BGP Duplicated AS path

[rbncarvalho]

Hello guys,

Today at work something awkward caught my attention.

The situation falls to BGP, when I issue the command sh ip bgp, the output gives me this:

RT52162#sh ip bgp
BGP table version is 56, local router ID is 10.10.0.16
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network                     Next Hop            Metric LocPrf Weight  Path
*  0.0.0.0                     172.**.***161                    50       0          15**5 15**5 65010 ?
*>                                 172.**.***.169                              0          15**5 65010 ?
*> 10.**.0.16/32        0.0.0.0                                            0            32768 ?
*> 10.**.0.0/16          0.0.0.0                                            0            32768 ?

The bold part of the output it's exactly the same, and this connection runs on ATM.

I can't figure out this issue, so if anyone could explain me why this is happening or how can I troubleshoot it, I have other sites configured the with the same configuration and it gives me the exact same output.

[cadet alain · Accepted solution]

Hi,

The neighbor 172.xxx.xxx.161 is doing AS_PATH prepending by prepending its AS number one time when advertising the default route so that is is considered a backup neighbor.

Regards

Alain

[rbncarvalho]

Thanks,

You've just solved a mistery!! :D