I did Vol1 RIP labs yesterday and I wanted to show you some cool stuff. How to do conditional default routing, this is lab stuff but some of it is definetely useful for real life deployments as well. I will be demonstrating RIP but the concepts are the same for other IGPs as well. I will show how to do it in two different ways. Lets start out with the topology.
This network has two exit paths to the Internet which is simulated by two routers with a loopback of 126.96.36.199. R2 has a static default route to ISP B which is the secondary exit. This route has an AD of 130 so it will only be used when the primary exit via RIP to ISP A is down. I have preconfigured the routers with addresses and the static route on R2 for ISP B. You can download the .net file and initial configs here.
Lets start by enabling RIP on R1 and R2. No routes will be learned since we only have a local link between them. We will now configure R1 to advertise a default route.
Let’s check R2 if we can see the route.
Yes it is there. This is not a very dynamic setup however. R1 will announce the route even if it looses the link to ISP A. Remember that RIP does not need to have a default route in its own RIB for it to announce it to neighbors. We will prove this by shutting down link to ISP A.
Interface has been shutdown. Is route still available at R2?
Yes it is. We now have a blackhole. Traffic will reach R1 and then it will be blackholed. Lets look at a way of solving this.
We can create a route-map and tie this to the advertisement of the default route. The route-map will wheck that the ISP A link (188.8.131.52/24) is in the routing table before advertising the default to R2. If the link to ISP A goes down the default should disappear, lets try this out.
We then shutdown the interface on R1.
We check R2, before and after shutdown of interface to ISP A.
If we debug RIP on R1 we can see that it poisions the route and sends it to R2 (hop count 16).
Now we have a way of doing a conditional default if the interface goes down. How can we solve a situation where we have issues but the interface stays up? Maybe we are connected via a fibre converter, not an optimal solution but sometimes we don’t get to decide.
Time to get funky, IP SLA is our friend. The basic idea is still the same. We will create a dummy route in R1 routing table. This dummy route will only be installed as long as R1 can ping ISP A IP address. If it can’t it will remove the dummy route and stop announcing the default route. We start by configuring R1.
We have a prefix-list that matches the dummy route. We use IP SLA to ping the other side, this is more reliable than relying on link down on the interface. Then we have the dummy route that is tied to track 1. Track 1 tracks the status of the SLA ping. We then have a route-map that looks for the dummy route prefix. If this prefix is missing we will stop announcing the default route. Lets look at R1 to see that the SLA is succeeding and that the dummy route is installed.
The reason we see some failures is because I had the interface shutdown when I brought the SLA probe online. Lets check that R2 still has a default route.
Indeed it does. Now lets see what happens when R1 can’t ping ISP A. I will temporary remove the IP on ISP A so that the interface still stays up but R1 won’t be able to ping any longer. We will run a debug of track to see what happens.
R1 can’t ping ISP A any longer so it stops announcing the deafult route as we can see on R2.
So now we have a more dynamic way of sending a conditional default. You can create even more exciting scenarios than this I am sure. If you want to lab it up just download the files from the beginning of this post.