BGP:Please suggest next step to troubleshoot this

Answered Question
Nov 20th, 2009
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Hi,  can you help me with this:

the "inacessible" status in red below below puzzles me. Do you know how I should be able to troubleshoot this further to find out why it states inaccessible?

See that Rack19R4 router is learning the route from eBGP. Why it does not pass the learned 112.0.0.0 route to Rack19R1?


Rack19R1#
Rack19R1#show run | s bgp
router bgp 100
no synchronization
bgp cluster-id 150.19.1.1
bgp log-neighbor-changes
network 150.19.1.0 mask 255.255.255.0
neighbor 155.19.0.5 remote-as 100
neighbor 155.19.13.3 remote-as 100
neighbor 155.19.146.4 remote-as 100
neighbor 155.19.146.4 route-reflector-client
neighbor 155.19.146.6 remote-as 100
neighbor 155.19.146.6 route-reflector-client
no auto-summary

Rack19R1#show ip bgp 112.0.0.0
BGP routing table entry for 112.0.0.0/8, version 0
Paths: (2 available, no best path)
  Not advertised to any peer
  54 50 60, (Received from a RR-client)
    204.12.19.254 (inaccessible) from 155.19.146.4 (150.19.4.4)
      Origin IGP, metric 0, localpref 100, valid, internal
  54 50 60, (Received from a RR-client)
    54.19.1.254 (inaccessible) from 155.19.146.6 (150.19.6.6)
      Origin IGP, metric 0, localpref 100, valid, internal
Rack19R1#
RSRack14AS>4
[Resuming connection 4 to R4 ... ]

Rack19R4#show ip bgp 112.0.0.0
BGP routing table entry for 112.0.0.0/8, version 9
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Advertised to update-groups:
        1
  54 50 60
    204.12.19.254 from 204.12.19.254 (31.3.0.1)
      Origin IGP, localpref 100, valid, external, best
Rack19R4#show run | s bgp
router bgp 100
no synchronization
bgp log-neighbor-changes
network 150.19.4.0 mask 255.255.255.0
neighbor 155.19.146.1 remote-as 100
neighbor 204.12.19.254 remote-as 54
no auto-summary
Rack19R4#

Correct Answer by Jerry Ye about 7 years 4 months ago

In R4, do the following and test again


neighbor 155.19.146.1 next-hop-self


and then


clear ip bgp * soft


Regards,

jerry

Correct Answer by marikakis about 7 years 4 months ago

From what I see, R4 does pass the route to R1 via iBGP. The problem is that the BGP next-hop is inaccessible from R1.  You can either redistribute the connected routes of the external connections in your IGP or use next-hop-self command on the iBGP session on R4 towards R1 (i.e. neighbor 155.19.146.1 next-hop-self).  On R1 I see that the route is received from two different sessions, and routes from both neighbors have the same problem (i.e. inaccessible next-hops).

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Correct Answer
marikakis Fri, 11/20/2009 - 14:42
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From what I see, R4 does pass the route to R1 via iBGP. The problem is that the BGP next-hop is inaccessible from R1.  You can either redistribute the connected routes of the external connections in your IGP or use next-hop-self command on the iBGP session on R4 towards R1 (i.e. neighbor 155.19.146.1 next-hop-self).  On R1 I see that the route is received from two different sessions, and routes from both neighbors have the same problem (i.e. inaccessible next-hops).

Paolo Bevilacqua Fri, 11/20/2009 - 14:47
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Dissecting analisys as the usual, welcome back Maria!

marikakis Fri, 11/20/2009 - 14:59
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Paolo, we come and go, but we always come back! 'Dissecting analysis'? I tried really hard to keep my post short this time! Better luck next time!

Correct Answer
Jerry Ye Fri, 11/20/2009 - 14:45
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  • Cisco Employee,

In R4, do the following and test again


neighbor 155.19.146.1 next-hop-self


and then


clear ip bgp * soft


Regards,

jerry

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