eBGP

Routers that belong to different autonomous systems use an exterior session of Border Gateway Protocol (BGP) to name the destinations they can reach. Each autonomous system is one network with its own routing policy. The session sits at the edge where those networks meet.

The matching interior session, Interior Border Gateway Protocol (iBGP), carries those routes among routers inside one network.

What an Exterior Session Announces

• Destinations: Each side tells the other which destinations it can reach, so traffic can be sent toward those networks.
• Separate networks: The routers are not in the same autonomous system. Each network keeps its own view of what it is willing to announce.
• A boundary: The session is the handoff between networks. It does not, by itself, distribute those routes to every router inside either network.

How Interior Sessions Carry Routes

An interior session does not replace the exchange between separate networks. It spreads routes that an exterior session has already made available.

• Accepted routes: Once a router learns a destination from a neighboring autonomous system, an interior session can carry that route to other routers in its own network.
• One network: Those routers share a single autonomous system. The interior session stays inside that boundary.
• Shared reachability: Routers that never speak directly to the outside network still learn which destinations the exterior session announced, and which router can reach them.

Exterior and Interior Sessions

The two sessions split one job. Exterior sessions cross from one autonomous system to another. Interior sessions keep the learned routes inside a single network.

Comparison	Exterior Session	Interior Session
Where it runs	Between routers in different autonomous systems	Among routers inside one autonomous system
What it shares	Destinations one network can reach, told to a separate network	Those routes, carried to routers in the same network
Boundary	Crosses from one network to another	Stays inside a single network