External BGP is the method that different Autonomous Systems (ASs) use to interconnect with one another. eBGP usually takes place over WAN links, where there may be a single physical path between eBGP peers. Alternatively, they may have multiple eBGP peer connections to provide redundancy. Redundant peers use distinct BGP sessions so that, if one session fails, another can take over.
BGP uses an AS path to track the path of a prefix through the various ASs that send or receive the prefix announcement. When a prefix is announced to an eBGP peer, the local AS number is prepended to the AS path. This helps to prevent routing loops by rejecting any prefix announcements that include the local AS number in the AS path. Prefix announcements learned via eBGP are also analyzed using the BGP best path selection process.
For eBGP peerings, the BGP ID and peering IP address is typically the local IP address of the interface that is being used to connect to the eBGP peers. However, if more than one physical interface is being used for eBGP peering, it is also common to use a loopback IP address as the BGP ID, but still use the physical interface IP address as the peering IP address.