unable to establish ebgp neighbourship in case of default route

[Sanjib Pradhan]

Dear  All,

We have two router on different location. Both routers are running EBGP as well. When i am trying to establish a neighbourship between two router by configuring default route, me unable to establish. But when i configured stastic route from one side, able to establish the neighbourship between two router. Please somebody help me out......why it is happening like ?????

[Bruno Rangel]

Hi  Sanjib

A BGP speaker will not initiate the TCP session to establish a BGP peering if the peer is reachable only over a default route, you’ll need a more specific route than default – that can be learned statically or dynamically (including via another BGP session ) a BGP speaker will accept/respond to a TCP session and will establish a BGP peering even if the peer is reachable over a default route.

Though the more specific route is needed only on one side, it is always recommended to exist on both sides, once established, the BGP peering will not be broken if the more specific route is lost and connectivity remains over the default route.

Cheers
Bruno Rangel

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[Jose Jara]

Hi Sanjib,

Bruno is correct. In fact, you may test this behavior configuring a route to the other peer to null0 just to bypass the BGP check and after that, once the BGP Open has been sent to the other peer, remove it to allow the peers to complete the 3-way handshake and you'll see how the BGP session is established.

Best Regards,

Jose.

[Peter Paluch]

Hello Jose,

You may be interested in knowing that the behavior of IOS and IOS-XR is said to differ in this aspect. IOS appears to verify the reachability of the BGP peer only in the moment of establishing the session. However, if the BGP peering is established, and afterwards, a more specific route towards the BGP peer is removed and only a default route remains, the BGP peering will remain up and running as long as the default route provides a working connectivity to the BGP peer.

IOS-XR, on the contrary, appears to be performing these checks continuously. Anytime during the BGP peering buildup or existence, if the only route towards the BGP peer is a default route, the BGP session will be torn down.

I recall discussing these differences on Cisco Live! 2011 with Mr. Oliver Bohmer.

Best regards,

Peter

[Jose Jara]

Peter,

thank you for the information, I really did not know. I have just touched XR and configured a couple of things. As always, good stuff. Rated as deserved!

Best Regards,

Jose.

[Bruno Rangel]

Hey guys very nice approach on this one.
Rated both as deserved +5


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