In the last video, we began our introduction to IP routing and started off with static routing. Although we configured static routing on one of the routers in our lab topology in our last video, our ping test was unsuccessful.

In this video, we will look at why that ping failed and also go on to configure a default route on one of our routers.

Further reading

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Transcript:

Welcome back to this video series on the CCNA R&S exam.

In the last video, we began our discussion on IP routing and configured static routing. In this video, we will conclude on static routing and also look at default routing on the Cisco router.

In the previous video, we added a router to our network and configured a static route for the 192.0.2.0/30 network on the OFFICE RTR. When we tried to ping that network, the ping was unsucessful.

Let’s debug icmp traffic on the BRANCH RTR so that we can see if those ping packets are getting to it. Be careful when you use this debug command because it can overwhelm your router and cause a denial of service.

If we ping again, we see that the ping packets are indeed reaching this router. So what then is the problem? If we check the routing table of this router, we see that it does not have a route to the source of the ping traffic, that is 41.1.1.2.

This brings us to an important realization: Just because traffic goes from A to B does not mean traffic will go from B to A. In summary, think of routing in unidirectional terms and there must be proper routing in both directions to guaranty communication.

We can resolve this issue by configuring a static route on the branch router for the 41.1.1.0/30 network.

Let’s test again. Cool!

One of the problems with static routing is that it is not scalable. Imagine if we had to configure a static route for every network on the Internet! Therefore, there is this concept of having a default route. It basically tells the router: Every network you don’t know how to reach, forward it to this default address.

One of the ways we configure default routes is to use the “ip route” command with all 0s in the network and mask portion. For example, we can configure a default route on the branch router to point to the ISP router. This route will only be used to forward traffic to a destination if there isn’t any more-specific route for that destination in the routing table.

If we check the routing table, you will now see that default route there, at the end of the table and notice that the default gateway is now set to the ISP router.

For example, let us configure a loopback interface on the ISP router with an IP address of 8.8.8.8/32. We will be able to ping this route from the branch router but not from the office router.

Let’s stop here for now. In this video, we have finalised our static route configuration. We have also looked at default routing. In the next video, we will begin discussing dynamic routing protocols.

I hope you have found this video helpful and I look forward to the next one in the series.