Multihoming Reaction To Failures In The Network - Juniper JUNOSE 11.2.X BGP AND MPLS Configuration Manual

JunosE 11.2.x BGP and MPLS Configuration Guide

Multihoming Reaction to Failures in the Network

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If the selected best path—and therefore the designated VE—has not changed, then
nothing happens. If the best path has changed, then this PE router brings 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.
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.
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 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.
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