8.4 BGP Basic Configuration
8.4.1 EBGP and IBGP
BGP supports two types of sessions:
  • EBGP session -- Occurs between routers in two different ASs. These routers are usually adjacent to one another, sharing the same media and a subnet.
  • IBGP session -- Occurs between routers in the same AS, as shown in the Figure, and is used to coordinate and synchronize routing policy within the AS. Neighbors may be located anywhere in the AS, even several hops away from one another.

An IBGP session occurs between routers in the same AS in an ISP. The IBGP session coordinates BGP routing policy within an AS.