Route exchange
BGP peers exchange updates over a lasting session.
Two BGP-speaking routers first establish a peer session over TCP. Once the session is ready, they exchange UPDATE messages: an update can announce a prefix with its path attributes, or withdraw a prefix that is no longer reachable.
Peer session
Neighboring BGP speakers maintain a control session for exchanging routing information.
UPDATE
Carries a reachable prefix, path information, and attributes, or reports a withdrawal.
AS_PATH
Records the autonomous systems crossed by the announcement and helps detect routing loops.
LOCAL_PREF and MED
Attributes that can influence which learned route an organisation prefers.
The global routing problem
No single router owns the whole Internet.
Networks operated by providers, companies, universities, and content platforms connect as autonomous systems. Each system manages its own routing decisions while exchanging reachability information with neighbors.
BGP carries announcements about prefixes. An announcement says, in effect, that a network can reach a range of addresses through a particular path. Operators can accept, reject, or prefer announcements according to policy.
Build the model
Four pieces of BGP
Autonomous system
A network under one administrative routing policy, identified by an ASN (Autonomous System Number).
Prefix
A range of IP addresses announced as reachable.
Path
The sequence of autonomous systems an announcement has crossed.
Policy
Rules that filter announcements or prefer one route over another.
Mini experiment
Evaluate a path
Select a path announcement and identify the decision it represents.
AS 64510 learns that AS 64520 is reachable through its directly connected neighbor.
An announcement describes reachability; it does not carry the application data.
Knowledge check
Test your global routing model
Answer five questions. The answer order changes each time.