ip next hop unreacheable , routes still advertised ..

[thomas.fayet]

Hello community .

I have a strange behavior  and a simple proble . I configured the following static route

ip route 10.84.22.0 255.255.254.0 10.84.23.254

That I advertsied in eBGP :

router bgp 65000

network 10.84.22.0 mask 255.255.254.0

the IP next hop 10.84.23.254 is cascaded on my customer LAN . At nominal time the router advertsed the route in BGP

pjnb1376#sh ip bgp nei 57.213.169.169 ad

           Next Hop            Metric LocPrf Weight Path

*> 10.84.22.0/23    10.84.23.254             0         32768 i

My issue :

When the lan interface of the router goes down , the router still advertise the route !!! Even if the IP next hop

10.84.23.254  is not reacheable anymore ....

The box is a Cisco 1941 using

1900-universalk9-mz.SPA.151-4.M1

Is this a bug a normal behavior present in Cisco 15.x Mainline now ?

Thanks

Da

[thomas.fayet]

Hello,

Here is the result of the show ip route loops command :

ppos179#sh ip route loops

->default:ipv4:base 10.84.32.0/23 -> base 10.84.33.254 static 00:00:29 N

Can someone please explain what is the meaning of this result ?

Please remember the 10.84.32.0/23 subnet is not vanished from the router routing table , even after the IP next hop 10.84.33.254 becomes unreacheable.

Thanks

T

[thomas.fayet]

Anyone ????

Thanks

[Peter Paluch]

Thomas,

Can you please try removing the line

ip route 10.84.22.0 255.255.254.0 10.84.23.254

from your configuration and replace it with the following command?

ip route 10.84.22.0 255.255.254.0 Gi0/0 10.84.23.254

The idea behind this change is to make the route depend both on the egress interface and the next hop. If the egress interface fails, the route is immediately considered invalid and the router is not going to look up a different recursive resolution for the 10.84.23.254 next hop address. This should solve the issue - hopefully.

Please keep me informed! This is an interesting case I haven't stumbled across yet. Something regarding the routing table must have changed in recent IOS versions.

Best regards,

Peter

[thomas.fayet]

Peter ,

Indeed , I already fixed the issue by adding the lan interface as another "condition" to the route .

Still , even without this "trick" I dont understand why the router keeps the static route on the routing table when the ip next hop is down ....

Thanks for the great support !!!!

T.

[Peter Paluch]

Hi Thomas,

The route remains in the routing table because the next hop of this route essentially still remains reachable - through the route itself. Now, logically, a route should remain in the routing table only if its next hop can recursively be traced to an outgoing interface that is up/up. That in your case, the route was not dropped can partially be explained by the fact that a lookup in the routing table still produced an existing route - the very route itself, but it is not how things should work.

I would personally suggest raising this as a TAC ticket if possible.

Best regards,

Peter