BGP uses message types.
OPEN: Sent after TCP connects. Contains:
- BGP version (usually )
- Your ASN
- Hold time (how long to wait for keepalives)
- Router ID
- Optional capabilities
UPDATE: Carries routing information. Contains:
- Withdrawn routes (no longer reachable)
- Path attributes (AS_PATH, NEXT_HOP, etc.)
- NLRI (Network Layer Reachability Information) listing prefixes
KEEPALIVE: Sent every of the hold time. Empty message. Confirms session is alive.
NOTIFICATION: Signals an error. Session closes after sending. Includes error code explaining what went wrong.