Juniper BGP - CONFIGURATION GUIDE V 11.1.X Configuration Manual page 620

JUNOSe 11.1.x BGP and MPLS Configuration Guide
When a VE device receives an advertisement for a layer 2 NLRI that matches its own
site ID but the site is not multihomed, then the pseudowire between it and the
transmitting PE router transitions to a site collision (SC) state and is not considered
to be up.
Multihoming Reaction to Failures in the Network
The redundant connectivity provided by a multihoming configuration protects against
several types of network failure.
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BGP Multihoming for VPLS
up pseudowires to and from the new designated VE device and tears down the
pseudowires to and from the previous designated VE device.
If this PE router does not select a best path after running the process, then the
local PE router does not consider the remote site to exist.
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.