So we're going to continue our discussion of OSPF and we're going to discuss OSPF LSA and Area Types. Now LSAs are Link State Advertisements and OSPF Routers use these Link State Advertisements to pass information about their directly connected networks. Now in reality, LSAs can advertise much more than just networks. It can advertise Routers or Summary Routes or Autonomous System Border Routers or External Routes, but by and large OSPFs routers use these to tell other routers, hey here are the networks that I know about. And you'll recall during the neighbor set up process during the adjacency initialization process that the routers will exchange LSU packets and these LSUs are like envelopes if you will and inside these envelopes are multiple LSAs. So I send back an entire LSU that contains you know fifteen LSAs for example and then the router will open up that LSU and say, oh well here's all of my Link State Advertisements. I know about this router and this Route and this Summary and this External System and so on and so forth. Now for the exam and in OSPF life in general, you'll need to know about five different LSA types and there's one other that Cisco specific that's used in specific circumstances and we'll get into that when we discuss OSPF Area Types here in a couple of slides. So let's just run down the different LSA Types, talk about what they're used for and where you might see them and, and then we'll move onto OSPF Areas. So the first LSA Type, they're numbered one through five, is a Router LSA. It's an LSA of Type one and it is simply a Router Announcement. Hey, here's an LSA, I know about me, I know about my neighbors, I know about all these other routers in my area. Type one LSAs are only flooded inside the area. So if you have OSPF Network that is divided into multiple areas, you know, you have Area 0 and Area 1 and Area 2. If a router in Area 2 goes down then all of the routers in Area 2 will flood the Type one LSA saying, hey this router went away, I, it's no longer being advertised. That LSA will not pass the Area Border Router and go into Area 0 or Area 1 for example. Next is the LSA Type two. It is a Network LSA and it is advertised to be a, the Designated Router for a Broadcast Network. You'll recall for example, an Ethernet Network or on a network type where we use a Designated Router, that the Designated Router and the Backup Designated Router, if one is elected, are the only ones that advertise this particular LSA and they advertise it for the entire network. They're telling the entire area hey, this is the network, it's over here, here's this Broadcast Network and that way you don't have fifteen routers that happen to be on that same Ethernet segment advertising this same network throughout the area. Just like Type one LSAs, they stay inside the area only, they never pass the Area Border Route and in fact Type one and Type two LSAs are the only LSAs that don't pass the ABR and this is the main reason for partitioning off areas. If you have, you know, this is my Core Network over here and I'm going to put it's fifteen routers in one area. Here's my DMZ over here and here's my Internet access layer and it's over here. If you partition those off, that way, if one of the routers in your Core goes down it doesn't flood that update to all 200 routers throughout your enterprise, it's confined just to the Core Network, because you know, if you're Core Network is in Birmingham, Alabama and you have a branch in Madison, Wisconsin, well that, that branch in Madison, Wisconsin's not going to care that one of your Core routers went down. So you just use it to isolate the Type one and Type two LSAs. There's other uses for partitioning off areas and you know, we'll get into that here in just a bit. Type three LSA is a Summary LSA and the Summary LSA is used by the Area Border Router to summarize routes before they're injected into an area. And you may have noticed this in our previous lab, our Lab 2 where we had the three different areas and the six routers, you may have noticed in our Lab 2, where we had the multiple routers and the multiple areas. When I displayed the Route Table on each of the routers, you saw a Summary LSA being broadcast. You saw that, hey these routes are available through me and in fact that was part of the lab, we had to correct a topology configuration issue so that the East Router could contact the West Router because the West Router was saying, hey, all of these networks, you have to go through me to get to them. That was a Type three LSA. We'll see this again because I've got a little lab set up to show you the different Area Types and how setting different area flags can affect the way the routes are propagated throughout the network. It's a really simple lab, there's not really any objectives, other than just to show you how it works. Type four LSAs are ASBR Summary LSAs, they're the same thing as Type three LSAs but they advertise Autonomous System Border Routers. So if for example you have a RIP Router injecting External Routes into your OSPF routing domain, Type four LSAs are what would advertise those External Routes. And finally we have a Type five LSA which is an Autonomous System External LSA. It is flooded by the ASBR and it advertises the External Routes just like the Type four, the only difference between the Type four and the Type five is, the Type four LSA comes from an Area Border Router and it summarizes those external routes. Type five LSAs are done by the ASBR, they flood across the entire routing domain and unless they're blocked or filtered, every router will see these Type five LSAs. Now the one special type of LSA that we talked about on the previous slide is a Type seven LSA. Now Type seven LSA as used by routers in a Not So Stubby Area to advertise External Routes. This will make more sense when we see it on the next slide and when we see it in the lab, but just know that, that's the one special type. It's a Cisco specific LSA but it's still used by a number of industry standard routers. We'll pick up and talk about OSPF Area Types in the next video.
|Course:||Implementing Cisco IP Routing (642-902 ROUTE)|
|Duration:||10 hrs / 105 lessons|
|Captions:||Available on CD and Online University|
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