iBGP routers

Unanswered Question
Jul 16th, 2007

It seems to me iBGP routers are almost always the eBGP routers. Is that correct? That means the ASBR will run eBGP facing the other AS, and iBGP in its own AS?



I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (2 ratings)
royalblues Mon, 07/16/2007 - 14:22


IBGP and EBGP are not a router property but more a peering property.

IBGP is a BGP session between 2 neighbors in the same AS, whereas EBGP is a session between peers in different AS.

It is perfectly possible for a router to have a EBGP neighbor as well as an IBGP neighbor. EBGP session will populate routes from an external autonomous domain.

ASBR are totally an OSPF property and is a router that redistributes routes from external AS.

HTH, rate if it does


Edison Ortiz Mon, 07/16/2007 - 14:36

iBGP and eBGP peering can be run on the same router.

When BGP is used between autonomous systems (AS), the protocol is referred to as External BGP (EBGP). If a service provider is using BGP to exchange routes within an AS, then the protocol is referred to as Interior BGP (IBGP).

For instance:

Router A

router bgp 100

neighbor remote-as 100 (this is an iBGP peer, same AS #)

neighbor remote-as 200 (this is an eBGP peer, different AS #)

As for ASBR, this concept is used in OSPF to define a router that bring external routes into the OSPF domain.

For instance,

router ospf 1

redistribute connected subnets (the redistribute command will automatically classify this router as ASBR).

Harold Ritter Mon, 07/16/2007 - 15:36

In an SP context, routers run eBGP to routers in other ASes to receive routes from these other ASes and they also run iBGP between routers in the same AS in order to exchange the external information originally received via eBGP.

The only router that is likely to run only iBGP is the route reflector, which is used to alleviate the need for a full mesh between all BGP routers in an AS.

Hope this helps,


This Discussion