Designated VE Device Selection for a Multihomed Site
BGP on each PE router in the VPWS 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:
1. Select a path with a reachable next hop.
2. Select the path with the highest weight.
3. If path weights are the same, select the path with the highest local preference value.
4. Prefer locally originated routes (network routes, redistributed routes, or aggregated
routes) over received routes.
5. Select the route with the shortest AS-path length.
6. 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.
7. If the origins are the same, select the path with lowest MED value.
8. If the paths have the same MED values, select the path learned by means of EBGP
over one learned by means of IBGP.
9. Select the path with the lowest IGP cost to the next hop.
10. 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.
11. Select the path received from the peer with the lowest BGP router ID.
12. Select the path that was learned from the neighbor with the lowest peer remote
address.
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 the 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.
Copyright © 2010, Juniper Networks, Inc.646
JunosE 11.2.x BGP and MPLS Configuration Guide
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