So we're moving into the more advanced OSPF topics and at the outset of this, we're going to talk a little bit about OSPF network types on NBMA which is Non-Broadcast Multi Access. Now you'll recall we talked a little bit about NBMA networks when we talked about EIGRP. Unfortunately unlike EIGRP which just kind of works if you've got a, the broadcast keyword set up on your Frame Relay Network. OSPF has a few different network types and a few different network configurations and you have to kind of know what to expect when you're going through them. OSPF actually has five types of network topologies when it comes to NBMA networks. Three are RFC standards and two are Cisco Proprietary. Now hopefully I don't have to point out that Cisco Proprietary Network topologies for OSPF require all Cisco routers, but I'll, I'll point it out anyway because you wouldn't believe the number of people that don't know the definition of the word Proprietary. We'll start off talking about the RFC standard type of networks and then we'll move onto the Cisco Proprietary Networks and in the lab which we'll do immediately following this slide show, we'll set up a multi-area OSPF network using three different types of network topology and you'll get to see all of them in action. The first RFC standard type of topology we're going to talk about is Broadcast. Now Broadcast is pretty much like what it sounds. It's the default for Ethernet, it's any type of network topology where you can actually send broadcasts. The Broadcast type network topology uses a Designated Router and a Backup Designated Router and all of the routers just kind of find one another. You saw it when we set up our basic OSPF Setup back when we did the first lab, you know, we just configured OSPF on the right interfaces and the neighbor relationships formed and, and away we go. Now by far it is the simplest and technically Point-to-Point networks use Broadcast to find their neighbors, but Point-to-Point is actually different RFC standard and we'll talk about it here in just a couple of minutes, but just know that Broadcast type networks are used when they're all on the same Ethernet segment. The next type of RFC standard network type we're going to talk about is Non-Broadcast and if Broadcasts just kind of work, you can imagine that Non-Broadcasts don't just work by themselves. Non-Broadcast network type is default for Multi-point Interfaces or Frame Relay Interfaces. The Non-Broadcast RFC standard still uses a DR and a BDR because again on these Multi-point Interfaces your WAN network is basically one big layer three network, however, since you can't send broadcasts across these networks, you have to use Manual Neighbor Statements to get OSPF up and running. You have to go into the hub of your hub and spoke router or into one of your hub and spoke routers and say, there's my manual neighbors and we'll set that up when we set up our lab here in just a few minutes. Now it's important to know that even if you use the Broadcast Keyword on your Frame Relay Interfaces when you're setting up your Frame Relay Maps for example, because it is on a Multi-point Interface, OSPF is going to say, well I can't use broadcasts and it's not going to use broadcasts and you can go in and force it to use broadcasts but we, we won't do that because, well then it's just kind of too easy, I mean, come on, where, where's the challenge in that? So carrying on with our RFC standard, the third type of RFC standard network is a Point-to-Point network and you'll recall where we talked about Broadcast Networks and how they just all kind of work. Point-to-Point Networks pretty much kind of work too, it's the default for all the Point-to-Point protocols, HDLC, PPP are Frame Relay Point-to-Point. If OSPF sees that configured as your network type, then it will automatically default to the Point-to-Point Network. The Point-to-Point Network does not use a DR or a BDR and if you'll recall from our discussions of what the DR and BDR actually does, you'll recall that on a Point-to-Point Network, it's not really needed. There's only two nodes on the link so I mean, yeah, you can be the DR and be the BDR, I'll be the DR and you can be the BDR, doesn't really matter because when one router sends a broadcast across the Point-to-Point to link to find it's neighbor, of course, it's going to find it and you know, there's, there's no flooding possible there like there would be on an Ethernet network. So now we'll move into the Cisco Proprietary Network types and it's really two variations of the same type of Cisco Network. That is Point-to-Multipoint broadcast and Non-Broadcast. And I kind of lumped them together because they pretty much act the same, the only difference between the two is one can pass broadcasts and one can't but OSPF still doesn't pass broadcasts even if the Multi-point topology supports it. For the Point-to-Multipoint broadcast or Non-Broadcast Networks, say that five times fast, it is used on Frame Relay Partial Mesh Networks and you'll recall the Partial Mesh is where you don't have every network where they can talk to every other network but most of your networks talk like this. It does not use a DR or a BDR either because again, every network can talk to every other type of network and again you also have to use Manual Neighbor Statements as you would in the Non-Broadcast type of network. And that's it, it's pretty simple and we'll see all of these network types in use in our lab, but for now that concludes our discussion of the OSPF network types on NBMA.
|Course:||Implementing Cisco IP Routing (642-902 ROUTE)|
|Duration:||10 hrs / 105 lessons|
|Captions:||Available on CD and Online University|
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