BGP – Configuring Route Reflectors

Router reflection is an alternative to confederations as a strategy to reduce IBGP meshing.
BGP specifies that a BGP speaker cannot advertise routes to an IBGP neighbor if the speaker learned the
route from a different IBGP neighbor.

A route reflector is a BGP speaker that advertises routes learned from each of its IBGP neighbors to its
other IBGP neighbors; routes are reflected among IBGP routers that are not meshed. The route reflector’s neighbors are called route reflector clients.
The clients are neighbors only to the route reflector, not to each other. Each route reflector client
depends on the route reflector to advertise its routes within the AS; each client also depends on the
route reflector to pass routes to the client.

A route reflector and its clients are collectively referred to as a cluster. Clients peer only with a
route reflector and do not peer outside their cluster. Route reflectors peer with clients and other
route reflectors within the cluster; outside the cluster they peer with other reflectors and other routers
that are neither clients nor reflectors. Route reflectors and nonclient routers must be fully meshed.

Clients and nonclients have no knowledge of route reflection; they operate as standard
BGP peers and require no configuration.

Route reflectors advertise routes learned from:

A nonclient peer only to clients
A client peer to all nonclient peers and to all client peers except for the originator of the route
An EBGP peer to all nonclient peers and all client peersIllustrates a simple route reflection setup.Configured as a route reflector, Router Harvard reflects routes among its
clients within Cluster 23: Routers Plymouth, Westford, and Acton.
These route reflector clients see router Harvard and each other simply as IBGP neighbors.
Router Newport in AS 325 and router Mason in AS 413 see router Harvard simply as
an EBGP neighbor in AS 29.
To configure router Concord as a route reflector:
router bgp 29
neighbor 10.7.1.3 remote-as 29
neighbor 10.7.1.3 route-reflector-client
neighbor 10.7.1.4 remote-as 29
neighbor 10.7.1.4 route-reflector-client
neighbor 10.7.6.2 remote-as 29
You do not configure a cluster ID, because router Concord is the only route reflector in this cluster.
To configure router Acton :
!
router bgp 29
bgp cluster-id 23
neighbor 10.3.1.1 remote-as 29
neighbor 10.3.1.1 route-reflector-client
neighbor 10.1.2.3 remote-as 29
neighbor 10.1.2.3 route-reflector-client
neighbor 10.3.3.4 remote-as 29
neighbor 10.2.5.1 remote-as 29
You must configure a cluster ID, because router Acton and router Harvard are both route reflectors
in this cluster.
To configure router Harvard as a route reflector:
!
router bgp 29
bgp cluster-id 23
neighbor 10.3.1.2 remote-as 29
neighbor 10.3.1.2 route-reflector-client
neighbor 10.1.2.1 remote-as 29
neighbor 10.1.2.1 route-reflector-client
neighbor 10.3.3.2 remote-as 29
neighbor 10.2.5.2 remote-as 29
!
You must configure a cluster ID, because router Harvard and router Acton are both route reflectors
in this cluster.
bgp client-to-client reflection
- Use to reenable the reflector to reflect routes among all clients.
- Client-to-client reflection is enabled by default. If the route reflector’s clients are
  fully meshed, you can disable reflection because it is not necessary.
- If client-to-client reflection is enabled (the default), clients of a route reflector
  cannot be members of a peer group.
- Example
```
no bgp client-to-client reflection 
```
- Changes apply automatically to any routes received after you issue the command.
  To advertise or withdraw routes that are already present in the BGP RIB, you
  must use the clear ip bgp command to issue a hard clear or an outbound
  soft clear.
- Use the no version to disable route reflection; use only if the route reflector’s clients are fully meshed.
bgp cluster-id
- Use to configure a cluster ID on the route reflectors if the BGP cluster has more than one
  route reflector. For clusters with a single reflector, the cluster ID
  is the reflector’s
  router ID and does not have to be configured.
- You specify a cluster ID number or an IP address of a router acting as a route reflector.
- The new cluster ID is used in update messages sent after you issue the command.
  To force BGP to resend all routes with the new cluster ID, you must use the clear ip bgp
  command to perform a hard clear or a soft clear.
- Use the no version to cause BGP to use the router ID as the cluster ID.
neighbor route-reflector-client
- Use to configure the local router as the route reflector and the specified neighbor as one
  of its clients. The reflector and its clients constitute a cluster. BGP neighbors that
  are not specified as clients are nonclients.
- Route reflectors pass routes among the client routers.
- Route reflection eliminates the need for all IBPG peers to be fully–meshed.
  The members of a cluster do not have to be fully meshed, but BGP speakers outside
  the cluster must be fully meshed.
- If client-to-client reflection is enabled (the default), clients of a route reflector cannot
  be members of a peer group.
- If you specify a BGP peer group by using the peerGroupName argument,
  all the members of the peer group inherit the characteristic configured with this command.
  You cannot override this inheritance for a peer group member.
- Changes apply automatically to any routes received after you issue the command.
  To advertise or withdraw routes that are already present in the BGP RIB, you must use
  the clear ip bgp command to issue a hard clear or an outbound soft clear.
- Use the no version to indicate that the neighbor is no longer a client. Use the default version
  to remove the explicit configuration from the peer or
  peer group and reestablish inheritance of the feature configuration.