Juniper JUNOSE SOFTWARE FOR E SERIES 11.3.X - BGP AND MPLS CONFIGURATION GUIDE 2010-10-12 Configuration Manual page 691

Copyright © 2010, Juniper Networks, Inc.
site always have a local route and therefore all advertise a locally-originated route.
These PE router also receive the advertisements from the other connected PE routers.
If the multihoming selection process preferred local origination, each of these routers
would select itself as the best path.
The process does not consider IGP cost in order to prevent improperly designated VE
device selection by the remote PE routers.
When the remote PE router establishes or refreshes a pseudowire to the local PE router,
it verifies whether the prefix is in the range required for the site ID based on the block
offset and label range advertised by the designated VE device. If the prefix is out of range,
then the pseudowire status is set to OR (out of range).
For VPWS only, the layer 2 multihoming decision process considers the status vector bit
in the layer 2 route. The status vector bit gives the combined status of the following:
The attachment circuit between the CE device and the local PE router
The MPLS connectivity between the local PE router and the remote PE router
When the status vector bit indicates that the connectivity between the CE device and
the remote CE device is down, then the PE router runs the layer 2 multihoming decision
process again, but this time excludes all advertisements that indicate this lack of
connectivity between the two site IDs (CE-IDs). The status vector bit is not considered
for VPLS multihoming.
This evaluation of connectivity must be supported by all PE routers in the VPWS network.
Because some implementations do not support the status vector bit, The E Series routers
also advertise the down bit in a VPWS network. This bit is then used by the other routers
to evaluate connectivity.
One of the following cases applies for each PE router when it completes the BGP path
selection process for a layer 2 advertisement on the VPWS.
The PE router originated one of the multihomed advertisements and selected its own
advertisement as the best path.
This PE router hosts the designated VE device. Selection as the designated VE device
triggers the creation of pseudowires to and from the other PE routers in the VPWS.
When the remote customer site is also multihomed, then the designated VE device
triggers the creation of pseudowires to and from only the designated VE device for the
remote site.
The PE router originated one of the multihomed advertisements but did not select its
own advertisement as the best path.
This PE router is one of the redundant PE routers for the multihomed site; it does not
host the designated VE device. If its status has just transitioned from being the
designated VE device, then the PE router tears down all the pseudowires that it had
to and from the other PE routers in the VPWS network.
The PE router receives the multihomed advertisements and selects a best path; it does
not originate any of these advertisements because it is not connected to the multihomed
customer site.
Chapter 14: VPWS Overview
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