Information Technology Reference
In-Depth Information
has the same outbound policy is assigned to the peer group, which can greatly reduce
redundant configuration on routers that have a large number of BGP peers. Example 3-4
shows a sample BGP configuration without peer groups.
BGP Neighbor Configuration
Example 3-4
router bgp 100
...
neighbor 10.1.1.1 version 4
neighbor 10.1.1.1 remote-as 100
neighbor 10.1.1.1 password cisco
neighbor 10.1.1.1 route-reflector-client
neighbor 10.1.1.1 update-source loopback0
...
Five lines of configuration are needed for every route reflector client in Example 3-4. In
Example 3-5, this configuration is done using peer groups.
BGP Neighbor Configuration with Peer Groups
Example 3-5
router bgp 100
...
neighbor RR_CLIENTS peer-group
neighbor RR_CLIENTS version 4
neighbor RR_CLIENTS password cisco
neighbor RR_CLIENTS route-reflector-client
neighbor RR_CLIENTS update-source loopback0
neighbor RR_CLIENTS remote-as 100
neighbor 10.1.1.1 peer-group RR_CLIENTS
...
In Example 3-5, the initial peer group configuration takes six lines. The addition of a new
route reflector client takes only a single line of configuration. This reduces the configura-
tion's size, increases configuration readability, and reduces the probability of configuration
errors. Inbound configuration can be applied to the peer group and to the individual neigh-
bors. The inbound policy for members of a peer group does not need to be consistent.
If peer groups were purely a configuration enhancement feature, the restriction of all peers
sharing the same outbound policy would not make sense. However, the main benefit of peer
groups—the ability to replicate updates across peers—is derived from this requirement.
Because all the peers have the same outbound policy, the update messages they send are the
same. This means that the BGP update message is generated once for each peer group and
then is reused for all the neighbors.
In a nonpeer group environment, the BGP process must walk the entire BGP table for every
peer, creating updates for each peer independently. If there are 100,000 prefixes and 100
iBGP peers, the router walks through 10,000,000 prefixes.
