ECS4660-28F_Management Guide-R03

C HAPTER 51 | IP Routing Commands
Border Gateway Protocol (BGPv4)
– 1825 –
If there is only one route reflector in a cluster, that router would still have
to process the same number of routing messages that would be required if
it were in a fully meshed network. It is therefore preferable to use more
than one route reflector in a cluster to reduce the overall number of iBGP
sessions a single reflector has to handle.
If multiple route reflectors are configured in the same cluster, they must be
fully meshed with each other. However, the route reflector clients only need
to be connected to its designated route reflector. Once all iBGP routing
sessions are established, routing advertisements must follow these rules:
◆ Announcements received by a route reflector from another reflector are
passed to its clients.
◆ Announcements received by a route reflector from a reflector client are
passed to other route reflectors in the cluster.
◆ Announcements received by a route reflector from an eBGP speaker are
passed to all route reflectors in the cluster and to its own clients.
It can now be seen that routing information learned from an iBGP speaker
can be passed to another iBGP speaker. This breaks the normal rules for a
fully meshed iBGP autonomous system, and other steps are now required
to avoid routing loops. These include the addition of the following new
attributes:
◆ Originator-ID – When a route reflector learns about a route from one of
its clients, it adds this attribute to the announcement before reflecting
it to other speakers. If a route reflector receives an announcement
about a route with an Originator-ID that matches its own router ID, it
should drop it.
◆ Cluster-List – This is a list of the clusters through which a route
announcement has passed. When a route reflector passes on an
announcement, it must prepend the local Cluster-ID to this list. The
Cluster-List thereby serves a similar function to the AS-Path attribute in
detecting routing loops.
Configuration Guidelines
1. Route reflection from this router is enabled by default. If it has been
disabled, use the bgp client-to-client reflection command to restore
route reflection via this router.
2. If more than one route reflector is used, use the bgp cluster-id
command to configure the cluster identifier.
3. Use the neighbor route-reflector-client command configure a
neighboring router as a client.