Juniper JUNOSE, Configuration Manual

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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 VPWS 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 VPWS 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 VPWS, then a VPWS
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 status vector bit for VPWS (or the down
bit for VPLS).
The layer 2 multihoming decision process is run only in the non-core VPWS (or VPLS)
layer 2 unicast address families. This second decision process treats prefixes with
the same site ID but different RDs as unique prefixes.
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 a PE router in a VPWS 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 L2VPN multihoming decision
process enables the RR to reflect all routes that have different route distinguishers
BGP Multihoming for VPWS
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Chapter 12: VPWS Overview