Route verification on IOS

Unanswered Question
Mar 1st, 2007

We have some layer 2 switches between our routers so they are not directly attached and will not detect a failure between the layer 2 switches. Will IOS blackhole the traffic or is there a command to make IOS verify that the next hop of a route is valid?

I have this problem too.
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purohit_810 Thu, 03/01/2007 - 13:02

You can do trace or Ping...

If you doing trace and where it is topping. from there you don't have routing. Look up on that device routing tabele by

sh ip route {remote subnet}

PIf you are doing ping , it will give you

reply from ; remohost is unreachable.

You can make your routing diagram by SHOW CDP NEIGHBOR DETAILS Command.


Dharmesh Purohit

glenn.newman Thu, 03/01/2007 - 14:28

I need the router to be able to check if the next hop is reachable and use a lower preference route if it isn't.

Jon Marshall Thu, 03/01/2007 - 14:40

Hi Glenn

Perhaps you could explain your setup in a bit more detail.

If you have a router R1 that connects to a switch which connects to another switch and then to another router R2 and the link between the switches goes down then generally speaking the R1 will know as it will lose it's neighborship with the other router eg

say you were running EIGRP between R1 & R2 and you lost the link between the switches R1 would lose it's adjancency with R2 and all the routes it has via R2 will be removed from it's routing table.

If there was a lower preference route via another router it would then use this instead.

I guess i don't have a full picture of your topology so it's difficult to say more.



glenn.newman Thu, 03/01/2007 - 15:17

Your scenario is correct, but we are not running a routing protocol as this is an Internet link to a provider through metro ethernet - just static routes. I thought there was a command to make IOS verify the next hop.

sundar.palaniappan Thu, 03/01/2007 - 15:49

The feature you are looking for is Reliable Static Routing Backup Using Object Tracking. This link has configuration tasks and some examples too...

The Reliable Static Routing Backup Using Object Tracking feature uses Cisco IOS IP SLAs, a network monitoring feature set, to generate ICMP pings to monitor the state of the connection to the primary gateway. Cisco IOS IP SLAs is configured to ping a target, such as a publicly routable IP address or a target inside the corporate network. The pings are routed from the primary interface only. A track object is created to monitor the status of the Cisco IOS IP SLAs configuration. The track object informs the client, the static route, if a state change occurs. The preconfigured floating static route on the secondary interface will be installed when the state changes from up to down.


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