External Border Gateway Protocol (eBGP)
External Border Gateway Protocol (eBGP) is a
BGP term or process used to refer 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.
Example topology:
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.
