Peer-groups
A peer-group member is synchronized with the leader if all UPDATEs sent to the leader have also been sent to the peer-group member
The more peer-group members stay in sync the more UPDATEs BGP can replicate.
A peer-group member can fall out of sync for several reasons
Slow TCP throughput
Rush of TCP Acks fill input queues resulting in drops
Peer is busy doing other tasks
Peer has a slower CPU than the peer-group leader