How to detect ethernet path failure and clear dynamic routes on that path on cisco router

Posted on 2009-02-19
Last Modified: 2012-06-27
I have routers that are interconnected via two independent fast-ethernet paths for redundancy and load sharing (Per-Destination).

I have used the ip sla and track features in the IOS to auto-fail a default route based on reachability of the far end ethernet interface IP address and it works very well.

However, I have dynamic routes advertised on both ends that need to see each other.  When one path goes down, the default route fails over within 3 to 5 seconds.  But the dynamic routes ( using RIPv2) don't fail -over for the full 180 seconds time-out that is the RIP route time-out timer.

Is there a way to signal / remove RIP on the failed path interface so the dynamic routes get pushed over to the redundant path quickly?  The RIB should have both routes and the fail-over should happen as quickly as it takes the RIB to update to the FIB.

I know if I could shutdown the interface on the failed path, this would force a routing update on the RIB and FIB.   But then I could no longer monitor the path to see when it comes back up.

Use VLANs on the path maybe?  One VLAN to carry traffic and another strictly for monitoring?
I could shutdown the traffic VLAN forcing the routing re-set while leaving the monitoring VLAN up to detect when the path comes back up

Any ideas?
I have no idea on how to or if an interface can be shut down via a track object
Question by:jrhue
    LVL 2

    Expert Comment

    Sounds like you need a 'fast converging' routing protocol, which clearly RIP is not.  If it is not too much trouble for your environment, you should implement either EIGRP (which I prefer) or OSPF
    with these two, you will not only have the inherent fast convergence, but you can further fine-tune their behavior to suit your environment.  If you need further help wth these, yu know where to post.....

    Author Comment

    What would the converging time be with EIGRP assuming the physical interface did not go down ( due to it being ethernet and it traveleing through a few switches before reaching the other end.)?

    LVL 2

    Accepted Solution

    By definition, EIGRP has the following default behavior:

    HelloTimers: 5 seconds
    hold-down timer: 3 * Hello timer = 15 seconds

    so with default configuration, if RouterA doesn't hear from RouterB in 15 seconds (over the interface the neighborship is established), it will declare that route as unavailable and proceed with converging with next best route

    You can modify these values under the interfaces in question as follows:

    int Gi0/0
      ip hold-time eigrp <AS#> <# in seconds>
      ip hello-interval eigrp <AS#> <# in seconds>

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    Author Comment

    OK.  I did not know you could configure the routing timers on a per interface basis.

    So if I use default-route-originate on my router's connected to our Direct Internet Connections (statically routed -- not using BGP ) and set the hello-interval to say between 1 and 3 seconds, I should be able to see a fail-over occur in about 4 to 10 seconds.

    My only other concern would be stability of using non-static default routes and the CPU surge on approx 2400 routes being re-converged to the redundant ethernet link.  ( I am currently using 7500 series routers with RSP4's, VIP2, VIP4 with dCEF running.

    I have used OSPF and RIP2 but never EIGRP.  Are there any gotchas / caveats I should look out for with this protocol?  I am going to set it up on my lab network and experiment later today.


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