Information Technology Reference
In-Depth Information
In a peer group environment, the BGP process walks the entire BGP table only once for
each peer group. A peer group leader is elected for each peer group based on the lowest IP
address. The BGP process walks the table for the peer group leader, creating the BGP
update messages. These update messages are replicated for all other members in the peer
group. If the 100 iBGP peers are all in the same peer group, the router walks 100,000
prefixes instead of 10,000,000. This optimization reduces both the processor and memory
requirements.
A peer group member must be in sync with the peer group leader for replication to take
place. A peer group member is synchronized with the leader if the set of BGP paths adver-
tised to the leader has also been advertised to the peer group member. A peer group member
that is initialized after the peer group has begun to converge will not be in sync with the
peer group. This requires the router to format update messages for the nonsynchronized
peer just like a nonpeer group member until that peer becomes synchronized with the peer
group leader.
The command to examine the replication statistics is
show ip bgp peer-group
, as shown
in Example 3-6.
Command Output for
show ip bgp peer-group
Example 3-6
Router#show ip bgp peer-group
BGP peer-group is regular_group, peer-group leader 10.1.1.2, external
Index 1, Offset 0, Mask 0x2
BGP version 4
Neighbor NLRI negotiation:
Configured for unicast routes only
Minimum time between advertisement runs is 30 seconds
Update messages formatted 2714375, replicated 4174650
BGP peer-group is filter_group, peer-group leader 10.1.1.17, external
Index 2, Offset 0, Mask 0x4
BGP version 4
Neighbor NLRI negotiation:
Configured for unicast routes only
Minimum time between advertisement runs is 30 seconds
Update messages formatted 1904858, replicated 3145550
In the example, for peer group regular_group, the number of update messages that are
formatted is 2714375, and the number replicated is 4174650. The number of peers is not
shown; however, there are 55 peers in regular_group. If the number of replicated messages
is divided by the number of formatted messages, the result is the replication rate—in this
case, 1.54. In an optimal situation, the replication rate is 1 less than the total number of
peers. The number of updates per peer is 125,255. If the replication had been perfect,
125,255 updates would have been formatted for the peer group leader, and 6,763,770
updates would have been replicated for the other 54 peer group members.
