If a router suspects that its peer at the other end of an established session has experienced a complete failure of both its control and data planes the router should divert traffic away from the failed peer as quickly as possible to minimize traffic loss. There are various mechanisms that the router can use to detect such failures, including:
BGP session hold timer expiry (for more details about this mechanism, see Keepalive message)
peer tracking
BFD
fast external failover
When any one or these mechanisms is triggered the session immediately returns to the Idle state and a new session is attempted. Peer tracking, BFD and fast external failover are described in more detail in the following sections.