network and therefore can forward frames to the next hop in the shortest path. 
     SPB is defined in IEEE-802.1aq: IEEE standard for Shortest Path Bridging, and operates in conjunction with IEEE-802.1ah: IEEE standard for Provider Backbone Bridging, sometimes referred to as Mac-in-Mac encapsulation. Both SPB and SPBM forward packets on shortest path trees with minimum path cost as a first order tie-breaker, where for any pair of nodes A and B, the unicast path for A to B is the exact reverse of the path from B to A (reverse path congruency), and all multicast traffic between the two nodes follows the unicast path (multicast and unicast congruency). These are extensions to fundamental Ethernet forwarding properties in IEEE bridged networks. 
     SPB technology allows a network administrator to easily form mesh networks that distribute load more evenly across the network topology since it can mitigate bottlenecks at core links for traffic that only needs to go from one distribution switch to another. Shortest Path Bridging (SPB, SPBM) technology is being adopted in Ethernet based data networks to enable Layer-2 and Layer-3 network virtualization. These networks are expected to continue to deliver business critical services even when a variety of network faults occur (or when maintenance operations are performed on the network). 
     SUMMARY 
     An access network provides connectivity to end stations that provide computing services to users. Typically, an end station communicates with another end station at a remote access network, which may be another user, a server, storage device, or gateway to such entities or services. A transport network provides connectivity and message traffic transport between the access networks. The access network is therefore supporting a number of end users via end stations in a corporate site, local area network, or other campus or enterprise setting. Since an interruption of connection between the access network and transport network may inhibit operation of the access network, and hence all end stations connected to it, it is beneficial to provide an alternate mechanism for throughput from the transport to the access network. Configurations disclosed herein provide multiple connections via a plurality of network devices, such as a network switch, from the transport network to the access network. In an example arrangement shown below, a transport network employs a dual homing arrangement to the access network to provide connectivity from multiple network switches. 
     Dual homing is a mechanism by which an access network connects to and uses a pair of devices in the transport network as if it were connecting to a single device. The two devices (network switches) in the transport network exchange information between them which allow them to present the access network to the rest of the transport network as if the access network was connected to a single device in the transport network. Failure of the connection of one of the transport devices to the access network or even the complete failure of one of the transport devices will not cause loss of connectivity between the access network and the transport network. The access network therefore exhibits dual homed access, which is an access network that uses dual homing to connect to a pair of transport devices, and the transport devices define a dual homed edge, or a pair of partner devices, in the transport network that provide dual homing service to an access network. 
     In the examples disclosed herein, the transport network is an SPB network employing EVPN (Ethernet Virtual Private network), an Ethernet bridging service provided by a transport network which connects two or more access networks. The disclosed dual homed access configurations, routing logic and deployment are applicable to other multiple homed schemes for fault tolerance. The bridging service includes the ability to forward data packets from one access network to another using the address information on the packet. A tunnel may be employed across the transport network to connect access networks. Such a tunnel defines a communication path and mechanism used between devices in a transport network—wherein a data packet is encapsulated inside another data packet using header (and trailer) information. A tunnel is identified by the network address of the sender and receiver. 
     Configurations herein employ a virtual tunnel across the transport network from an originating switch to both of the partner switching devices serving the dual homed access network. Each of the partner switch devices serving the access network are also a dual homed EVPN edge, which is a dual homed edge providing EVPN services, and provides transport to the access network using EVPN dual homing. In the access network, a destination defines an end station, which is equipment that is connected to part of an access network that can send or receive data packets, and may be an interactive user device. In accordance with IEEE-802.1ah, edge devices employed in SPBM networks may be referred to as BEB, as a network device that conforms to the edge device specifications in IEEE-802.1ah and IEEE-802.1aq. 
     Unfortunately, conventional arrangements for defining multiple paths from a transport network to an access network suffer from the shortcomings of potential routing loops, increased hops to the access network, and inability or inconsistency with forwarding to different types of access networks. Configurations herein are based, in part, on the observation that conventional solutions involve redirecting traffic even when there are no faults in the access network. With the disclosed approach, redirection occurs only if there is a fault in the path to the access network. Conventional approaches do not take advantage of the ability of SPBM to support multiple paths for a given tunnel and do not employ Shared Virtual SPBM tunnels, and may not allow for the interception of traffic addressed to a shared virtual BMAC as the disclosed approach does. In contrast, the disclosed approach reduces traffic latency and also leads to more efficient link utilization by avoiding additional hops to the partner device when both dual homed switching devices are operational. 
     Accordingly, configurations herein substantially overcome the above described shortcomings by providing a comprehensive solution for dual homed access from an SPBM enabled network for the available types of access networks and transport networks. The disclosed approach provides access to various types of access networks, and avoids restricting access networks to which dual homing can be supported. Various IEEE-standard compliant based access networks may thus be dual homed. Typical conventional arrangements are vendor proprietary or supported only a subset of the standard. 
     Alternate configurations of the invention include a multiprogramming or multiprocessing computerized device such as a workstation, handheld or laptop computer or dedicated computing device or the like configured with software and/or circuitry (e.g., a processor as summarized above) to process any or all of the method operations disclosed herein as embodiments of the invention. Still other embodiments of the invention include software programs such as a Java Virtual Machine and/or an operating system that can operate alone or in conjunction with each other with a multiprocessing computerized device to perform the method embodiment steps and operations summarized above and disclosed in detail below. One such embodiment comprises a computer program product that has a computer-readable storage medium including computer program logic encoded thereon that, when performed in a multiprocessing computerized device having a coupling of a memory and a processor, programs the processor to perform the operations disclosed herein as embodiments of the invention to carry out data access requests. Such arrangements of the invention are typically provided as software, code and/or other data (e.g., data structures) arranged or encoded on a computer readable medium such as an optical medium (e.g., CD-ROM), floppy or hard disk or other medium such as firmware or microcode in one or more ROM, RAM or PROM chips, field programmable gate arrays (FPGAs) or as an Application Specific Integrated Circuit (ASIC). The software or firmware or other such configurations can be installed onto the computerized device (e.g., during operating system execution or during environment installation) to cause the computerized device to perform the techniques explained herein as embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages of the invention will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1  is a context diagram of a computer networking environment suitable for use with configurations herein; 
         FIG. 2  is a flowchart of network configuration and forwarding in the environment of  FIG. 1 ; 
         FIG. 3  shows an access network configuration as in  FIG. 2  employing single and dual homing; 
         FIG. 4  shows multicast transport in the configuration of  FIG. 3 ; 
         FIG. 5  shows link failure in the configuration of  FIG. 4 ; 
         FIG. 6  shows unicast redirection upon a link failure as in  FIG. 5 ; 
         FIG. 7  shows unicast redirection as in  FIG. 6  to a spanning tree (STP) access network; 
         FIG. 8  shows multicast transport to a spanning tree network as in  FIG. 6 ; and 
         FIGS. 9-15  show a flowchart of network configuration and transport logic disclosed in  FIGS. 1-8 . 
     
    
    
     DETAILED DESCRIPTION 
     Disclosed below is an example configuration of an SPBM network. In a computer networking environment, dual homing provides multiple (dual) access paths to an access network from a transport network, and thus provides alternate paths for accommodating downtime. Various examples of dual homing configurations and corresponding routing logic are shown; other arrangements will be apparent to those of skill in the art. In the disclosed arrangement, the dual homed switching device may be an Avaya Modular Ethernet Switching product such as the ERS8600, marketed commercially by Avaya Inc., of Basking Ridge, N.J. 
     A particular feature of fault tolerance in an SPBM network is being able to support dual homing. Dual homing involves connecting an access network to two different edge devices in the transport network and operating the edge devices in the transport network in a manner that ensure that the access network continues to be serviced in the event of either 1) failure of the link(s) from the access network to one of the two edge devices in the transport network, 2) complete failure of one of the two edge devices, or 3) a network maintenance operation that cause ones of the two edge devices to become partially or completely unavailable for an extended interval of time. Achieving these goals requires the disclosed capabilities on the edge devices of a transport network. Features employed to achieve these goals include 1) prevention of data packet duplication, 2) prevention of looping of traffic, which incidentally tend to be extremely debilitating to a network, 3) efficient utilization of the network bandwidth, 4) avoiding unnecessary latency for data traffic, and 5) avoiding restrictions on the type of access networks to which dual homing is supported. 
       FIG. 1  is a context diagram of a computer networking environment suitable for use with configurations herein. Referring to  FIG. 1 , a dual homed network environment  100  includes a transport network  110  and a plurality of access networks  120 - 1  . . .  120 - 2  ( 120  generally), as a network  120  at the edge is typically referred to as an access network. The transport network  110  is therefore an intermediate network connecting two or more separate networks ( 120 -N) at its edge. The access networks  120  couple to end stations  124 - 1  . . .  124 - 3  ( 124  generally) for access by user devices  124 ′ responsive to users  125 . The transport network  110  performs transmission of message traffic (packets) between switching devices  130 - 1  . . .  130 - 3  ( 130  generally). Switching devices  130  connected to an access network  120  are edge devices  130 ′, and are designated as an origin or destination across the transport network  110  depending on message traffic direction. 
     In the example environment  100  shown, the transport network  110  is an SPB/SPBM network according to IEEE-802.1aq, and the access networks  120  may be a Split Multi-Link Trunking (SMLT) network, a spanning tree (STP) network, or other type of access network. Configurations herein identify the type of access network  120  and perform switching logic corresponding to an access network type to provide comprehensive dual-homed support to the access network  120  independently of the type of transport employed in the access network  120 . 
     A Split Multi-Link Trunking (SMLT) access network  120  allows multiple physical network links between two network switches and another device (which could be another switch or a network device such as a server) to be treated as a single logical link and load balance the traffic across the available links. For each packet that needs to be transmitted, one of the physical links is selected based on a load-balancing algorithm (usually involving a hash function operating on the source and destination Media Access Control (MAC) address. A spanning tree (STP/MSTP) network operates according to protocols defined in IEEE-802.1Q for controlling layer-2 networks that use Ethernet bridging. Other types of access networks  120  may be dual homed (i.e. be served by multiple edge devices  130  for redundancy and fault tolerance). 
     In the example arrangement, employing an SPBM network as the transport network  110 , the edge devices  130 ′ are BEBs, which are IEEE-802.1ah/aq compliant edge devices. Edge devices  130 - 1  and  130 - 2  provide dual homing to access network  120 - 1  via links  122 - 1  and  122 - 2 . In the transport network, a static route  112 - 1  connects switching device  130 - 3  to switching device  130 - 1 , and a static route  112 - 2  connects switching device  130 - 3  to switching device  130 - 2 . In accordance with dual homing, a virtual link  114  connects switching device  130 - 3  to both  130 - 1  and  130 - 2 . The virtual link  114  employs a virtual address that defines separate paths to the access network  120 - 1  through switching devices  130 - 1  and  130 - 2 , thus allowing a single routing entry to designate multiple physical paths (through devices  130 - 1  and  130 - 2 ). The virtual address is an address that is used by more than one device in the network as a sender address and is used by more than one device in the network to receive data frames bearing this address as the receiver address 
     Since the same dual homed destination may be traversed by different physical paths, each of the dual-homed pair  130 - 1 ,  130 - 2  are designated as “primary” and “secondary” edge devices, each being a partner of the other. Similarly, routing logic  134  in the origin switch device  130 - 3  designates a path identifier of either primary or secondary, which denotes the physical path to be followed absent any failures triggering intervention of dual homing. Since the path identifier is independent of the virtual route  114 , an individual routing entry is followed by switching device  130 - 3 . If individual path designation is preferred, the routing logic  134  may invoke, i.e. it need not consider which path (through  130 - 1  or  130 - 2 ) be followed. 
     In the example arrangement, using an SPBM network as a transport network, a. dual homed SMLT edge device refers to the switching device  130  on the dual homed SPBM edge that uses SMLT redundancy to connect to access networks  120  on the edge of the SPBM network  110 . A dual homed STP/MSTP edge refers to switching device that uses STP/MSTP redundancy to connect to access networks  120  on the edge of the SPBM network  110 . In each of these cases, the switching devices  130 - 1 ,  130 - 2  providing the dual homed access are defines as a primary BEB and s secondary BEB, meaning that the primary BEB in a dual homed SPBM edge that is configured by the operator to be “primary” and the secondary BEB is configured by the operator to be “secondary.” A partner BEB is the other of the primary and secondary BEB in a “dual homed SPBM edge.” 
       FIG. 2  is a flowchart of network configuration and forwarding in the environment of  FIG. 1 . Referring to  FIGS. 1 and 2 , the method of connecting a network switching device between a transport network  110  and an access network  120 , the access network  120  for providing connectivity between the transport network  110  and end stations  124  coupled to the access network  120  includes, at step  200 , identifying a transport network  110 , such that the transport network  110  is configured to interconnect a plurality of access networks  120  for internetwork traffic between end stations  124  connected to the access networks  120 , and identifying a switching device  130 - 1 , in which the switching device  130 - 1  is coupled to the access network  120 - 1 . A configuration defines a partner device  130 - 2 , the partner device  130 - 2  coupled to the same access network  120 - 1  as the switching device  130 - 1 , as depicted at step  201 . The switching devices  130  configure a dynamic route  114  between the switching devices  130 - 1 ,  130 - 2  and an origin device  130 - 2  in the transport network  110 , such that the origin device  130 - 3  is also coupled to the partner device  130 - 2 , in which the partner device  130 - 2  shares the dynamic route  114  with the switching device  130 - 1 , as shown at step  202 . The transport network  110  maintains connectivity, via the dynamic route  114 , from the origin device  130 - 3  to the access network  120 - 1  via at least one of the switching device  130 - 1  and the partner device  130 - 2  independently of failure of either the switching device  130 - 1  or partner device  130 - 2 , or failure of a link to the switching device or partner device. 
       FIG. 3  shows an access network configuration as in  FIG. 2  employing single and dual homing. Referring to  FIGS. 3 and 4 , each dual homed switching device  130 - 1 ,  130 - 2  may also connect to single homed access networks  120 - 3 ,  120 - 4 , serving end stations  124 - 3  and  124 - 4 , respectively. Routing logic  134  as defined herein likewise servers to provide transport to single homed access networks  121  concurrently to dual homed access to the dual homed access network  120 - 1 . 
       FIG. 4  shows multicast transport in the configuration of  FIG. 3 . Referring to  FIG. 4 , a multicast transmission  140  emanates from end station  124 - 2 . In the example arrangement, each partner pair  130 - 1  and  130 - 2  of a dual homed configuration need direct each multicast transmission  140  only once to each dual homed recipient  124 - 1 . The routing logic  134  establishes primary  142  and secondary  144  paths to each of the dual homed switching device pair  130 - 1 ,  130 - 2 . Each switching device  130  is configured as the primary  130 - 1  or secondary  130 - 2  for the dual homed configuration for a particular access network  120 - 1 . The end station  124 - 2  sends the multicast transmission  140 , and the routing logic determines, from load and flow data, the primary  142  or secondary  144  path. The dual homed arrangement provides a dual homed pair  135  of links  137 - 1 ,  137 - 2  to from the primary and secondary switching devices  130 - 1 ,  130 - 2 , respectively, on primary  142  and secondary  144  paths The routing logic  134  writes the determined path (primary or secondary) into a path identifier  148  in the message  140  in addition to the destination address of an end station  124  of the destination access network  120 - 1 . Upon receipt, each of the switching devices  130 - 1 ,  130 - 2  examines the path identifier  148 , and the primary switching device  130 - 1  forwards traffic  140  on the primary path  142 , while the secondary switching device  130 - 2  forwards traffic  140  on the secondary path  144 . Multicast traffic  140  to single homed access networks  120 - 3 ,  120 - 4  continues unimpeded via the corresponding single homed access network  120 - 3 ,  120 - 4 . 
       FIG. 5  shows link failure in the configuration of  FIG. 4 . Referring to  FIGS. 4 and 5 , in the event of failure  145  of one of the dual homed links ( 137 - 1  in the example shown), the link  137  to the partner ( 137 - 2  in the example) is employed for primary and secondary message traffic  140  to the access network  120 - 1 . A failure notification  147  is received by the functional or unaffected partner as an indication that the partner  137 - 2  should take over for the primary node and forward all traffic on both the primary and secondary paths  142 ,  144  to the access network  120 . The switching device  130 - 1  experiencing the failure  145  generates the failure notification  147  from both an intra-device protocol between dual homed partners  130 - 1 ,  130 - 2  and from MAC learning of reachable addresses which indicate unavailable routes, both discussed further below. 
       FIG. 6  shows unicast redirection upon a link failure as in  FIG. 5 . Referring to  FIGS. 4-6 , In the multicast example of  FIG. 5 , both dual homed partners  130 - 1 ,  130 - 2  receive the multicast message on the primary  142  and secondary  144  paths. In contrast, in a unicast transmission, the routing logic  134  selects only one of either the primary  142  or secondary  144  paths for transport to the destination end station  124 - 1 . If the selected path  162  is via the switching device  130 - 1  experiencing failure, the partner switching device  130 - 2  has not received the transmission  160 . Accordingly, following the failure message  147 , the failed partner  130 - 1  forwards primary path  142  traffic to the partner switching device  130 - 1  for completion on the secondary path  144 , shown as selected path  162 . 
       FIG. 7  shows unicast redirection as in  FIG. 6  to a spanning tree (STP) access network. Referring to  FIGS. 4, and 7 , an alternate access network  120  configuration which may be configured for dual homed access is a spanning tree, or STP arrangement. In a dual homed spanning tree configuration, local switching devices  172 - 1 . 172 - 4  ( 172  generally) from a serial interconnection over which message traffic  170  passes to a switching device  172 - 3  serving the destination end station  124 - 13 . In a dual homed spanning tree arrangement, an STP block  175  is implemented along the interconnection to prevent loops back to the dual homed pair  130 - 1  . . .  130 - 2 . Each dual homed switching device generally maintains switching connectivity for a respective side of the block  175 . If the routing logic  134  directs message traffic  170  to the partner switching device  130 - 1  serving the other side of the block, a BEB interconnection  177  forwards the message traffic  170  to the partner  130 - 2  for delivery to the end station  124 - 13 . 
       FIG. 8  shows multicast transport to a spanning tree network as in  FIG. 6 . Referring to  FIGS. 4, 6, and 8 , multicast message traffic  180  traverses both partner switching devices  130 - 1 ,  130 - 2 . Switching logic  134  employs the primary path  142  for transport to each partner  130 - 1 ,  130 - 2  for forwarding to the respective STP portion  177 - 1 ,  177 - 2  on respective sides of the block  175 . 
       FIGS. 9-15  show a flowchart of network configuration and transport logic disclosed in  FIGS. 1-8 . The transport scenarios depicted in  FIGS. 3-8  show a comprehensive routing and forwarding approach by examples of various types of dual homed access networks.  FIGS. 9-15  show a sequence of conditions and operations that implement the various examples of  FIGS. 3-8 . The routing logic  134 , incorporated in each switching device  130  serving a single or dual homed switching device  130  implements these transport scenarios, as well as others which will be apparent from the disclosed scenarios. 
     Referring to  FIGS. 9-15 , at step  300 , configuration logic identifies the access network  120  as single or dual homed. Network configuration includes designating the switching devices  130  serving a dual homed access network  120 - 1  identifies one of the switching devices as primary and the other as secondary, both being dual homed partners of the other. Dual homed networks, which in the examples shown may be SMLT networks or STP (spanning tree) networks, provide multiple switching devices  130  at the transport network  110  edge (edge devices) to the access network  120 . If the access network  120  is dual homed, this includes configuring a static route  112 - 1  to the access network  120 - 1  between the switching device  130 - 2  and the origin device  130 - 3 , as depicted at step  301  and configuring a second static route  112 - 1  to the access network between the partner device  130 - 2  and the origin device  130 - 3 , as shown at step  302 . A third dynamic  114 , or virtual route is configured at step  303  that avoids redundant routing entries to the access network  120 - 1  through both the switching device  130 - 1  and the partner device  130 - 2  by maintaining separate entries for the static route, the second static route and the dynamic route, as shown at step  304 . The dynamic route  114  defines a tunnel to each of the switching device  130 - 1  and the partner device  130 - 2 , in which each respective tunnel defines a separate physical path  112 - 1 ,  112 - 2  to the access network  120 , and in which the dynamic route  114  is identifiable by a single routing entry for avoiding redundant routing entries to a particular switching device  130 , as depicted at step  305 . 
     The first static route  112 - 1  is a defined physical path from the origin device  130 - 3  to the access network  120 , via the switching device  130 - 1  and the second static route  112 - 2  defines a different physical path from the origin device  124 - 2  to the access network  120 - 1  via the partner device  130 - 2 , as shown at step  306  and also in  FIG. 1 . 
     In the case of a single homed access network  120 - 3 ,  120 - 4 , from step  300 , the routing logic  134  determines that the access network ( 120 - 3  or  120 - 4 ) is served by one of either the primary edge device  130 - 1  or the secondary edge device  130 - 2 , as depicted at step  307 , and receives the transmission at the access network  120 - 1  via only the determined edge device  130 - 1 ,  130 - 2  as shown at step  308 , also shown in  FIG. 3 . Single homed networks  120 - 3 ,  120 - 4  are deemed to have accepted the risk of reduced fault tolerance present with a single homed configuration, otherwise they would have opted for the dual homed arrangement. 
     A check is performed, at step  309 , to identify the transmission as unicast or multicast. At step  310 , In the case of a multicast transmission, control passes to step  311  where the transmission is identified as a multicast transmission  140  having a plurality of recipients. The routing logic  134  at the origin  130 - 3  determines that the transmission is addressed to recipients in the access network for at least the dual homed access network  120 , as shown at step  312 . The routing logic  134  or a configuration parameter  148  computes a designation of a primary edge device for the switching device  130 - 1 , as shown at step  313 , and computes a designation of a secondary edge device for the partner device  130 - 2 , as shown at step  314 . The primary and secondary designations are generally static once applied to a switching device  130  for the dual homed configuration. The switching device  130 - 3  at the origin designates the multicast transmission  140  as either primary or secondary  148  to correspond to the primary  130 - 1  and secondary edge devices  130 - 2 , respectively, as shown at step  315 , and routes, by the corresponding edge device  130 - 1 ,  130 - 2 , the transmission to the access network  120 - 1 . Thus, for a given multicast transmission  140 , the primary/secondary designation  148  ensures that the routing logic  134  sends one and only one message instantiation to the access network  120 - 1 , as the partner device will ignore the other (primary/secondary) designation. 
     The above approach may be illustrated by the following rules, which are used to prevent packet duplication and looping of multicast traffic. 1. Multicast traffic  140  in the SPBM core  110  using the primary BVLAN  142  is forwarded to a dual homed SMLT access network  120 - 1  by the primary BEB  130 - 1 . 2. Multicast traffic in the SPBM core  110  using the secondary BVLAN  144  is forwarded to dual homed SMLT access network  120 - 1  by the secondary BEB  130 - 2 . Further, a single homed access network  120 - 3 ,  120 - 3  can receive both primary  142  and secondary BVLAN  144  multicast traffic  140  from whichever BEB  130 - 1 ,  130 - 2  that it connects to, also shown in  FIG. 4 . 
     A check is performed, at step  317  for a spanning tree access network, and if, at step  318 , the access network  120 - 1  is a spanning tree (STP) network, then control passes to step  319 , where the routing logic  134  determines that the access network  120 - 1  is of a network type of spanning tree, such that the spanning tree forms a serial path  172 - 1  . . .  172 - 4  through the access network between the switching device  130 - 1  and the partner device  130 - 2 , also shown in  FIG. 8 . The routing logic  134  defines a subset of end stations  124  in the access network for service by the switching devices  130 - 1 ,  130 - 2 , as depicted at step  320 . For each switching device  130 , the routing logic defining a subset  177 - 1 ,  177 - 2  of end stations  124  in the access network for service by each of the partner devices  130 - 1 ,  130 - 2 , as shown at step  321  and defines a blocking  175  in the access network for prevention loops between each of the subsets  177 , such that the blocking  175  partitions each of the defined subsets  177  against receipt of traffic by the switching  130 - 1  or partner  130 - 2  device not defined to service that subset  177 , as disclosed at step  322 . 
     The corresponding rule in the routing logic therefore suggests that a dual homed STP/MSTP based access network  172 - 1  . . .  172 - 4  can receive both primary and secondary BVLAN  142  multicast traffic  180  from both the BEBs  130 - 1 ,  130 - 2  that it connects to, and relies on the port blocking  175  in the access network  172  to prevent network loops. 
     A feature of the dual homed approach is fault tolerance in the event of failure of one of the partner devices  130 - 1 ,  130 - 2 . At step  323 , in the event of failure, control passes to step  324 . The routing logic  134  detects that a link  137 - 1  from the corresponding edge device  130 - 1  to the access network  120 - 1  is unavailable, as disclosed at step  325 . The affected switching device  130  forwards a notification  147  of link failure to one of the partner device  130 - 2  or the switching device  130 - 1  unaffected by the link failure  145 , as shown at step  326 . The routing logic  134  directs routing of the transmission to the access network  120 - 1  by the unaffected edge device  130  independently of the computed designation  148  of primary/secondary, as depicted at step  327 . Thus, in the case of failure of one of the dual-homed switching devices  130 - 1 ,  130 - 2 , the other partner forwards all multicast traffic for both the primary  142  and secondary  144  paths. The routing logic  134  therefore implements the rule that a given dual homed SMLT access network  120 - 1  receives primary BVLAN  142  multicast traffic  140  from the secondary BEB  130 - 2  if and only if the link(s)  137 - 1  that connect the access network  120 - 1  to the primary BEB  130 - 1  are all down. Further, the complementary rule provides that a given dual homed SMLT access network  120 - 1  receives secondary BVLAN  144  multicast traffic  140  from the primary BEB  130 - 1  if and only if—the link(s)  137 - 2  that connect the access network  120 - 1  to the secondary BEB  130 - 2  are all down, also shown in  FIGS. 4 and 5 . 
     Link  137  failure detection further includes, at step  328 , determining that one of the switching device  130 - 1  or the partner device  130 - 2  is down. The routing logic  134  computes unavailability of a partner link  149  between the switching device  130 - 1  and the partner device  130 - 2 , as depicted at step  329 , in which the partner link  149  is for communication between switching devices  130  serving the same access network  120 - 1  for such messages as the unavailability notification  147 . The routing logic also indexes a link state database indicative of switching device  130  connectivity to determine unreachability, as depicted at step  330 . The link state database may be provided by routing topology information commonly propagated among switching devices. 
     The routing logic  134  implements a rule for determination of a partner BEB down. The partner link  177  for communicating between two network deices  130 - 1 ,  130 - 2  that are peers in a “Dual homed edge” is maintained by a specialized protocol. The link  137  propagation is addressed by ISIS, A networking protocol defined by an ISO standard, and is a typical protocol for implementations of IEEE-802.1aq as disclosed herein. Such links employ an ISIS System-Id, which is a unique identifier used by each network device within a network device that uses ISIS. Unavailability is based on whether the following criteria is met—the primary  130 - 1  (and/or secondary  130 - 2 ) BEB determines that it has lost all connectivity with its partner by 1. The partner link  149  between the two BEBs  130  is down and 2. ISIS has determined that it has lost reachability to the partner.  130  This is possible because the Partner ISIS System-Id is configured on the local device and it can monitor reachability to that System-Id. Losing all connectivity with the partner means that 1. Either the SPBM core  110  has segmented with the primary  130 - 1  and secondary BEBs  130 - 2  ending up in different segments or 2. The partner is down. 
     A check is performed, at step  331 , to identify an SMLT access network  120 - 1 . If so, then the routing logic  134  determines destination addresses for end stations  124  in the access network  120 - 1  affected by the unavailable link  137 - 1 , as disclosed at step  332 , and redirects destination address information to reference the switching device  130 - 2  or partner device still serving the access network  120 - 1 , as shown at step  333 . 
     The routing rules for redirecting unicast traffic using a SPBM tunnel in the partner link  177  are as follows: If a dual homed SMLT access network loses its link(s) to either the primary  130 - 1  or the secondary BEB  130 - 2 , the EVPN MAC addresses that have been learnt from that access network are reprogrammed to point to the SPBM tunnel  177  between the primary  130 - 1  and the secondary BEB  130 - 2 . This will allow traffic arriving from the SPBM core  110  at the BEB  130 - 1  to be redirected to the partner  130 - 2  in event of the failure of link(s)  137 - 1  between the BEB  130 - 1  and a dual homed SMLT access network, also shown in  FIG. 6 . 
     When the unavailability condition of the link  137  no longer exists, the unaffected switching device  130 - 2  detects reavailability of the detected unavailable link  137 - 1 , as shown at step  334 , and releasing redirectional control of both the primary and secondary dynamic links  142 ,  144 , as depicted at step  335 . The routing logic  134  informs the edge device  130 - 1  affected by the unavailable link  137 - 1  to resume forwarding traffic on the dynamic link  142  to the access network  120 - 1 , as shown at step  336 . The routing logic  134  for determination of Partner BEB UP includes the following rules: a primary or secondary BEB determines that its partner is UP if at least one of the following conditions is met. 1. The control channel link  177  (IST) between the two BEBs is up OR 2. ISIS has determined that it has reachability to the partner. This is possible because the Partner ISIS System-Id is configured on the local device and it can monitor reachability to that System-Id. 
     Following determination of partner switching device  130  availability, the routing logic  134  implements the following rule releasing the Shared Virtual BMAC for the partner BVLAN  142 ,  144  so that primary  142  and secondary  144  paths will again be intercepted by their respective switching devices  130 - 1 ,  130 - 2 . A primary BEB release its control of the Shared Virtual BMAC on the secondary BVLAN  144  after it determines that the secondary BEB  130 - 2  is UP. A secondary BEB  130 - 2  releases its control of the Shared Virtual BMAC on the primary BVLAN  142  after it determines that the primary BEB  130 - 1  is UP. 
     In the case of dual homed spanning tree unicast traffic, at step  337 , the switching device  130 - 1  receives message traffic  170  on the dynamic link  114 , as depicted at step  338 , also shown in  FIG. 7 . The routing logic  134  determines that the destination of the message traffic  170  is to an access network  172  serviced by the other edge device  130 - 2  of the switching device  130 - 1  or partner device  130 - 2 , as shown at step  339 . Each device  130  exchanges destination address information between the switching device  130 - 1  and the partner device  130 - 2  using link  177 , as disclosed at step  340 , and delivers the message traffic  170  using the exchanged destination address information, as depicted at step  341 . Routing rules include, for dual homed access networks that use STP or MSTP (instead of SMLT) for redundancy, topology changes in the access network may cause MAC addresses to move from the primary BEB side of a STP/MSTP blocked link  175  to the secondary side or vice-verse. Since all data traffic from a dual homed access network (SMLT based or STP/MSTP based) uses the Shared Virtual BMAC based tunnels  142 ,  144 —the knowledge of the change in access topology is localized. Since the two BEBs  130 - 1 ,  130 - 2  exchange their EVPN MAC tables using the control protocol, they are able to redirect the unicast traffic  170  from the core  110  to the correct BEB  130 - 1 ,  130 - 2  even if it arrives at the BEB and finds that CMAC is learnt by the partner BEB  130 -N on the other side of a blocked STP/MSTP link  175  in the access network. 
     Unicast spanning tree forwarding further includes routing message traffic  170  using an identifier designating the dynamic route  142 , as depicted at step  342 , and delivering the message traffic  170  redundantly to the switching device  130 - 1  and the partner device  130 - 2  via the virtual link  114 , as shown at step  343 . Each switching device  130 - 1 ,  130 - 2  identifies the message traffic as destined for the access network serviced by both the switching device  130 - 2  and partner device  130 - 2 , as disclosed at step  344 , and forwards the message traffic to the access network  120 - 2  for delivery to the recipient at a corresponding end station  124 , as disclosed at step  345 . As indicated above, forwarding on the dynamic path  142  includes appending the path identifier  148  to the massage header, the path identifier indicative of either the primary or secondary edge devices, such that the virtual tunnel  114  is agnostic to the path identifier  148 . 
     The routing logic  134  implements a rule directed to intercepting unicast traffic Addressed to Virtual BMAC  142 : The primary BEB  130 - 1  can intercept secondary BVLAN traffic destined for the Virtual BMAC  142  and forward it towards the dual homed access network  120 - 1 . Similarly, the secondary BEB  130 - 2  can intercept primary BVLAN traffic destined for the Virtual BMAC  144  and forward it towards the dual homed access network  120 - 1 . This works because in the case of layer-2 bridged data traffic because the Virtual BMAC is only used for traffic that has to be sent towards dual homed access network(s) and by definition both the BEBs are connected to the dual homed access network(s). 
     This approach works for unicast traffic in the case of traffic routed into an ISID because it is only done if the underlying VLAN is configured for active-active Router Redundancy (a.k.a. RSMLT) on the primary  130 - 1  and secondary BEB  130 - 2  pair. In this case both the BEBs are capable of routing the traffic addressed to the other router. This interception of traffic using the Virtual BMAC leads to lower latencies and more efficient link utilization in the SPBM core—because the traffic travels on one less core link before it is sent to the access network  120 - 1 . 
     At step  348 , In the case of unicast failure, the routing logic  134  attempts to forward the message traffic  160  to the access network  120 - 1  for delivery to the recipient. The available switching device  130 - 1 ,  130 - 2  detects that a link  137 - 1  from the servicing edge device  130  to the access network  120 - 1  is unavailable, as disclosed at step  349 . The partner link  177  forwards the transmission to one of the partner device  130 - 2  or the switching device  130 - 1  that is unaffected by the unavailable link  137 - 1 , as shown at step  350 , and delivers the forwarded transmission to the access network  120 - 1 , as depicted at step  351 . The link failure notification and reavailability notification is similar to that of the multicast, and thus control reverts to step  324 . 
     The corresponding routing rule may be stated as follows: after a primary BEB  130 - 1  determines that it has lost all connectivity to its partner  130 - 2 , it takes over the Shared Virtual BMAC path  144  that uses the secondary BVLAN. Now the primary BEB owns both paths of the shared Virtual BMAC  142 . The primary BEB  130 - 1  also starts forwarding both primary and secondary BVLAN multicast traffic to all of its dual homed access networks  120 . 
     Those skilled in the art should readily appreciate that the programs and methods for connecting a network switching device between a transport network and an access network as defined herein are deliverable to a user processing and rendering device in many forms, including but not limited to a) information permanently stored on non-writeable storage media such as ROM devices, b) information alterably stored on writeable storage media such as floppy disks, magnetic tapes, CDs, RAM devices, and other magnetic and optical media, or c) information conveyed to a computer through communication media, as in an electronic network such as the Internet or telephone modem lines. The operations and methods may be implemented in a software executable object or as a set of encoded instructions for execution by a processor responsive to the instructions. Alternatively, the operations and methods disclosed herein may be embodied in whole or in part using hardware components, such as Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs), state machines, controllers or other hardware components or devices, or a combination of hardware, software, and firmware components. 
     While the system and method of connecting a network switching device between a transport network and an access network has been particularly shown and described with references to embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.