Join Lazaro Diaz for an in-depth discussion in this video Understanding autonomous system numbers, part of Advanced Topics in Cisco Routing: RIPv2, EIGRP, and OSPF.
- Now, we've mentioned the autonomous system number that we use in the EIGRP, and I'm drilling it, and drilling it, and drilling it, that you need to have the same numbers so all routers can talk to each other by number. But what is exactly an EIGRP? The best way to do that is to show you. Let's go ahead and open up Router 1, and what I'm actually going to do, I'm going to break this network. I'm going to go ahead, and I want to take out EIGRP. "No router EIGRP 100" And that's going to break that.
I'm going to do our shortcut to copy. I'm going to show the start, to make sure that it's not there. I'm going to enter my way down, and we see that we have no routing protocol, whatsoever. And if I were to look at my routing table, do show IP route, we see that we have no Ds anymore. So we're out by ourselves, so let's configure EIGRP. "Router EIGRP" and now the famous autonomous system number.
As you can see there, you have a range from 1 to 65,535 autonomous systems. Wow, that is a lot of autonomous system numbers. But what does it mean? What is an autonomous system number? Well, it's a number that, when all routers have the same number, they're sharing the same routing table That's what that means. Now we're all in the same neighborhood, we all see what everybody else sees, so we understand that. There is no redistribution, I don't got to give you a specific network that I'm in.
You're going to learn it from my updates immediately, because once I configure EIGRP in the same autonomous system and I put in the networks, one of the things that you want to see is "neighbor adjacency found." Let is build an adjacency with it's neighbor. That's a good thing. If you put in the wrong autonomous system number, you're not going to see those adjacencies come up, so let's do that. Let's not put the same autonomous system number, let's see what happens. Let's put in 3,000, let's put in something really high, and let's advertise the networks.
"Network 192.168.100.0, enter." And then we also have 10s, 10114 and 101128, but again, it's a distance vector routing protocol, classful boundary. We can never, ever, ever forget "No auto-summary," must do that. Because that command tells it, please, please, send the summary mask along out on your updates. If you don't type that, it won't know where to send the information, so it's very crucial that you do that.
I'm going to copy that, and I'm going to do our shortcut here, "do show IP route." And again, just so you know, this do command is new. It's so we don't have to go back to prelage mode, and we can do everything right from where we're at. That's why I do the do. You deduce your IP route, you see. I did not build any adjacencies, so that's not working. So how do we fix that? Very simple. Back up one, and we say "no router EIGRP 3000," get rid of that, let's do it correctly.
"Router EIGRP 100, "network 192.168.100.0" and "network," and get ready. Woo, there you go. Now all of the sudden, it builds that adjacency. It's very quick, it's a very quick routing protocol. Can't forget, our famous "no auto-summary." Boom.
Now, just going to do a control-z, come back to prelage mode, and doing "show IP route," instead of "do show IP route," and there you see we're back in routing again. So that's what the autonomous system number is. We've got to make sure that we're all on the same one, and we're sharing the same routing table. Again, when do you draw the line, of doing multiple autonomous systems? Completely up to you. How complex is you network? But that in a nutshell is your autonomous system number.
- Configuring static and default routes
- Understanding dynamic routing
- Issuing verification commands
- Comparing RIPng and RIPv2
- Using RIP timers
- Configuring an EIGRP feasible successor route
- Exploring EIGRP options
- Introducing the OSPF process ID
- Exploring wildcard masking