GNS3 and IP SLA with default route tracking

As I have mentioned before, I am a big fan of GNS3. I also said that I would be posting GNS3 labs from time to time with an eye toward gaining skill in the CCNA R&S arena, as well as the CCNP track. If you don’t have GNS3, go to the The GNS3 download page to get it. Without further ado, here is today’s learning experience.

One really cool thing that Cisco devices (routers – in this blog) can do is to perform various scheduled network status checks. What I want to do today is show how you can use an IP SLA together with a tracking object to make routing decisions based on downstream availability.

I have built the scenario shown here:

001-Cisco-EIRP1-network

As noted on the diagram shown above, R3 has an ip sla configured (number 10). It is scheduled to verify reachability to the loopback interface that is on R1 by pinging it. R3 is then tracking this ip sla. The tracking returns a result of either true or false, depending upon whether or not the ping was successful. There is a default route configured on R3 which is present in the routing table contingent upon the status of the tracking (object 1). If the target address becomes unreachable (for instance, if you log into R1 and shutdown the Loopback 0 interface), the result of the tracking will switch from true to false, and the default route will disappear from the routing table. Here is what show ip route looks like on R3 if tracking it true:

002-show-ip-rout-Cisco-EIRP1-network

As you can see, the Gateway of last resort exists, as well as a candidate default route as shown at the bottom of the output.

Now I will log in to R1 and shutdown interface Loopback 0, the target IP address, and then view the routing table on R3 again. Let’s see the difference.

003-show-ip-rout-Cisco-EIRP1-network

First off, you note that the change in network topology, which was being tracked, was noticed by R3. This caused the conditional default route to disappear from the routing table. Notice in the output of show ip route on R3 that the gateway of last resort is no longer set, and the candidate quad-zero default route is no longer in the output. This shows one of the powerful capabilities offered by ip sla and tracking.

I have provided all the files necessary for you to build this scenario for yourself. I hope you enjoy this lab, and if you discover any suggestions or have any suggestions, I would love to hear from you.

The zipped files are available here: Feel free to experiment with the configurations and shoot me any suggestions you may have on changing it up or improving it. I will continue to post GNS3 scenarios in future blogs. If there is a scenario you would like to see, leave a comment and I will work on creating it.

Mark Jacob
Cisco Instructor – Interface Technical Training
Phoenix, AZ

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