Juniper BGP - CONFIGURATION GUIDE V 11.1.X Configuration Manual page 689

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.
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
Chapter 12: VPWS Overview
BGP Multihoming for VPWS
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