BGP Sessions (Internal and External)

BGP supports these session types between neighbors:

• Internal (sometimes referred to as IBGP or iBGP) - Runs between routers in the same autonomous system.

• External (sometimes referred to as EBGP or eBGP) - Runs between routers in different autonomous systems.

When you send routes to an external peer, the local AS number is prepended to the AS path. Routes received from an internal neighbor have the same AS path that the route had when it was received from an external peer.

BGP sessions might include a single metric (Multi-Exit Discriminator or MED) in the path attributes. Smaller values are preferred. These values are used to break ties between routes with equal preference from the same neighbor AS.

Internal BGP sessions carry at least one metric in the path attributes that BGP calls the local preference. The size of the metric is identical to the MED. Use of these metrics depends on the type of internal protocol processing.

For BGP implementation, external peers are directly attached to a shared subnet and advertise next hops that are host addresses on the subnet. If you enable the multihop option in the BGP peer template during configuration, this constraint is relaxed.

Internal groups determine the immediate next hops for routes. The next hop received with a route from a peer is used as a forwarding address and to look up an immediate next hop in IGP routes. Internal groups support distant peers, but need to know the IGP whose routes they are using to determine immediate next hops.

Where possible, for internal BGP group types, a single outgoing message is built for all group peers based on the common policy. A copy of the message is sent to every peer in the group, with appropriate adjustments to the next hop field to each peer. This minimizes the computational load needed to run large numbers of peers in these types of groups.