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BGP Session

When two BGP speakers have both been configured to be BGP peers of each other, they will establish a BGP session to exchange routing information. A BGP session is simply a TCP connection over which routing information is exchanged according to the rules of the BGP protocol.

Because BGP relies on TCP to provide reliable and flow-controlled transmission of routing information, the BGP protocol itself is very simple. However it also implies that two routers can be BGP peers of each other only if they are reachable from each other in the sense that they can exchange IP packets.

In practice this means that either of the following must be true:

The BGP peers must be connected to a common IP subnet.
The BGP peers must be in the same AS, which runs an IGP enabling the BGP peers to reach each other.

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