External Border Gateway Protocol (eBGP) is a BGP term or process used when refering to BGP peers or neighbors that are in a different Autonomous System and Number (ASN).
In other words, eBGP is used to exchange route information between different autonomous systems. When only one link connects two autonomous systems, the IP addresses of the connected interfaces are used to establish a BGP session between the two. You can use any other IP address on the interfaces, but the address must be reachable without using internal Gateway Protocol IGP configuration.
There are basic needs to be met when using eBGP to inject routes owned by one Autonomous System (AS) through the enterprise network and into another AS, such as:
i, A local router’s ASN must match the neighboring router’s reference to that ASN, also its neighbor remote-as asn command.
ii, The neigboring BGP router IDs must not be the same..
iii, Each BGP neighboring router must be part of a TCP connection with each other. The remote or neighboring router’s IP address used in that TCP connection must match with the local router configuration in a BGP neighbor remote-as command.
R1(config-if)#ip address 192.168.100.1 255.255.255.252
R1(config)#router bgp 556
R1(config-router)#neighbor 192.168.100.2 remote-as 255
R2(config-if)#ip address 192.168.100.2 255.255.255.252
R2(config)#router bgp 255
R2(config-router)#neighbor 192.168.100.1 remote-as 556
From the above configuration, R1 and R2 belong to different autonomous systems— 556 and 255, respectively.
The peer relationship between R1 and R2 in the above example is established because, R1 peering IP address is in the same subnet as its own physical interface.
When BGP neighbor are not directly connected, you will have to use the eBGP multihop command to establish connectivity.