Juniper BGP - CONFIGURATION GUIDE V 11.1.X Configuration Manual page 619

The result of this process is the best path to the multihomed customer site through
each PE router connected to the site. One best path is selected for each router. The
process establishes whether the route advertised by each PE router is suitable for
advertising to peer routers.
Next, BGP runs the layer 2 multihoming selection process on this set of best paths
to determine the one best path to the customer site. The result of this process
establishes that the best path is suitable for establishing a pseudowire from the remote
PE router to the PE router. That PE router is accordingly selected as the designated
VE device.
The multihoming selection process is similar to the standard BGP process, but it
omits two steps:
The process does not prefer locally originated routes. Local origination is of no
value in establishing the designated VE device. The PE routers connected to the
customer 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 the remote PE routers
from selecting different designated VE devices in the event of a misconfiguration,
such as having the same site priority on different multihomed 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).
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 VPLS.
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 VPLS. 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 VPLS 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 9: VPLS Overview
BGP Multihoming for VPLS
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