BGP advertised-routes commend

[fabio.marino]

Hi all,

I have a doubt.

Scenario is:

R1 -> R2 -> R3

y.y.y.y -> next hop for R2 towards R3

R2 learns the network x.x.x.x with a static route towards R1.

There is then a BGP session between R2 and R3 and I am announcing the network x.x.x.x to R3.

On R2, issuing the command show ip bgp neighbors y.y.y.y advertised-routes | i x.x.x.x I got as output, in the Next Hop column, the value 0.0.0.0.

Why?

Thanks.

[sesoerensen]

Hi Fabio, would you mind pasting the bgp configs and show output from R2 and R3?

Cheers,

Søren

Sent from Cisco Technical Support iPad App

[stephen.stack]

Hi,

The reason x.x.x.x is in R2's BGP table with a next-hop value of 0.0.0.0 is becuase the x.x.x.x is locally originated via a static route and is advertised into BGP via a network or aggregate command in the BGP configuration. So, you said x.x.x.x is configured as a static route on R2 point to R1, right? So, i assume to you a network statement in BGP to advertise it onto R3.

Regards

Stephen

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[fabio.marino]

Yup your assumption is correct.

Do you have more example in which the next hop is 0.0.0.0? Or is it valid only for locally originated routes?

Thanks.

[John Blakley · Accepted solution]

Next hop of 0.0.0.0 indicates that this router is doing the originating advertisement. You can also pass routes from other hosts that would have a next-hop of that advertising routers address:

   Network          Next Hop            Metric LocPrf Weight Path

*> 0.0.0.0          10.125.100.54            0         32768 i

*> 1.1.1.0/24       10.23.0.2                          50000 215 1 i

*> 2.2.2.0/24       10.23.0.2                          50000 215 1 i

*> 3.3.3.0/24       10.23.0.2                          50000 215 1 i

*> 4.4.4.0/24       10.23.0.2                          50000 215 1 i

*> 10.12.10.0/24  0.0.0.0                  0         32768 i

If we trace the 4.4.4.0 subnet, you'll notice that the originating AS is 1:

215 1

    10.23.0.2 from 10.23.0.2 (172.28.1.2)

      Origin IGP, localpref 100, weight 50000, valid, external, best

So, heading over to the advertising router in 1, you'll see that the 4.4.4.0 subnet has all 0.0.0.0:

*> 1.1.1.0/24       0.0.0.0                  0         32768 i

*> 2.2.2.0/24       0.0.0.0                  0         32768 i

*> 3.3.3.0/24       0.0.0.0                  0         32768 i

*> 4.4.4.0/24       0.0.0.0                  0         32768 i

*> 10.12.0.0/24     0.0.0.0                  0         32768 i

HTH,
John

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