eBGP and iBGP

If an AS has multiple BGP speakers, it could be used as a transit service for other ASs. As you see below, AS200 is a transit AS for AS100 and AS300.

It is necessary to ensure reachability for networks within an AS before sending the information to external ASs. This is done by a combination of internal BGP (iBGP) peering between routers inside an AS and by redistributing BGP information to Internal Gateway Protocols (IGPs) running in the AS.

As far as this paper is concerned, when BGP is running between routers belonging to two different ASs, we call this exterior BGP (eBGP). When BGP is running between routers in the same AS, we call this iBGP.

How Does BGP Work

How Does BGP Work ?

BGP uses TCP as its transport protocol (port 179). Two BGP routers form a TCP connection between one another (peer routers) and exchange messages to open and confirm the connection parameters.

BGP routers exchange network reachability information. This information is mainly an indication of the full paths (BGP AS numbers) that a route should take in order to reach the destination network. This information helps in constructing a graph of ASs that are loop−free and where routing policies can be applied in order to enforce some restrictions on the routing behavior.

Any two routers that have formed a TCP connection in order to exchange BGP routing information are called peers, or neighbors. BGP peers initially exchange their full BGP routing tables. After this exchange, incremental updates are sent as the routing table changes. BGP keeps a version number of the BGP table, which should be the same for all of its BGP peers. The version number changes whenever BGP updates the table due to routing information changes. Keepalive packets are sent to ensure that the connection is alive between the BGP peers and notification packets are sent in response to errors or special conditions.