Juniper JUNOSE SOFTWARE FOR E SERIES 11.0.X - BGP AND MPLS CONFIGURATION GUIDE 2009-12-30 Configuration Manual page 612

JUNOSe 11.0.x BGP and MPLS Configuration Guide
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 the RR to reflect all routes that have different route distinguishers
to all other RR clients even though only one of these routes is used to trigger the
VPLS pseudowire to the multihomed site.
Selecting the Designated VE Device for a Multihomed Site
BGP on each PE router in the VPLS network determines the best path to the
multihomed site by comparing path attributes. The PE routers receiving the advertised
routes first run the standard BGP selection process. The routes from the connected
multihomed PE routers all share the same site ID, but can have different route
distinguishers and block offsets; the routers are advertising different prefixes. The
following sequence is applied to all routes on a per-prefix basis:
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BGP Multihoming for VPLS
Select a path with a reachable next hop.
Select the path with the highest weight.
If path weights are the same, select the path with the highest local preference
value.
Prefer locally originated routes (network routes, redistributed routes, or aggregated
routes) over received routes.
Select the route with the shortest AS-path length.
If all paths have the same AS-path length, select the path based on origin: IGP is
preferred over EGP; EGP is preferred over Incomplete.
If the origins are the same, select the path with lowest MED value.
If the paths have the same MED values, select the path learned by means of
EBGP over one learned by means of IBGP.
Select the path with the lowest IGP cost to the next hop.
Select the path with the shortest route reflection cluster list. Routes without a
cluster list are treated as having a cluster list of length 0.
Select the path received from the peer with the lowest BGP router ID.
Select the path that was learned from the neighbor with the lowest peer remote
address.