EBGP Route Selection

Routes are selected based on the administrative distance of the routing protocol running on that route. Routing protocols with lower administrative distances are given priority over routing protocols with higher administrative distances. EBGP has an administrative distance of 20. OSPF has an administrative distance of 110.

This diagram shows three ASs and the routing protocols used by the BGP routers.

EBGP Route Selection Configuration

The RTC router in AS300 advertises route 1000::/64 to both AS100 and to AS200:

RTA (AS100) receives updates about route 1000::/64 from both OSPF and eBGP. The route learned from eBGP is selected and added to RTA’s routing table, because the administrative distance of eBGP is less than the administrative distance of OSPF.

On RTA:

On RTB:

On RTC:

To check the routes on router RTA, use the show ipv6 route command.

Since RTC is directly connected to RTA, the route from OSPF is actually a better route than the route learned by BGP. To ensure that the route between RTA and RTC is selected for the routing table, you can use the distance command to change the default administrative distance of the BGP route to a higher administrative distance than the OSPF route. For example:

You can also use the backdoor neighbor command to set the BGP route as the preferred route. For example:

On RTA:

To check the routes on router RTA, use the show ipv6 route command.