Gateway: A
Abstractly, each router has a BGP routing table in the form of:
\[(\text{prefix in another AS},\ \text{best ngh (next gateway hop) to reach prefix})\]
First we will go over eBGP. We know BGP uses TCP to send messages and eBGP is no exception. The TCP connection exists between:
each two gateways in the same AS, and
each two ‘‘adjacent’’ gateways in different ASes.
These gateway pairs sent
\[(prefix,\ AS\text{-}path,\ next\text{-}hop)\]
The BGP next-hop attribute is the next hop IP address that is going to be used to reach a certain destination.