In this video, we're going to discuss a little bit about OSPF Neighbors. Now like EIGRP and other Link State Routing Protocols, OSPF maintains a Neighbor table and it uses this Neighbor table to build a map of the entire network or of the entire area that, that particular router is in. It uses Hello packets like EIGRP and like other Link State Protocols. However the Hello packets, unlike EIGRP, contain a little more information. The information contained within these Hello packets are the Area ID for the interface that is sending the Hello packet. We also have the Hello and Dead Interval for this particular section of the network. Like with EIGRP, OSPF expects to see Hello packets on a certain interval, by default, it's ten seconds. If it does not see these Hello packets for the Dead Interval which by default is four times the Hello interval or 40 seconds, then the OSPF process considers that Neighbor to be down and it triggers a route re-convergence and recalculates the Network Topology in it's own little mind or own little memory there using the SPF algorithm. We also have the Router Priority in the Hello packet. Now Router Priority is used for the election off Designated Routers and Backup Designated Routers as well as for the Master-slave relationship which we'll talk about when we talk about the various Neighbor states here on the next slide. The Hello packet also contains the Designated Router and Backup Designated Router for that network segment if a network segment requires a Designated Router or a BDR. And last but not least, the Hello packet contains what Neighbors that router sees and that becomes important when we talk about the Neighbor states here in just a second. Now unlike EIGRP, just because Neighbors exchange Hello packets, doesn't mean they'll form an Adjacency. EIGRP is pretty simple, it's like the coyote and the sheepdog in the old Warner Brothers cartoons, morning Ralph, morning Sam. Hey you want to be Neighbors? Yeah, what's your K values? Here's my K values, great we're Neighbors, everything's happy. OSPF's a little more picky. There's a great many things that have to match between OSPF Neighbors before that Neighbor Adjacency will form and honestly getting Neighbor Adjacency's to form is sometimes the hardest part of OSPF. Everything else works like pretty much any other routing protocol, you summarize at certain points and you configure the timers and the interfaces and what not, just like you would EIGRP or RIP or any other routing protocol but these OSPF Neighbors, sometimes they just don't want to form and it's one of these following things that usually prevents it. Number one is the subnet mask and I've done this, in fact I did it building one of the labs for, for this course. I had a WAN link and one side I had a .252 subnet mask and on the other side of the WAN link I had a Class C subnet mask. Well that Neighbor Adjacency just didn't form and it took me about fifteen minutes to figure out why. The packets would go one way but wouldn't go the other and finally I, I just looked at the config and, and stared and compared between the two routers, oh, well you know, forehead slap moment there. So the subnet mask has to match between the OSPF Neighbors. Also the subnet address, they have to be on the same network with the same subnet mask. Again, seems kind of a no brainer but you'd be surprised at the number of times I've seen two routers that have a different networks on either end of a WAN link or more often on a broadcast medium such as a switch network that don't have the same subnet. Also the Hello and Dead Intervals have to match. Unlike EIGRP, OSPF checks the Hello and Dead Interval and if they don't match, you won't form the Neighbor Adjacency. That kind of saves from having a flapping router but again it, it makes the Neighbor negotiation process tricky sometimes. Also the OSPF Area ID has to match. Now again, kind of a no brainer but you can't have an interface in one area form a Neighbor Adjacency with a different area. Also if there's any Authentication set up, you remember we talked about MD5 Authentication with EIGRP. You can set up that same type of Authentication with OSPF. If Authentication is enabled, then the Authentication keys have to match, obviously, that's the whole point of the Authentication keys and finally the Stub Area Flag has to match. You can't have one router think I'm in a stub area and it's Neighbor's say well no you're not in a stub area. Again, if these two flags don't match, then the Neighbor Adjacency doesn't form. Now we've talked about the Neighbor states and in the next slide and in the next video we'll go through the entire Neighbor Negotiation Process and talk about what happens during each step of the Neighbor Negotiation.
|Course:||Implementing Cisco IP Routing (642-902 ROUTE)|
|Duration:||10 hrs / 105 lessons|
|Captions:||Available on CD and Online University|
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