alternative path to a directly connected network

Unanswered Question
Aug 13th, 2010


I have a scenario (see attached image) where I have 3 routers, and 3 networks:

router 1<---Network A---> router 2 <--- Network B ---> router 3 <--- Network C ---> route 1

My question is, Router 2 for example is directly connected to Network A, but if it's interface that is connected to Network A is down it can reache that Network through router 3 that reaches it through router 1.

This is easly done, I have my automatic routes in router 2 routing table because it is directly connect to network A, then I add a static route to network A through router 3 with an higher metric, and when the router 2 interface that is connected to network A goes down, the static route takes place and everything works. However if the interface connected to network A in router 2 does not go down, but instead the problem occurs for example in the connection between the switches then router 2 does not detect this does not apply the static route.

What I would like is to apply the static route no when the physical interface link goes down, but when the other router becomes unreachable.

Is there any way to do this?

This should be made with configurations only in router 2 because that is the only router controled by me at this point, all the switches in my example are just for demonstrating the scenario, in fact I do not know what is between my two routers, there is a cloud of network devices from the ISP .

Is there any way of aplying the static route when the logical link goes down?

thank you

I have this problem too.
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nibauramos Sat, 08/14/2010 - 11:54

Hello, and thank you for your response.

I didn't know about that feature, so I investigated if I could use it in my scenario or not, unfortunately I wasn't very successful.

I didn't quite understand how to use that feature, however I found this link: but even there I had problems because some of the commands just don't work. I checked and my current IOS image seems to support all that is needed: c1841-advsecurityk9-mz.124-24.T3.bin

What I understood is that we can define a tracking object that checks if for example a route is, or is not valid, using words from the link you gave: "A tracked IP-route object is considered up and reachable when a routing-table entry exists for the route and the route is not inaccessible."

However my problem is that my router has (forgeting all of the other networks) one network directly connected, and a default route to my ISP, and sometimes (and I think this might be the strange part...), accessing the network that is directly connected might not work if accessed through my router interface in that network, but will work if accessed indirectly through my ISP.

The problem is that my network interface that connects to that network is never down, so the router always has a route to that network, with an administrative distance of 0, always believing that the best path to that network for which he has an interface is through that very interface

Is there a solution for this?

My current solution is: If the link is not working... I "unplug" the cable from the router making the interface down somewhat primitive but works.

Thank you

Richard Burts Sat, 08/14/2010 - 14:30

Yes, unplugging the interface is a bit primitive - but it would certainly be effective. I believe that the track route encounters the same problem as what you are trying to solve. If there is a problem in the switch or a problem with the neighbor router then the route stays in your routing table and the track route will always report success.

I suggest that there may be an alternative. You can track reachability as well as track route. Look in the sources that you used and see if they describe tracking reachability as well as track route. In tracking reachability you basically set up a ping to some address (probably an address on the neighbor router). As long as the ping is successful the route is considered good and if the ping fails then it indicates that there is a problem through the interface and the static route is withdrawn.



nibauramos Sun, 08/15/2010 - 11:03


I'll try that, but if it doesn't work I'll just drop it because it isn't a very common scenario and I don't have the need for such network availability, it is acceptable to lose connectivity until someone remotely shutdowns that interface.

However I'll ask something that has happened to me in the past and is happening with this again, I found in the link I left in my post some commands to implement the tracking for reachability however I found not much documentation about it, when this happens I tend to explore the commands directly, I go to the routers console and start typing the commands and using the "?" to find their arguments that many times are easly understood.

However for example in my link ( it says to do the following:

Track 1 rtr 1 reachability
Rtr 1
  Type echo protocol ipIcmpEcho
Rtr schedule 1 life forever start-time now

the first command Track 1 rtr 1 reachability, I type track 1 then "?" and rtr is no listed in the options I type rtr anyhow and then "?" and it says unrecognized command, however if i ignore it and type the rest of the command it in fact accepts it... however then all the other commands it states that they don't exist, and has the "?" doesn't help... I'm kinda lost there

I'm gonna search a little bit more, If I found a solution I'll post here.

Thank you

nibauramos Sun, 08/15/2010 - 11:26


I've been digging a little bit, and noticed this link had escaped me in:

It is a great example implementing a similar scenario, however it doesn't work for what I was trying to do, I'm deducing that has stated in that example you can select the best route by testing some ip address in each network, but the problem is that I am trying to define the route to a directly connected device, and even in that example they don't consider that, I believe this is not possible to do For example, in that link I just left here they test both connections pinging the modems in each network they are testing, in my case I wouldn't have that, at best I could ping one host in the destination but my ping had to specify that in the first track I want to ping it going through one interface, and on the second track the ping would have to test reachability through the other interface... maybe this is possible, however the command RTR does not exist in my router, I think it is because I don't have the Enterprise Base IOS has stated in the requirements in that tutorial (even though when I go to Cisco Feature Navigator and search for RTR and my image name: c1841-advsecurityk9-mz.124-24.T3.bin it says it is supported ).

Well I'll just drop this, anyhow, thank you for all the help, learn a few things!

Richard Burts Sun, 08/15/2010 - 13:19

I do not believe that it is a matter of feature set that explains why you do not have the rtr command. The rtr command was used in early implementations of Object Tracking. As it was enhanced the command was changed. I believe that if you experiment with the commands under ip sla that you will find the functionality that you need.




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