Peer-groups
UPDATE generation without peer-groups
The BGP table is walked once, prefixes are filtered through outbound policies, UPDATEs are generated and sent…per peer!!
UPDATE generation with peer-groups
A peer-group leader is elected for each peer-group. The BGP table is walked once (for the leader only), prefixes are filtered through outbound policies, UPDATEs are generated and sent to the peer-group leader and replicated for peer-group members that are synchronized with the leader.
Replicating an UPDATE is much easier/faster than formatting an UPDATE. Formatting requires a table walk and policy evaluation, replication does not.