Juniper JUNOSE 11.2.X BGP AND MPLS, Configuration Manual

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You specify on each PE router connected to the CE device in the VPLS that the site is
multihomed and you configure a priority. The priority serves as a site preference and is
propagated by BGP in the local-preference attribute.
You configure the same site ID (sometimes referred to as a VE ID) on these connected
PE routers. Each of these routers then advertises reachability for the multihomed site;
the VPLS NLRI contains the site ID. The site ID shared by the connected PE routers should
be different than the site IDs configured on the remote PE routers in the VPLS network;
if the site ID is not different, then the pseudowire will be in a site collision state. The
remote routers then use the site ID to identify where to forward traffic destined for the
customer site.
Although the site ID is the same for all connected PE routers, the block offset, label range,
and route distinguisher can be different for each PE router. The BGP path selection process
uses the block offset and label range only to determine whether a layer 2 advertisement
is relevant to the multihomed customer site. A route distinguisher is helpful to uniquely
identify a particular PE router when you are troubleshooting a network.
The PE routers run the BGP path selection process on the locally originated and received
layer 2 route advertisements to establish that the routes are suitable for advertisements
to other peers, such as route reflectors. For this selection process, the routes advertise
different prefixes, distinguished by the site ID, block offset, and route distinguisher.
The remote PE routers then run a modified selection process on these selected routes
for L2VPN multihoming. Because all the prefixes advertised by multihomed local PE
routers share the same site ID, the set of routes advertised for a multihomed site effectively
consists of multiple routes to a single prefix, distinguished by the site ID alone. Therefore
the result of the second selection process is the single best path to the multihomed site.
The PE router that originates this advertisement then becomes the designated VE device
for the multihomed customer site. When the designated VE device is determined for both
the local and remote customer sites for the VPLS, then a VPLS pseudowire is created
between the designated VE devices.
The BGP best path selection process is run only in the core VPN address family. This first
selection process does not consider the down bit for VPLS (or the status vector bit for
VPWS).
The layer 2 multihoming decision process is run only in the non-core VPLS (or VPWS)
layer 2 unicast address families. This second decision process treats prefixes with the
same site ID but different RDs as a single prefix.
When the PE router receives a layer 2 BGP advertisement that has the down bit set,
inbound policy sets the local preference attribute to zero. The selection process can then
choose an existing route from an alternate PE router, if available.
When a PE router in a VPLS domain is also a BGP route reflector (RR), the path selection
process to determine the VE device for the multihomed site has no effect on the path
selection process performed by this PE router for the purpose of reflecting layer 2 routes.
Layer 2 prefixes that have different route distinguishers are considered to have different
NLRI for route reflection. This result of the standard BGP path selection process enables
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JunosE 11.2.x BGP and MPLS Configuration Guide