VPLS Multihoming
VPLS multihoming allows you to connect a customer site to multiple PE routers to provide redundant connectivity while preventing the formation of Layer 2 loops in the service provider’s network. A VPLS site multihomed to two or more PE routers provides redundant connectivity in the event of a PE router-to-CE device link failure or the failure of a PE router.
When multihoming a VPLS site (potentially in different autonomous systems [ASs]), the PE routers connected to the same site can either be configured with the same VPLS edge (VE) device identifier or with different VE device identifiers. In the latter case, you must run STP on the CE device, and possibly on the PE routers, to construct a loop-free VPLS topology.
If the PE routers are connected to the same site and assigned the same VE device identifier, a loop-free topology is constructed using a routing mechanism such as BGP path selection. When a BGP speaker receives two equivalent network layer reachability information (NLRI) advertisements, it applies standard path selection criteria such as local preference and AS path length to determine which NLRI to choose; it selects only one.
Because a PE router picks one of the received NLRI advertisements with a particular VE device identifier, it establishes pseudowires to only one of the remote PE routers, the PE router that originated the winning advertisement. This prevents multiple paths from being created in the network between sites, preventing the formation of Layer 2 loops in the network. If the selected PE router fails, all PE routers in the network automatically switch to the backup PE router and establish pseudowires through the backup PE router.
Two VPLS NLRIs are considered equivalent from a path selection perspective if the following are the same:
- Route distinguisher
- VE device identifier
- VE block offset
If two PE routers are assigned the same VE device identifier in a given VPLS, they must also advertise the same VE block size for a given VE offset. The PE routers can be configured with the same route distinguisher or with distinct route distinguishers.
We recommend that you configure distinct route distinguishers for each multihomed router. Configuring distinct route distinguishers helps with faster convergence when the connection to a primary router goes down. It also requires the other PE routers to maintain additional state information.
 | Note: Traffic loss can occur when the old pseudowires are brought down and new ones established. |
