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.
VPLS Supported Features
The JunosE implementation of VPLS provides the following features:
585Copyright © 2010, Juniper Networks, Inc.
Chapter 11: VPLS Overview
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