BGP Explained: How It Works and What to Look For
Border Gateway Protocol exchanges routing information between autonomous systems.
Published by Protocol Explorer · Reviewed
How BGP works
Peers establish a TCP connection, exchange OPEN messages and then use UPDATE messages to announce or withdraw routes. KEEPALIVE messages maintain the session.
What to look for in an exchange
Compare network prefixes and path attributes such as AS_PATH and NEXT_HOP. BGP chooses routes according to policy as well as protocol rules.
Follow an example step by step
- Find the TCP session between peers and the OPEN messages that establish BGP parameters.
- Read UPDATE messages for announced or withdrawn prefixes and associated path attributes.
- Compare subsequent KEEPALIVE or NOTIFICATION messages with the state of the peering session.
Common interpretation mistake
The shortest AS path is not always the selected route because routing policy can take precedence.
Key point to remember
A BGP route announcement is a statement of reachability, not a measurement that packets are successfully delivered.
Explore the protocol
The BGP overview is free without an account. Its full interactive lesson is included in Protocol Explorer Pro, a one-time $4.99 purchase; an account is required to buy Pro and restore the licence across devices.
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Specification
Read the primary specification for BGP for the authoritative protocol details. The interactive scenario is a simplified teaching model, not a capture of your network.
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