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c.captari
Beginner

BGP route received twice - Whats the reason?

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

2 ACCEPTED SOLUTIONS

Accepted Solutions
Peter Paluch
Hall of Fame Cisco Employee

Hello,

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

Best regards,

Peter

View solution in original post

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.

View solution in original post

6 REPLIES 6
kozorezdi
Beginner

hi! it is a normal situation when you can see both prefexes: received-only and modified(in your case - localpref 150).

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

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.

Peter Paluch
Hall of Fame Cisco Employee

Hello,

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

Best regards,

Peter

View solution in original post

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.

View solution in original post

Peter Paluch
Hall of Fame 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