IBGP LOOP

[Thomas Leong]

Hi Everyone,

May i know how IBGP causing loop? if all the router is at one AS Number. Thanks !

[pman]

Hi,
Have you been diagnosed with a loop in your network topology ?

If so, I would appreciate it if you could expand a little on the topology, and how you came to the conclusion that there is indeed a Loop.

If not, and you want to know how iBGP avoided loop then:

BGP prevents loops in two ways:

1. Do not accept routes with your own AS from eBGP peers.

2. Horizon rules for iBGP Peers:

       a)Do announce eBGP routes to all peers.

       b)Do announce iBGP routes to external peers.

       c)Do not announce iBGP routes to internal peers.

[balaji.bandi]

This only happends when you have full mesh connectivity, IGP to BGP and BGP to IGP restribution take place.

which can avoice using Route-map to stop doing that.

Note : again depends on the network design.

[Thomas Leong]

Basically im watching a ccnp course from udemy, which is chinese one. he didnt exactly talk about how ibgp causing the loop. Just saying use full mesh to prevent the loop

[Martin L]

I think the instructor meant to say that because of loop=prevarication mechanism of BGP (listed above), you must use full mesh topology (or route-reflector) to have proper connectivity and reachability.  

Regards, ML
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[paul driver · Accepted solution]

Hello

For full ibgp mesh bgp uses as stated split horizon rule which won’t advertise ibgp routes to other ibgp peers 

for non full mesh ibgp peering then Route-Reflectors are applicable and they utilise attributes such as:

Originator ID = which is the rtr that originated/advertised a prefix 




Cluster-id = which is the ID of the cluster list of the RR

If either of these attributes related to the rtr that created them are seen in a prefix on the same rtr then it’s discarded thus negating any loop