Welcome into our overview of OSPF. If you're watching these videos in order then we've just gone through EIGRP and talked about everything you could possibly want to know about EIGRP from a CCNP standpoint. Now we're going to move into the wonderful world of OSPF. OSPF stands for Open Shortest Path First and it is the most widely used Link State Protocol out there. Really, the only other Link State Protocol that's used on production networks is IS-IS which is thankfully not covered at the CCNP level. That's more at the CCIE or the CCNP service provider level. All of the other routing protocols that we talk about, EIGRP, RIP, even BGP the backbone of the Internet is a Distance Vector Routing Protocol. Now the big difference between OSPF and EIGRP and it's really the differences between all Link State and Distance Vector Routing Protocols is that OSPF keeps a database of the entire network in the router's memory. Now as we'll see in a couple of slides, that's not entirely 100 percent correct but if you have a Default OSPF configuration and you don't structure it and set up areas and everything like that, then yes, every router on your network will know about every other router, will know about every subnet and anytime anything changes every router has to find out that, hey this link between these two routers went down. We'll get into again some of the ways to mitigate that here in a little bit but for now we'll just carry on. There are three steps of Link State Logic and we'll examine these three steps as they relate to OSPF in this slide deck as well as in our lab later on when we, when we set up our OSPF network. The first is Neighbor Discovery. Like EIGRP, OSPF keeps a Neighbor Database to keep track of what routers and what subnets that everybody knows about. However, unlike EIGRP the Neighbor processes are not quite so simple. After we discover the Neighbors we exchange our Link State database, hey here are all the networks I know about, tell me all the networks you know about, so on and so forth and then finally is Path Analysis. Once I know about all the routers, all the links, all the subnets, once I know everything about everything, then I run my Shortest Path First Algorithm and figure out, for me, what is the best way to get from where I'm at to where I want to go. The best example I can think of this is if, say for example, I'm sitting in Birmingham, Alabama and I want to drive to Nashville, Tennessee. Well anyone that's from South knows well you get on 65 North and you just go up 65 North. But let's say that I had never driven to Nashville, so what's the first thing you do? Well, you go to Google Maps, or MapQuest or any other of the mapping sites and you put in hey, I want to start in Birmingham, Alabama and I want to end in Nashville, Tennessee. Well Google Maps knows about all of the routes between here and Nashville. Okay. But you're traveling by car, you want to get there the quickest, what's the best way to get there? Well you just get on 65 North and then you go and that's essentially what's happening inside your router every time a link changes. Let's say that you know, a bridge collapses on 65 North and you're in Culman, Alabama and so, well you can't take 65 North all the way. You got to get off at this exit and you got to go down this way and take this alternate path, you know, it's slower, it's lower bandwidth, you know, fewer cars can fit down this two lane road, but you can still get there, you just have to go a different way. And you know, if there were real time updates for things like that, if, if the Alabama Department of Transportation told Google, hey this path is no longer valid, then Google could recalculate and say, hey you need to go this other direction and that's what the Shortest Path First Algorithm and the Shortest Path First Protocol does. Like EIGRP, OSPF uses three tables, the Neighbor table, the Topology table and the Route table. You may think, well then what's the difference? It's the same three tables, well there is a big difference in the Neighbor and the Topology table in OSPF. Again OSPF knows about everything, so the Neighbor and the Topology table are a lot bigger. Whereas with EIGRP you recall when we looked at the Neighbor in the Topology table inside EIGRP, they were really small, they knew about the directly connected Neighbors, they knew about the directly connected networks and beyond that, you know, it was just kind of, well I know the network is that way and so I, I just send my packets that way and hope for the best. Now unlike other Distance Vector Protocols, OSPF only sends triggered updates to announce network changes. It's not like RIP where it sends the entire routing table out to everybody every 30 seconds and this triggered update is triggered by a Link State Change, hence the name Link State Routing Protocol. If a WAN link between two routers go down, if a LAN link between the router and the switch goes down, if a network becomes inaccessible, OSPF floods that information to every router in the area, every router in the network, depending on the configuration. Every body gets this updated information, runs the OSPF Protocol and says well, I can't take that route anymore, so I got to go this other way to get to that network. Now as a fail safe, OSPF also sends periodic updates on longer intervals and this is called the Link State Update, it's that interval, is 30 minutes by default. Every 30 minutes, OSPF sends out all of it's Topology information saying hey just in case you missed one of my other Link State Updates, you know, the packet got dropped or, or whatever happens, just in case you missed one of these updates, here's my entire Topology and Routing table. And again, by default, that is 30 minutes. We'll continue on with our discussion of OSPF overview 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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