BGP route not advertised (inaccessible)

I don't get this. Most of the stuff I have been reading states that an inaccessible next-hop will prevent a BGP route from being advertised. Ok, so there must be some other reason for the "inaccesible" because the next-hop is in the routing table!

Rack1R5#show ip bgp 136.1.3.0/24
BGP routing table entry for 136.1.3.0/24, version 3
Paths: (1 available, no best path)
  Not advertised to any peer
  300 100
    136.1.245.2 (inaccessible) from 136.1.245.2 (150.1.2.2)
      Origin IGP, localpref 100, valid, external

Routing entry for 136.1.245.2/32
  Known via "ospf 1", distance 110, metric 64, type intra area
  Redistributing via rip
  Advertised by rip metric 1
  Last update from 136.1.245.2 on Serial0/0/0.245, 00:06:02 ago
  Routing Descriptor Blocks:
  * 136.1.245.2, from 150.1.2.2, 00:06:02 ago, via Serial0/0/0.245
      Route metric is 64, traffic share count is 1

 

Rack1R5#ping 136.1.245.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 136.1.245.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 28/29/32 ms
Rack1R5#

 

So, the question is: why is that "inaccesible" there?

Comments

  • When I filter out the 136.1.245.2/32 route (it is being generated because of ospf point-to-multipoint), then the behavior of BGP returns to normal.

    ip access-list standard NO_R2_S0/0/0
     deny   136.1.245.2
     permit any

    !

    router ospf 1

     distribute-list NO_R2_S0/0/0 in

    So, this removes the /32 route from the routing table. Now recursive routing shows the host address 136.1.245.2/32 as directly connected.

    Rack1R5#show ip route 136.1.245.2
    Routing entry for 136.1.245.0/24
      Known via "connected", distance 0, metric 0 (connected, via interface)
      Redistributing via rip
      Advertised by rip
                    bgp 200
      Routing Descriptor Blocks:
      * directly connected, via Serial0/0/0.245
          Route metric is 0, traffic share count is 1

    This in turn fixes the "inaccesible" issue with BGP:

    Rack1R5#show ip bgp 136.1.3.0/24
    BGP routing table entry for 136.1.3.0/24, version 12
    Paths: (1 available, best #1, table default)
      Advertised to update-groups:
         5          6        
      300 100
        136.1.245.2 from 136.1.245.2 (150.1.2.2)
          Origin IGP, localpref 100, valid, external, best

    I don't get it.

     

     

  • Solspin,

    Having no auto-summary will require exact route in the IGP. In your BGP, you're looking for /24 whilst on your IGP/OSPF you're having /32.

    thx

    dsu

  • Great. Thanks for the explaination.

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