Juniper JUNOSE SOFTWARE FOR E SERIES 11.3.X - BGP AND MPLS CONFIGURATION GUIDE 2010-10-12 Configuration Manual page 628

JunosE 11.3.x BGP and MPLS Configuration Guide
592
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 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.
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.
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