Multihoming Reaction to Failures in the Network

The redundant connectivity provided by a multihoming configuration protects against several types of network failure.

• CE-Link failure between the CE device and the PE router—BGP on the PE router is notified when the circuit goes down. BGP then modifies the circuit status vector bit in the MP_REACH_NLRI to indicate that the circuit is down.

  If all VPLS local attachment circuits are down, then BGP modifies the down bit in the VPLS advertisement Layer2-Extended-Community to state that the site is down. When the bit is modified, BGP advertises the route to all remote PE routers to inform them that the circuit (and site) is down. The remote PE routers each run the best path selection process again and adjust the VPLS pseudowires as needed.

• Failure of MPLS reachability to the remote PE router—BGP on the PE router is notified that MPLS connectivity to the BGP next hop is gone. BGP then modifies the circuit status vector bit in the MP_REACH_NLRI to indicate that the LSP is down. When the bit is modified, BGP advertises the route to all remote PE routers to inform them that connectivity is down from the local site to the remote site.

  The down bit is set if no remote PE router is reachable by MPLS. This enables the remote PE routers to consider the other multihomed PE router as the designated VE device for the multihomed-site.

  The remote PE routers each run the best path selection process again and adjust the VPLS pseudowires as needed.

• PE router failure—When either the PE router or its BGP process fails, peer PE routers detect expiration of the holdtimer and bring down their peering sessions, and remove layer 2 advertisements from the PE router. Alternatively, the PE routers can detect unreachability to the BGP next hop that represents the failed PE router. In this case the peer routers mark the layer 2 routes advertised by PE router as unreachable. The peer PE routers each run the best path selection process again and adjust the VPLS pseudowires as needed.

  A similar response results when you adjust the multihoming priority of the PE routers connected to the multihomed site, effectively performing and administrative failover to another PE router. BGP sends a layer 2 update with the new local preference attribute to all peer PE routers. The peer PE routers each run the best path selection process again and adjust the VPLS pseudowires as needed.

To modify their pseudowires, the peer routers correct their MPLS forwarding tables and set up new entries in their pseudowire tables.

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