We're continuing on with our discussion of OSPF Overview. So now we're going to talk about OSPF areas. If you design an OSPF network correctly you have Hierarchical areas within OSPF and remember the statement that I said earlier, where every time a link changes that every router in the network has to know, well that's not exactly 100 percent true, well this is how that comes to not be true. If you break your network up into areas and this is a very simple network, but let's say in Area 1 and Area 2, each has 50 routers in it and Area 0 with a backbone area has ten routers in it. Well let's say that one of these routers in Area 2 goes down. Only the routers in Area 2 need to know about this change happening. The routers in Area 0 don't need to know, the routers in Area 1 don't need to know. Only these routers in Area 2 and this is how you contain those updates so that you're entire network doesn't go nuts with traffic every time a link goes down. You section your network off into areas like so and then as we've discussed, when one of these networks go down in Area 2, you don't kill your network with the traffic. So given our concept of OSPF areas, let's talk about the different router types in OSPF. First type we'll talk about is an Internal Router and these would be the 50 routers that's inside Area 1 and Area 2. If the router is only a member of a single area, all of it's interfaces are in Area 2, then that router is an Internal Router to Area 2. The next type we'll talk about is an Area Border Router and these are these routers that you see right up here that have one leg or one interface in Area 1 and one interface in Area 0. They are bordering areas, hence the name Area Border Router. Now the important thing to remember about ABRs is that these routers up here are the only place where Summarization can take place and you'll recall in EIGRP, we could go on any router we wanted to and set up Summarization on any interface we so desired. With OSPF you can't do that, all of the routers in each of these areas advertise all of their networks to everybody else and the only place Summarization takes place is at these Area Border Routers. The next type we have is the Autonomous System Border Router or ASBR. And it's not on this diagram but let's say we had another router over here that connected us to the Internet. It's got one leg over here on the Internet and one leg in Area 0 that borders this Autonomous System and this entire routing infrastructure over here is considered an Autonomous System. If you have a router that borders it, you have an Autonomous System Border Router. See these names are pretty straightforward if you, you think about it and the last type of router is a Backbone Router. Now the Backbone Router is really just an internal router but the Backbone Router is internal to Area 0. Area 0 is considered to the backbone area. Every area on your OSPF network has to border Area 0, has to be directly connected and again that's flexible. You could have another area, you know, 52 down here if you wanted to and it could use a trunk basically or a virtual link to go through Area 2 to get to Area 0. We'll set that up in the lab even though you really don't have to know how to set that up for the CCNP, we'll do it just to get a full understanding of how all the various areas in OSPF work. But suffice it to say that all areas have to connect to Area 0. That's how packets get from Area 1 to Area 2 to Area 52 and et cetera and et cetera. Now separate from the Router types, we have Router Roles and there's two Router Roles that can be assigned on an OSPF network. The first is the Designated Router, the Designated Router is used as the source of routing updates on multi- access topologies. We'll see this again in our lab but let's say you have fifteen routers down here and they're all connected on the same Ethernet switch. Well, you don't all fifteen routers in Area 1 sending out updates onto that same multi-access switch. One needs to broadcast it and the rest can see that broadcast and go, oh well here's all of the networks, we're all on the same switch and so we all have the same, pretty much the same Topology database. When OSPF comes up, it will elect a Designated Router and that router is what sends those updates onto that multi-access medium. The router with the highest priority which is a Configured Setting or the highest Router ID which should be a configured setting will win and I know we're introduced to new term Router ID here and we'll get to that when we talk about OSPF Neighbor Configuration. The next is the Backup Designated Router. The Backup Designated Router is the second highest priority or Router ID on the local network. Basically if a Designated Router goes down, this is the router that takes over immediately. It's kind of like the feasible successor in EIGRP, it's there, it's ready, it can handle the load, it can handle the role, it's just not doing anything until the first guy goes away. And that's it, that concludes our discussion OSPF Overview.
|Course:||Implementing Cisco IP Routing (642-902 ROUTE)|
|Duration:||10 hrs / 105 lessons|
|Captions:||Available on CD and Online University|
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