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Community Member

iBGP routers

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?

Regards,

Sarah

4 REPLIES

Re: iBGP routers

Sarah,

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

Narayan

Hall of Fame Super Bronze

Re: iBGP routers

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

neighbor 3.3.3.3 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).

Cisco Employee

Re: iBGP routers

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,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Community Member

Re: iBGP routers

That clarifies. Thanks a lot!

Cheers,

Sarah

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