Configuring IGRP routing with redistribution

The Interior Gateway Routing Protocol (IGRP) was designed to replace RIP in small to midsize networks. Of course, like RIP, it has its drawbacks. In this Daily Drill Down, Todd Lammle explains how to configure and use the IGRP protocol.

We finished talking about RIP in my last Daily Drill Down, “Dynamic routing with RIP,” which is kind of what Interior Gateway Routing Protocol (IGRP) was designed to do—finish RIP. Rest in peace, RIP! IGRP is a Cisco proprietary routing protocol that was created to replace RIP in small to medium-size networks. But there’s a catch—you have to run Cisco routers, and only Cisco routers, in your whole internetwork to be able to configure IGRP on that internetwork.

So what’s IGRP all about? Well, like RIP, it’s a distance vector routing algorithm, but instead of using hop count as a metric, IGRP uses bandwidth and delay of the line as metrics by default. And just like RIP, it sends periodic updates out all active interfaces, but instead of sending them every 30 seconds as RIP does, IGRP sends them out every 90 seconds.

I know I said I was finished talking about RIP, but just like IGRP, I’ve actually only kind of finished it. We will talk about IGRP, but first, I actually have some more configuration to show you, so let’s get started!

Okay, the 2500A and 2500B routers have already been configured using RIP routing with a default route to the Internet off of 2500B from my last Daily Drill Down. But our 2500C router still needs to be configured with both IP addresses and RIP routing, and our 2500A router needs to have the serial 1 interface configured. We’re going to set up the 2500C router to run only RIP in this example, but keep in mind that in reality, it can run any and all routing protocols. Because I want to give you a configuring redistribution demonstration in this Daily Drill Down, we’ll pretend it is an old router that can only run RIP routing.

Configuring the old gray mare—the legacy router
If you’re just joining us and you want to get brought up to date, you’ll need to see the complete configurations of routers 2500A and 2500B. You can do that by checking out both the “Configuring static and default routing” and the “Dynamic routing with RIP” Daily Drill Downs. Here we go.

As we can see from this router output, router 2500C needs to be configured.

Great. Notice that the 2500C routing table knows about all networks in the internetwork, as well as the default route to the Internet!

Configuring IGRP in an internetwork
Configuring IGRP on a Cisco router is pretty much the same as adding RIP routing. There is only one small difference: You configure an autonomous system number on each router that will share routing information. You must use the same AS number on each router if you want the routers to share routing information. In this network, all routers will use the same AS number of 10. This can be any number you want, and each router can be a member of multiple AS.

Here’s how to configure the 2500A router with IGRP with an AS of 10.

That’s all, folks! Configuring IGRP is pretty much the same as configuring RIP in that it’s a classful distance vector routing protocol. Now we’ll configure the 2500B router.

Now that both routers are configured with IGRP, let’s check out both of their routing tables.

We can see that the 2500A router is receiving IGRP updates from the 2500B router of subnet 96. RIP is advertising the default route, and router 2500C is running RIP, so the remote subnet 160 is being advertised via RIP. But if we were to turn off RIP on the 2500A router, we would lose both our Internet route and the route to subnet 160! We definitely have to fix that, and we will—in a minute, but first, let’s take a look at the 2500B’s routing table.

The 2500B router is receiving a RIP update from the 2500A router about subnet 160. But we want to determine if only IGRP found routes in our routers. We can do that with the redistribution command.

Redistribution is used to translate from one routing protocol to another. The most difficult process of redistribution is the metrics translation. Since RIP uses hop count and IGRP used bandwidth and delay of the line by default as well as reliability, load, and MTU (if the administrator configures the optional metrics), the redistribution command is responsible for converting metrics.

Remember that in our network, the 2500C router only runs RIP, so the 2500A router must translate between the RIP and IGRP routing protocols. Here’s what the configuration would look like.

This command told IGRP to advertise RIP found routes. The metrics are configured to replace the hop count used by RIP since IGRP does not use hop count.

Let’s take a look at the routing table for the 2500B router and see what it looks like now.

Notice anything? How about the fact that there are no longer any RIP found routes? The 2500A router is converting the RIP routes to IGRP and advertising them to the 2500B router! But we’re not out of the swamp yet. We still have one problem. Look at the 2500C router and see if you can figure out what it is.

If you have a problem with the fact that the 96 subnet is not being advertised to the 2500C router, congratulations—you’re on the money! The 2500A router is converting RIP advertised network into IGRP, but it’s not converting IGRP into RIP advertisements. This means that the 2500C router will not get the update for the 96 subnet. We can fix this little issue on the 2500B router.

The command redistribute igrp 10 metric 2 tells the 2500A router to change any IGRP advertised routes into RIP and advertise the routes with a hop count of 2.

So let’s take a look at the routing table for the 2500C router now.

All routes are now in all routers, and the 2500A router is providing the translation for the network between RIP and IGRP and IGRP and RIP. We’ve achieved Networld peace!

Now you have a nice example of how and when to use redistribution within your network, as well as how to configure IGRP on a Cisco router.

Redistribution can be really confusing when you’re trying to advertise a lot of different networks on your routers that are running multiple routing protocols within your internetwork. Here’s a tip: The best way to understand redistribution is to build a network like the one in this Daily Drill Down and configure the routing protocols using as many different configuration techniques as you can think of. You’ll become an expert in no time!

Editor's Picks

Free Newsletters, In your Inbox