Figure 45: Route Reflection: Physical and Logical Redundancy
Route Reflection and Looping
BGP prevents looping between ASs by evaluating the AS-path attribute to determine
a route’s origin. Border routers reject routes they receive from external neighbors if
the AS path indicates that the route originated within the border router’s AS.
Route reflection creates the possibility of looping within an AS. Routes that originate
within a cluster might be forwarded back to the cluster. Because this happens within
a given AS, the AS-path attribute is of no use in detecting a loop.
Route reflectors add an originator ID to each route that identifies the originator of
the route within the local AS by its router ID. If a router receives a route having the
originator ID set to its own router ID, it rejects the route.
You can also use a cluster list to prevent looping. Each cluster has an identifying
number, the cluster ID. For clusters with a single route reflector, the cluster ID is the
router ID of the route reflector; otherwise you configure the cluster ID. The cluster
list records the cluster ID of each cluster traversed by a route. When a route reflector
passes a route from a client to a nonclient router outside the cluster, the reflector
appends the cluster ID to the list. When a route reflector receives a route from a
nonclient, it rejects the route if the list contains the local cluster ID.
What about routes that a client forwards out of the cluster? No cluster ID is needed,
because clients can forward routes only to EBGP peers, that is, to peers outside the
AS. Looping between ASs is prevented by the AS-path list.
The following commands configure the route reflectors for the network topology
shown in Figure 46 on page 151. You configure the other routers, whether nonclients
or route reflector clients, as usual for IBGP and EBGP peers.
To configure router Salem as a route reflector:
host1(config)#router bgp 29
host1(config-router)#neighbor 10.2.5.5 remote-as 29
host1(config-router)#neighbor 10.2.5.5 route-reflector-client
host1(config-router)#neighbor 10.2.5.6 remote-as 29
host1(config-router)#neighbor 10.2.5.6 route-reflector-client
host1(config-router)#neighbor 10.2.5.7 remote-as 29
host1(config-router)#neighbor 10.2.5.8 remote-as 29
host1(config-router)#neighbor 10.2.25.5 remote-as 325
150 â– Managing a Large-Scale AS
JUNOSe 11.1.x BGP and MPLS Configuration Guide
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