So we're going to continue on with our discussion of some of our more advanced OSPF topics and we're going to talk about OSPF Virtual Links. OSPF Virtual Links are not something that you really want to use in your network on purpose. They're something that you might have to use for whatever reason and we'll go over the most common reason for that here in just a little bit, but OSPF Virtual Links allow you to break the rules and oh boy, you're thinking alright, I love breaking rules. Well, it doesn't allow you to break all the rules, it allows you to break one of the rules of OSPF and that rule is, all areas must connect directly to Area 0. If you'll recall from our concepts video or from our introduction to OSPF. If your Area doesn't connect to Area 0 then, well it just won't work right, the routes won't propagate correctly, you won't have reliable communications back to the network that's separated from Area 0. It's, it's just a huge mess. However, if you can't connect an area directly to Area 0 you can tunnel the data through another area. Again it's not something you'd really want to plan for, it's not something you design on purpose but as we all know, the best laid plans of mice and men will always go awry. And sometimes you'll end up with a network that's designed like this. Now let's say that you have the corporate office up here and you have another area that's out here, say the corporate offices in North America and this is in Australia. Well let's say that one of the Australia offices or sub businesses or whatever buys another company and rather than run a WAN link directly from that company all the way back to the corporate office, well it's just easier to connect them to the existing office in Australia and that way, you know, well in theory you can just send the data through this Area 1 and get it to this other office down here, this Area 2. Right. No, I mean, it's technology not magic, it doesn't really work that way. Even though it looks logical that it should. Well this is exactly what Virtual Links were meant to handle and basically what Virtual Links allow you to do, is take the data from Area 2, tunnel it through Area 1 and basically have this ABR act like it's directly connected to this ABR so that the traffic from here goes through this Virtual Tunnel and drops out on Area 0. Therefore you're not breaking the rules anymore and life is happy and grand. And honestly the set up is not that difficult. We'll see it in the lab here in, in the next video where we discuss the Area Types and Virtual Links. I've bundled them all into, into one lab here. But again, the caveat is, is that the Virtual Link is dependent upon the Router ID and you'll recall from our discussion of Router IDs that you need to specify the Router ID or the Router ID would change and if the Router ID changes, if for example, the Router ID of this guy here changes, well this router here no longer can contact this Router ID on this IP address, so this tunnel fails, this whole network or this whole area of networks goes down and you really can't figure out why. Because again the only time that the Router ID changes is when the router's reloaded or OSPF is re-initialized. That's usually done off hours, so you reboot the router, you know, you upgrade the IOS or whatever, you reboot the router, okay, everything's up and good, I can talk to my areas. You completely forget about this area down here that was tunneled through. The next morning everyone here in Area 2 says, hey I can't get to the Mail Servers, I can't get out to the Internet, I can't get to A B or C Application. And you're running around with your hair on fire trying to figure out what happened. So I guess the cautionary tale of all this is, if you're going to use Virtual Links then you need to specify the Router IDs of all the routers involved. And you'll notice when we went through advanced lab, that on every router I set up I went and specified the Router ID. It's a good habit to get into because you never know when you're going to have something like this thrown at you. So anyway, as I've said, we'll see this in a lab, but I just wanted to at least go over what Virtual Links were, how they worked and give you the basic overview of them, which we've done and that concludes our overview of Virtual Links.
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