Border Gateway Protocol (BGP)

BGP (Border Gateway Protocol) is the exterior gateway routing protocol of the Internet, used to exchange reachability (routing) information between Autonomous Systems (AS) — the current standard is BGP-4 (RFC 4271), a path-vector protocol.

Key characteristics

• Classless and path-vector: each BGP update advertises the complete AS-PATH to a destination network, so the receiver can detect and reject routing loops by checking whether its own AS already appears in the path.
• Policy-based: unlike interior protocols that optimise a metric, BGP lets an AS choose routes based on administrative policy (commercial relationships, traffic engineering, filtering) rather than shortest path.
• Runs over TCP (port 179): TCP provides reliable, ordered delivery, so BGP needs no own retransmission/sequencing; neighbours establish a TCP connection first, then open a BGP session.

Operation

1. Peering: Two BGP routers (peers) form a TCP connection and complete an OPEN handshake (version, AS number, hold timer, BGP ID).
2. Keep-alive: Periodic KEEPALIVE messages maintain the session; if none arrive within the hold time, the session drops.
3. Update: Only incremental, changed information is advertised (network layer reachability info + path attributes such as ORIGIN, AS_PATH, NEXT_HOP, LOCAL_PREF, MED, communities).
4. Notification: On errors, a NOTIFICATION message is sent and the session is closed.

Path attributes & route selection (simplified)

Attribute	Role
ORIGIN (IGP/EGP/incomplete)	How the route was learned
AS_PATH	Sequence of ASes traversed — loop prevention & policy
NEXT_HOP	IP address of the next router on the path to the destination
LOCAL_PREF	Preference for exit points within an AS (higher preferred)
MED	Suggestion to neighbouring AS which of several entry points to use (lower preferred)

The decision order is roughly: highest LOCAL_PREF → shortest AS_PATH → lowest origin type → lowest MED → lowest router ID.

iBGP vs eBGP

• eBGP: between routers in different ASes (external peers).
• iBGP: within the same AS — distributes externally learned routes to internal routers; iBGP routes are not re-advertised to other iBGP peers (hence the need for full-mesh or route reflectors).

Significance: BGP is the glue of the global Internet (~1 million+ routes in the default-free zone) — every packet crossing the Internet relies on the AS paths that BGP exchanges, and its policy control is what allows inter-domain business relationships to be enforced.