So we're carrying on with our discussion of OSPF LSA and Area types. We've talked about the LSA Types that you see there on your screen, now we're going to move into the different types of OSPF Areas and this has to be one of the more fun topics of discussion when you're talking about OSPF if, if OSPF can be fun at all. And it's probably kind of juvenile but it's simply because of the name of the some of these Areas and you, you'll see what I'm talking about here in a, in a few slides or in a few bullet points I should say. These are actually the names of these Areas, I'm not embellishing this or making any of this up. These are actually the names of these Areas, you go read the book, you go look it up Cisco.com or go look it up in Wikipedia or on the Internet, these are the Types of Areas that you'll find referenced and these are exactly what they're named. I'd be doing you a disservice if I didn't, if I didn't do that for you. So the first Type of Area we're going to talk about is a Standard Area and that Standard Area requires no special configuration, it doesn't do any summarization, it doesn't do anything other than filter Type one and two LSAs, those Network and Router LSAs at the Area Border Router. When you have routes inside this area and the Area Border Router advertises them back into the OSPF routing domain, it changes the next hop router when it advertises those Inter Area routes back into OSPF. You probably noticed this or you may have noticed this in the lab when we did the Show IP Routes on really any of the routers. You saw that the routes for the East and the West side were told hey if you want to get to these networks, you got to go through me. Really the only advantage of having a Standard Area set up is again, it filters those Type one and Type two LSAs, and those Type one and Type two LSAs a lot of times are what triggers the SPF algorithm to run. So you will split off your network logically, but you're not doing any summarization, you're not filtering any LSAs, you're not putting any summary routes or any default routes or anything like that. You're just splitting the network off. Next we have a Stub Area and a Stub Area has Type five LSAs, you remember those were the External Routes, outside your OSPF domain and it substitutes a default route for those Type five LSAs. You'll still get all the internal routes from all the other areas advertised into your area and you'll still have those routes summarized or not really summarized, but you'll have the next hop router changed when they're advertised back into OSPF. The next Area Type we'll talk about is the Totally Stubby Area. I told you some of these names would get interesting. The Totally Stubby Area filters Type three, Type four and Type five LSAs and substitutes a default route for all of those LSAs. It's used when the only way out of an area is to go out the default route or go out through the ABR. Like at a branch for example, I, I happen to work for a bank and each of our branches, even though we're not using OSPF, obviously in each of our branches the only way to get back to our Corporate Network is to go through the router that's at that site. Well there's no point in advertising all of our networks for all of our other locations in our Corporate Network and everything back into that site because the site doesn't care, it knows well, I, here's my internal networks and if I want to go anywhere else, I go through this router and it's the same concept with a Totally Stubby Area. If you want to get anywhere outside your area, you go through me and I'm just not going to tell you what else is out there, I'm just going to hand you this default route and normal routing takes over at that point. Again, this will be a little more clear when we set this up in the lab. So now onto some of the more interesting named areas or at least I consider them to be interesting named areas. Next we have the Not So Stubby Area. The Not So Stubby Area is exactly like a Stub Area but it allows you to connect an ASBR or an Autonomous System Border Router to your Stub Area. Now normally you wouldn't be connecting ASBR to your Stub Area, however if you've dealt with networking for more than a year or two, you realize that sometimes you just got to band-aid stuff together to make it work long enough for you to re-engineer it properly. You know, say your company goes through and, and purchases another company, well you have to somehow get their routes into your routing domains so that you can send traffic back and forth and we'll talk about that when we discuss Redistribution here in the next section. But in this case, the NSSA allows you to advertise those External Routes back into the main system. Now you may be asking yourself, well Stub Areas filter Type five LSAs, how am I going to get those external routes back into my main OSPF database? Back into Area 0? Well this is where that special LSA type comes in. The NSSA uses Type seven LSAs and Type seven again, is Cisco Proprietary. Basically when you configure a router in a NSSA with External Routes, it starts sending these Type seven LSAs back to the Area Border Router. The Area Border Router then converts those Type seven LSAs to Type five LSAs and advertises them back into the main OSPF routing domain. So again, it's a nasty hack and a kind of a band-aid in my opinion, but it gets the job done. The last one we're going to talk about is the Not So Stubby Area Totally Stubby Area. That might seem kind of like a contradiction but if you've looked at all of the other types of areas, you'll probably be able to figure out exactly what the NSSA Totally Stubby Area does. It's like a Totally Stubby Area but it allows an Autonomous System Border Router. So basically, you're not getting any External Routes whatsoever, you're getting a default route advertised to you, but I'm advertising external routes back into the main routing domain. Again it uses type LSAs converted to Type five and again to me it's kind of a nasty hack, but it works and if you've ever had to use it, I'm sure it works pretty well. And that concludes our discussion of OSPF, LSAs and Area Types.
|Course:||Implementing Cisco IP Routing (642-902 ROUTE)|
|Duration:||10 hrs / 105 lessons|
|Captions:||Available on CD and Online University|
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