Interface attempting to forward traffic before link is up

[eumaeus78]

We have 2 3745 routers. Each one has a single link to seperate 3725. The 3745s are also linked to each other as are the 3725s. Traffic is routing fine through one link when that link goes down the traffic switches to the other link with little or no loss. When the original link is brought back up, packets are lost until the interface is in a forwarding state. It appears that traffic is forwarded through the link as soon as the link gets power. It does not wait until the interface is in a full forwarding state. We are using static routes between the 3745s and 3725s.

Thank you for any advice you maybe able to offer.

[Richard Burts]

Can youl tell us a little more about the environment that you are operating in? What kind of interface is this (Ethernet, serial, something else)? Are the routers directly connected or are they connected through some intermediate box (switch or something)? If they go through a switch is it possible that you are hitting Spanning Tree convergence? (port fast might be a solution if this is the case).

If we knew more we might be able to find better suggestions.

HTH

Rick

[eumaeus78]

The routers are directly connected via 10/100 fast ethernet interfaces. There is no switch in between.

[Richard Burts]

If the routers are connected back to back over fast ethernet, is it possible that one router senses that its interrface is back up, inserts the static route, and starts to forward, but the router to which it is connected is not yet on line and can not receive packets yet.

Another factor that might come into play here is that when the interface comes up there are no entries in the ARP table for anything reached through that inteface. Any packet that the router attempts to forward before the ARP request is sent, the ARP reply received, and the entry added to the ARP table will be discarded. This could explain some amount of delay in beginning to successfully forward packets.

Thinking of this, it would be helpful if you would post the configuration of the interfaces and the configuration of the static routes.

HTH

Rick

[netia]

If the first assumption is correct and the static route is populated into the routing table because the link is up and the line protocol is down…. Look at the attachment.

Jeff

[eumaeus78]

That is exactly what the network looks like.

Thank you for the help and if you guys still want the configs let me know

[Richard Burts]

Whether to post the configs or not depends on whether you want to persue this further. It may be that as long as you understand why the behavior is this way that you can live with the behavior. (It seems to me to be a somewhat minor problem - how often does the router go down and how many packets really get lost?)

But if you want to work around this behavior I think there is an alternative to consider. There is a fairly new feature in IOS called Object Tracking which can be used to control static routes. If you use this feature it would test whether the next hop address was reachable and only let the route into the table when the next hop was reachable. This link will give you information to evaluate this technique:

http://www.cisco.com/en/US/products/sw/iosswrel/ps5413/products_feature_guide09186a00801d862d.html

Only you will be able to decide whether the additional complexity of configuring Object Tracking and the additional overhead of tracking the reachability of the next hop address are worth not losing packets when the router goes down and comes back up.

HTH

Rick

[eumaeus78]

Rick

Thank you very much for all of your help.

These routers are unfortunately mission critical and the loss of packets continues for 30 seconds or so. I will try the object tracking, and if that doesn't work for me I will post the configs. Again thank you very much for all of your help

[eumaeus78]

Rick,

I have attached a copies of the configs for the four routers. I am still unable to figure out how to reslove this problem. Any help would be greatly appreciated. Thank you very much and have a wonderful day.

It is only going to let me post 3 of the 4 configs, so I will post again with the 4th attached

[eumaeus78]

Post of 4th config

Again, Thank You and have a great day.