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Finite state machine
In order to make decisions in its operations with other BGP peers, a BGP peer
uses a simple finite state machine that consists of six states: Idle, Connect,
Active, OpenSent, OpenConfirm, and Established. For each peer-to-peer session, a
BGP implementation maintains a state variable that tracks which of these six
states the session is in. The BGP protocol defines the messages that each peer
should exchange in order to change the session from one state to another.
BGP state machine
Basic BGP UPDATES
Once a BGP session is running, the BGP speakers exchange UPDATE messages about
destinations to which the speaker offers connectivity. In the protocol, the
basic CIDR route description is called NLRI. NLRI includes the expected
destination prefix, prefix length, path of autonomous systems to the destination
and next hop in attributes, which can carry a wide range of additional
information that affects the acceptance policy of the receiving router. BGP
speakers incrementally announce new NLRI to which they offer reachability, but
also announce withdrawals of prefixes to which the speaker no longer offers
connectivity.
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