BGP – Part 2 (BGP Peers, BGP Sessions, BGP Messages)

What is BGP peer?

To use BGP, the neighbourship between the BGP routers must be established. You can use the statement BGP neighbour or BGP peer for this establishment. There are two types of BGP peer. These are iBGP(interior BGP) peer and eBGP(exterior BGP) peer. iBGP peer is the neighbour within the same AS. eBGP peer is the neighbour within another AS.   BGP, IBGP (Internal BGP) and EBGP (External BGP)In the topology above, these peering types are showned. As an example, here, router A and router B are iBGP peers. Because they are in the same AS. But router A and Router X are eBGP peers, because they are member of different ASs.

BGP Session Establishment and Session States

Without any connection attemp, the session is firstly in the idle state. With the first TCP message, the state changes to connect.   BGP Session EstablishmentFor BGP, firstly TCP connection must be established. This is called three-way handshake. This messages are SYN,SYN-ACK and ACK as you know. After TCP connection, one peer send Open Message and the other peer respond with another Open Message. During this process the state become OpenSent . [sc name=”ContentRMessage”] Other BGP Overview Lessons: BGP – Part 1 BGP – Part 2 (BGP Peers, BGP Sessions, BGP Messages) BGP – Part 3 (IBGP, IBGP Topologies and EBGP) BGP – Part 4 (BGP Administrative Distance and BGP Path Attributes) BGP – Part 5 (Packet Tracer BGP Configuration Example)