1.The router first must verify that it has a current route in the inet.0
routing table to the IP address in the BGP next-hop attribute field.
2. The router then compares routes for the highest local preference (the
only choice based on a higher, rather than lower, value).
3. The router evaluates the AS-path attribute next, where a shorter path is
preferred. This attribute is often a common tiebreaker for routes.
4. The router evaluates the origin code. The lowest origin code is preferred.
5. If any of the remaining routes are advertised from the same neighboring
AS, the router checks the multiple exit discriminator (MED) attributes for
a lowest value. The absence of a MED value is interpreted as a MED of 0.
6. If multiple routes remain, the router prefers any routes learned via an
EBGP peer over routes learned via an IBGP peer. If all remaining routes
were learned through EBGP, the router skips to Step 9.
7. If the remaining routes were learned through IBGP, the router uses the
path with the lowest IGP cost to the IBGP peer.
8. The router then examines the cluster-list attribute for the shortest length.
The cluster list is similar in function to an AS path.
9. The router prefers the route advertised from the peer with the lowest
router ID. However, for EBGP-received routes only, the router prefers the
current active route when comparing routes received from different
10. The router prefers routes from the router with the lowest peer ID.
• Idle: This is the first stage of a connection. BGP is waiting for a start
• Active: BGP is initiating a transport protocol connection in an attempt to
connect to a peer. If the connection is successful, BGP sends an open
• Connect: BGP is waiting for the transport protocol connection to
• OpenSent: BGP has sent an open message and is waiting to receive an
open message from the peer.
• OpenConfirm: BGP has acknowledged receipt of an open message from
the peer and is waiting to receive a keepalive or notification message.
• Established: The BGP session is established, and the peers are
exchanging update messages.