BGP route received twice - Whats the reason?

Answered Question
Nov 9th, 2009
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Hi.. what puzzles me about the following output is that the same route is being received from the same upstream router (172.17.251.10) twice. How is that possible. I thought the upstream router would send only it's best one.


Router3#show ip bgp 0.0.0.0

BGP routing table entry for 0.0.0.0/0, version 129

Paths: (3 available, best #1, table Default-IP-Routing-Table)

Advertised to update-groups:

1 2 5

65000 64512

172.17.251.10 from 172.17.251.10 (192.168.254.254)

Origin IGP, localpref 150, valid, external, best

Community: 65000:150

65000 64512, (received-only)

172.17.251.10 from 172.17.251.10 (192.168.254.254)

Origin IGP, localpref 100, valid, external

Community: 65000:150

65000 64512, (received & used)

172.17.251.1 from 172.17.251.1 (192.168.32.254)

Origin IGP, metric 0, localpref 150, valid, internal

Community: 65000:150




Correct Answer by adamclarkuk_2 about 7 years 6 months ago

It is soft-reconfiguration inbound at work here as Peter is suggestiing. This feature shows routes pre filtering / manipulation which is why the received only route has the default local pref value of 100 (pre route-map) and 150 (post route-map).


Check for the soft-reconfiguration inbound command attched to the neighbor statement under your BGP config.




Correct Answer by Peter Paluch about 7 years 6 months ago

Hello,


Are you possibly using the soft-reconfiguration inbound feature on your router?


Best regards,

Peter


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Overall Rating: 5 (3 ratings)
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kozorezdi Mon, 11/09/2009 - 23:10
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hi! it is a normal situation when you can see both prefexes: received-only and modified(in your case - localpref 150).

c.captari Mon, 11/09/2009 - 23:25
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I would've thought that the upstream router 172.17.251.10 should send only the best route. Not 2 routes:


65000 64512

172.17.251.10 from 172.17.251.10 (192.168.254.254)

Origin IGP, localpref 150, valid, external, best

Community: 65000:150

65000 64512, (received-only)

172.17.251.10 from 172.17.251.10 (192.168.254.254)

Origin IGP, localpref 100, valid, external

Community: 65000:150



Note that this is an EBGP connection to 172.17.251.10 . And local pref is being set on my router. The reason why local pref is 150 on only one is that the route-map terminates as soon as it has a match. That's why the second time the route remains with local pref 100

kozorezdi Tue, 11/10/2009 - 03:07
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hi!

no neighbor 172.17.251.10 route-map in

clear ip bgp 172.17.251.10 soft in

as result you will get the only one prefix from 172.17.251.10 by command: sh ip bgp 0.0.0.0 you can check it on GNS3 as well.

Correct Answer
Peter Paluch Tue, 11/10/2009 - 08:13
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  • Cisco Employee,

Hello,


Are you possibly using the soft-reconfiguration inbound feature on your router?


Best regards,

Peter


Correct Answer
adamclarkuk_2 Tue, 11/10/2009 - 10:22
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  • Silver, 250 points or more

It is soft-reconfiguration inbound at work here as Peter is suggestiing. This feature shows routes pre filtering / manipulation which is why the received only route has the default local pref value of 100 (pre route-map) and 150 (post route-map).


Check for the soft-reconfiguration inbound command attched to the neighbor statement under your BGP config.




Peter Paluch Tue, 11/10/2009 - 14:24
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  • Cisco Employee,

Hello,


Adam, thank you very much for confirming my suspicion.


Regarding the soft-reconfiguration, I find myself amazed at how many people are actually using this obsolete workaround years after all solid BGP implementations have started using the route refresh capability that is much more effective with respect to router resources, especially the memory.


There have been talks regarding the soft-reconfiguration and route refresh on these forums. I suggest reading, for example, this one:


http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=WAN%2C%20Routing%20and%20Switching&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40^1%40%40.2cd44d1c/4#selected_message


You will need to copy the entire URL and paste it into your browser as the automatically generated hyperlink here does not contain the entire URL and is thus invalid.


Best regards,

Peter


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