BGP Route Reflection and Peer Group

This example illustrates a practical use of route reflectors and peer groups. RT3 and RT4 form a route reflector cluster, where RT4 is the route reflector; RT5, RT6, and RT7 form another cluster where RT7 is the route reflector. RT4 and RT7 are part of a peer group called REFLECTORS; if there are other route reflectors, all should establish full-mesh IBGP peering. RT7 groups all its route reflector clients in a peer group called RR-CLIENTS, where common policies can be applied.

From RT5 and RT6’s perspective, the IBGP peering session with RT7 is a normal IBGP session – the client need not know that it is a client.

Below shows the BGP table on RT3 for all the routes in the network. Take notes upon the originator and cluster list for the different routes.

RT3#sh ip bgp 192.168.1.0
BGP routing table entry for 192.168.1.0/24, version 2
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Advertised to non peer-group peers:
  34.34.34.4
  1
    13.13.13.1 from 13.13.13.1 (1.1.1.1)
      Origin IGP, metric 0, localpref 100, valid, external, best
RT3#
RT3#sh ip bgp 192.168.2.0
BGP routing table entry for 192.168.2.0/24, version 8
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Not advertised to any peer
  2
    67.67.67.6 (metric 30) from 34.34.34.4 (4.4.4.4)
      Origin IGP, metric 0, localpref 100, valid, internal, best
      Originator: 6.6.6.6, Cluster list: 4.4.4.4, 7.7.7.7
RT3#
RT3#sh ip bgp 172.16.3.0
BGP routing table entry for 172.16.3.0/24, version 3
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Advertised to non peer-group peers:
  13.13.13.1 34.34.34.4
  Local
    0.0.0.0 from 0.0.0.0 (3.3.3.3)
      Origin IGP, metric 0, localpref 100, weight 32768, valid, sourced, local, best
RT3#
RT3#sh ip bgp 172.16.4.0
BGP routing table entry for 172.16.4.0/24, version 4
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Advertised to non peer-group peers:
  13.13.13.1
  Local
    34.34.34.4 from 34.34.34.4 (4.4.4.4)
      Origin IGP, metric 0, localpref 100, valid, internal, best
RT3#
RT3#sh ip bgp 172.16.5.0
BGP routing table entry for 172.16.5.0/24, version 7
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Advertised to non peer-group peers:
  13.13.13.1
  Local
    57.57.57.5 (metric 30) from 34.34.34.4 (4.4.4.4)
      Origin IGP, metric 0, localpref 100, valid, internal, best
      Originator: 5.5.5.5, Cluster list: 4.4.4.4, 7.7.7.7
RT3#
RT3#sh ip bgp 172.16.6.0
BGP routing table entry for 172.16.6.0/24, version 6
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Advertised to non peer-group peers:
  13.13.13.1
  Local
    67.67.67.6 (metric 30) from 34.34.34.4 (4.4.4.4)
      Origin IGP, metric 0, localpref 100, valid, internal, best
      Originator: 6.6.6.6, Cluster list: 4.4.4.4, 7.7.7.7
RT3#
RT3#sh ip bgp 172.16.7.0
BGP routing table entry for 172.16.7.0/24, version 5
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Advertised to non peer-group peers:
  13.13.13.1
  Local
    47.47.47.7 (metric 20) from 34.34.34.4 (4.4.4.4)
      Origin IGP, metric 0, localpref 100, valid, internal, best
      Originator: 7.7.7.7, Cluster list: 4.4.4.4
RT3#

A route may carry a cluster list that contains the Router IDs of all the route reflectors that it passed through.

When multiple route reflectors are configured in the same cluster, all the route reflectors must be configured with a common Cluster ID to detect routing loops that might occur between clusters. The Cluster ID is a number identifying a route reflector cluster. Note that it is a good practice to configure a Cluster ID even if using a single route reflector.
Ex: If RT5 were to be configured as a route reflector, the bgp cluster-id {number} BGP router subcommand is required on RT5 and RT7. Below shows the syntax of the command.

Router(config-router)#bgp cluster-id ?
  <1-4294967295>  Route-Reflector Cluster-id as 32 bit quantity
  A.B.C.D         Route-Reflector Cluster-id in IP address format

Router(config-router)#