bgp path selection issue

[uddika kahawatte]

hi,

i have the following cli show command output,

R2#show bgp ipv4 unicast
BGP table version is 11, local router ID is 192.168.220.252
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
* i192.168.30.0     192.168.110.70           0    100      0 63000 i
*>                  192.168.220.70           0             0 63000 63000 i
* i192.168.40.0     192.168.110.70           0    100      0 63000 63000 i
*>                  192.168.220.70           0             0 63000 i
R2#

why isn't the route through the shortest AS path not selected as the best route for 192.168.30.0. ?

thanks,

uddika

[Emmanouil Patiniotakis · Accepted solution]

It seems that one is ibgp learnt route. Please show the output form the command show ip bgp 192.168.30.0.

[uddika kahawatte]

R2#
R2#
R2#show ip bgp 192.168.30.0
BGP routing table entry for 192.168.30.0/24, version 7
Paths: (2 available, best #2, table Default-IP-Routing-Table)
  Advertised to update-groups:
     2         
  63000
    192.168.110.70 (inaccessible) from 192.168.111.251 (192.168.111.251)
      Origin IGP, metric 0, localpref 100, valid, internal
  63000 63000, (received & used)
    192.168.220.70 from 192.168.220.70 (192.168.220.70)
      Origin IGP, metric 0, localpref 100, valid, external, best
R2#
R2#

thanks, i noticed that R2 does not have the route for the next hop, 192.168.110.70.

[uddika kahawatte]

i guess the recommended configuration is to advertise routes to ibgp peers with next-hope-self, isn't it?

[Giuseppe Larosa · Accepted solution]

Hello Uddika,

yes use of next-hop-self is a good practice.

However, there are two other options:

a) to have each border router to advertise its own border link(s) subnet(s) in iBGP with a network command under BGP process

b) to use an IGP like OSPF with the passive-interface option to make border links connecting to other ASes known to other iBGP speakers as IP subnets.

Hope to help

Giuseppe

[shunmugas]

Hi,

        Already the route is learned via shortest as path only i think since the route is correctly learned from EBGP nly not IBGP..So why the additional config is required here? pls clarify

[milan.kulik]

Hi,

isn't there

'bgp bestpath as-path ignore'

configured on the router?

Best regards,

Milan

[uddika kahawatte]

no,.. i checked that.

[milan.kulik]

Hi,

in that case, can you paste more details?

sh ip bgp 192.168.30.0

output from both R2 and R1 plus BGP config section from both of them?

Best regards,

Milan

[paul driver]

Hello Try a soft reset and check again Clear ip bgp * soft Res Paul

[uddika kahawatte]

didn't work, even rebooted all router.