Juniper JUNOSE SOFTWARE FOR E SERIES 11.0.X - BGP AND MPLS CONFIGURATION GUIDE 2009-12-30 Configuration Manual page 684

JUNOSe 11.0.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 VPWS
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.
For a VPWS network, this PE router then sets the circuit status vector bit in the
MP_REACH_NLRI to indicate that the remote site is unreachable. BGP advertises
the route to inform remote PE routers fo this new condition.
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 VPWS local attachment circuits are down, then BGP modifies the down bit
in the VPWS 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 VPWS 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.
Unlike for VPLS, the down bit is not set when no remote PE router is reachable
by MPLS.
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 VPWS 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 VPWS pseudowires
as needed.