Problem in BGP's Next-Hop-Self

Answered Question
Apr 16th, 2007
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Dear all,


I have attached the scenario in which i am having problem with R3 & R4 BGP table in which 2 possible next-hops to reach 10.10.10.0/24 aren't being displayed instead only one is being displayed.... configuration has been attached with problematic output, please suggest any solution so that output mentioned in DESIRED OUTPUT FILE can be displayed.......


Regards,


Akhtar Rasool



Correct Answer by Harold Ritter about 10 years 1 month ago

Mujeeb,


BGP always select one best path. It is only this one best path that is advertised to the BGP neighbors.


The same applies when a route rflector receives many paths from its clients (and non-clients). It selects one best path and advertised it to all neighbors (except for the one that advertised the path considered to be the best path).


There is actually a proposed draft that would allow more than one path to be advertised to a BGP neighbor. For more information, please refer to the following draft:


http://tools.ietf.org/id/draft-walton-bgp-add-paths-05.txt


Hope this helps,

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Harold Ritter Mon, 04/16/2007 - 05:53
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  • Cisco Employee,

The reason R3 is only showing one path to 10.10.10.0/24 is that R4 is not sending it its path to 10.10.10.0/24 since it considers the path coming from R3 (local-pref = 300) the best path.


You would need to set the weight value on R4 for the 10.10.10.0/24 received from R2 in order to get the desired output on R3.


Hope this helps,

rmujeeb81 Mon, 04/16/2007 - 21:41
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Hello hritter,


Does this mean that iBGP peers always pass their best path information between them ?



Regards,


Mujeeb Sarwar


Harold Ritter Tue, 04/17/2007 - 03:28
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  • Cisco Employee,

A BGP peer will pass its best path to a neighbor given that it hasn't learnt that best path via that same neighbor.


Hope this helps,

rmujeeb81 Tue, 04/17/2007 - 03:47
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Hello hritter,


Thanks for your reply, I want to ask you that iBGP peers only pass their best path information between them. If an IBGP peer have two paths to reach a prefix and one of them is best then in that situation, will it pass both paths to its peer or only best path information ??


Regards,


Mujeeb

Correct Answer
Harold Ritter Tue, 04/17/2007 - 04:30
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  • Cisco Employee,

Mujeeb,


BGP always select one best path. It is only this one best path that is advertised to the BGP neighbors.


The same applies when a route rflector receives many paths from its clients (and non-clients). It selects one best path and advertised it to all neighbors (except for the one that advertised the path considered to be the best path).


There is actually a proposed draft that would allow more than one path to be advertised to a BGP neighbor. For more information, please refer to the following draft:


http://tools.ietf.org/id/draft-walton-bgp-add-paths-05.txt


Hope this helps,

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