The fundamentals

- [Instructor] What in the world could possibly go wrong with an OSPF configuration? A lot of things. You're already shaking your head. You've already seen quite a few of these in your CCNA and CCNP studies. And we're going to look at some more complex issues in this section. We're going to have some authentication labs for you. We're going to build a virtual link or two, and if they work, great. If they don't, we'll just troubleshoot 'em till they do, right? But first things first, we've got to work with adjacencies for a little while because if we don't have adjacencies, we don't have to worry about E1 versus E2 routes.

We don't have to worry about authentication. We don't have to worry about virtual links. We don't have to worry about anything because you're not going to have anything. Your network is dead in the proverbial water. We are going to prevent that from happening on your exam and in the real world with some simple adjacency lifesaving techniques. And we're going to perform them on this little old two router broadcast network using the 172.12.23.0 / 24 network unless specified otherwise, and as always, the router number is the last octet. Now the first thing I have for you on the board here, and we're going to jump to the live equipment to do the rest of it, but I just want to remind you that you can't spell OSPF without OSPF.

You got to make sure it's running on the interface upon which you're expecting an adjacency to form. And some of you are looking at that and saying, yeah, well, duh. But anybody who's ever worked help desk and had to talk anybody through a 15 minute conversation just to find out if they had the thing on, you always want to check the simpler stuff first. And here I've run show IP OSPF interface followed by the fast Ethernet 0/0 interface I wanted to check. That's a command we'll also be running on the live equipment. It's a great troubleshooting command and you just get that message that says OSPF not enabled on fast Ethernet 0/0 and it's easy to panic first time, and say, OSPF not enabled.

It just means that you haven't used your network command yet to enable it on that interface. We're going to get running right now on routers two and three, and hopefully get a little bit of an adjacency coming up here. We'll find out. You'll also see show IP OSPF interface for router three on the screen as well, but we're going to take care of that right now. We're going to write a couple of configs here. (typing) And that's everything that's required.

And I've got a little pop quiz for you. If you've seen the video a couple of times, I really expect you to get it right. (typing) When I hit Enter here, should I expect an adjacency to come up? Look at the router OSPF command I have, look at the network command I have, a couple of differences there. You're working on your np now and you're going to get some configs, maybe with some differences like there and be asked, you know, should we see an adjacency here? Do we need to fix anything? Well, that's the joy of live equipment.

Let's find out. Show IP OSPF neighbor. Don't see anything yet. And suddenly we see all kinds of stuff. Now this is a broadcast network, so it's going to come up pretty fast. You're not going to see every single OSPF state, when you start running show IP OSPF neighbor. But we have a message right there. I know A tad of it is off the screen. I like to make the font as large as I can for you, so I'll occasionally we'll lose a letter or two, but I can show you there OSPF adjacency process one and neighbor 172.12.23.3 from loading to full, loading done.

Well, that sounds pretty good and there it is. So there's our adjacency. So why didn't those differences in the config mess up our adjacency. I'm going to bring those up on the screen right now. There we have, I've actually flipped them here. I did them router two. This is router three, and so forth, but let's say one router has OSPF five, and it's using a wildcard mask, right, not a subnet mask, a wildcard mask of 0.0.0.0.255.

And the other router is using router OSPF one, and the network number is the same, but the mask is different. Why didn't those two differences cause a problem with the adjacency? Well, here's why. The number in router OSPF, that refers to our router process ID. And that is locally significant only. Router three doesn't have any idea what process idea numbers router two is using, couldn't care a less, and vice versa. It doesn't affect the adjacency at all.

Now the same goes for the wildcard mask in the network commands. We saw one router using 0.0.0.0.255, the other one using 0.0.0.3, and again, neither router is aware of the wildcard mask used on the other router. Those values are locally significant only. So the natural question is, Chris, well that's wonderful, but if these aren't troubles, why are you showing them to me in a troubleshooting course? Because it's so important to know when to troubleshoot and when not to troubleshoot. You don't want to be just a regular network admin, you want to be a world-class network admin.

You want to be a world-class troubleshooter. You want to be able to look at configs like that and say, well, that's not our issue. And you're going to see plenty of other ways to find out what the issue is. We're going to be doing plenty of shows and debugs here shortly. But, it's so important not to waste valuable time in the real world and in the exam room with troubleshooting a config that doesn't need to be troubleshot. Now, before we discuss a couple of other values that may or may not have to match. Let's look at this command again, show IP OSPF interface.

And I'll go fast 0/0 here. Lots of information here now. So we definitely have OSPF up and running. But tons of great information here. You don't need to all of this right now at this point in your studies, and frankly, there's some in the middle here, it may be a while, if ever, before you use. But I do want to show the info that you'll use regularly with this. First off, right up at the top, interface is up and up. We know that fast interface 0/0 is up refers to the physical state of the interface, line protocol to the logical, so we're all good there.

Here's the IP address of the interface and the area into which it was placed. Here's the process ID. Here's the router ID that OSPF rid. The network type is boradcast, which we would expect on a fast Ethernet segment. And it has a cost of one. What else we got here? We've got State BDR and Priority one here. This is letting us know that this router is the BDR, the backup designed router for this particular segment.

The priority refers to OSPF priority and that comes into play in the DRBDR election. And if you're a little rusty on that, fear not, we're going to get rid of that rust in this part of the course. Here's your ID for the designated router and the IP address. Now here, they're the same. But of course, we had a loop back on router three, and that loop back address would have been used for the OSPF rid, even if OSPF was not enabled upon it. Here's that same information for your backup designated router. There's the RID, ID, and there's the actual IP address.

Again they're the same here. Hello and dead timers, going to be talking about those a lot in this section. That's coming up And we've got 10 and 40 there, and that is set in seconds. And some information here, here's when we're expecting our next Hello in four seconds when we ran that command of course. And some info here in the middle you might not use all that often but there's some good stuff right down here. Neighbor command, adjacent neighbor count, and who your adjacencies are with and on top of that if they are the DR or the BDR, you're going to see that terminology right there.

And, let's just go back one more time to show IP OSPF. PF neighbor. Easy for me to say it. And going from left to right. This command you're very familiar with. There's the network ID, the priority of the neighbor, the state of the adjacencies along with the role of the neighbor, which is DR, designated router. We've got our dead time. What should that never go below? If I ran it right now? If I ran it a couple of times? There's 36. What should I not see that go below? And there you go.

You really shouldn't see it go below 31 technically, but 30 definitely. Because, so that Hello dead time, if we're expecting Hellos every ten seconds, that means that dead timer should be resetting about every ten seconds and we know that it starts at 40, because we just saw that. Then, finally the IP address of the interface with which which the adjacency has been formed, and then finally, that local interface. So, let's see. We're going to pause right there and at the beginning of the very next video, we're going to revisit this Hello and dead timers and start running some debugs as well.

See you there.