Peer Type
When one EBGP route and multiple IBGP routes are available, BGP selects the optimal route based on the peer type. If no EBGP route is available or multiple EBGP routes are available, BGP is unable to select the optimal route based on the peer type.
When multiple egress devices reside on a carrier network and receive
routes from the Internet, the egress devices select the optimal route
based on the peer type in most cases. In Figure 1, all devices reside in the
same AS, Switch A and Switch B function as egress devices and are
IBGP peers of all devices in the AS. In addition, Switch A and Switch
B receive routes from the Internet and advertise EBGP routes to all
their IBGP peers. Therefore, Switch A and Switch B have an IBGP route
and an EBGP route, and the two routes have the same AS_Path. In this
situation, Switch A and Switch B select the EBGP route as the optimal
route.
The EBGP route is selected as the optimal route, which prevents the traffic that leaves Switch A or Switch B for the Internet from being forwarded to the other egress device.
For more peer type-based route selection examples, see Local_Pref.