Welcome back into our discussion of OSPF Neighbors, we're about to jump into the whole OSPF Neighbor Negotiation Process and it's a lot more involved than the EIGRP. Again you'll recall we, we talked about that all these flags have to match and you see them here on the screen. They all have to match before this Neighbor Adjacency will form. Well the whole process of getting the routes between OSPF Neighbors is a process that, that you really need to learn for the exam because you'll be quizzed on what the various states are and what they do. You need to learn what's on the next slide, because it will be on the test, like they said in school. So let's jump right in and talk about the various Neighbor States. Now the first state that a router comes up in is the Down State and basically that means I've send Hello packets out but I've not seen any other Hello packets come back to me and so basically as far as I'm concerned the entire OSPF network is down, I don't know about any other networks, everything's down, it's just me all alone in the wild, wild world and, and space. No one can hear me scream and all the other little cliches that you use here. However, never fear, if you're sitting on an OSPF network, you will eventually get a Hello packet from one of your Neighbors or one of your potential Neighbors anyway. At that point the router enters the Init State. In the Init State, it says, I've seen a Hello packet but my Router ID is not one of the Neighbors that, that router knows about. So basically he sent me his Hello packet and then I've sent him my Hello packet and so we're in the Init State. We, we see each others Hello packets, we see that you're there but I, I don't really know about you, you know, do I really want to be your Neighbor? So it checks all of those things we saw on the previous slide and if all of that matches then Router B over there puts my Router ID in his Neighbor section in his Hello packet and sends it back to me. Once that happens, I enter the 2-Way State. Hey, bidirectional communication is established, I like you, you like me, let's all be a big happy family, like, like the big purple dinosaur said. Once you've entered the 2-Way State and you've decided you want to become Neighbors then the Route Exchange Process can begin to take place but, but we're not there yet, We've still got a few other states to go through. The next state is the Exstart State. The Exstart State is where the Master-Slave relationship is established and the only difference between the Master and the Slave in this relationship here is that the Master is the one that sends the DBD packets first and we'll talk about the DBD packets here when we talk about the Exchange State. Something to note is that this Master-Slave relationship is on a per interface basis or a per Neighbor basis more accurately. So therefore if I have five interfaces, I could be the Master on two of the interfaces and the Slave on two of the interfaces and I'm not a Master or a Slave on the fifth because it's on a multi-access network and I'm not a DR or BDR. So once you've established the Master-Slave relationship based on the Router ID, then you will enter the Exchange State. In the Exchange State, OSPF exchanges DBD packets and DBDs or Data Base Descriptors are snapshots of the route tables, kind of the cliff notes version is what I've heard some people say and basically the DBD is just a list of all of the networks that I know about. It's not the entire LSA, it's not the Link State Advertisement, it's not the Link State Table, I'm just telling you, here's the IP networks that I know about. The Master sends the DBD to the Slave first, the Slave then takes those networks, puts it in it's local DBD packet and sends it back to the Master. So once they've seen these DBDs then the routers look at it and say well I know about this network and this network and this network. Oh there's a fourth network in there that I don't know anything about. Now we're in the Loading Phase and the router says hey, this fourth network you told me about, I don't know anything about it. Give me all of the Link State information about that network and again it does this using the Link State Packets between the routers. I get the information from my Neighbor, it gets the full information, I then put it in my Link State Table, run the SPF algorithm, send the results to the Neighbor that we're communicated with or the Neighbor we're negotiating with at this point and he does the same thing. And once all of this loading has taken place, all the Link State Advertisements have gone back and forth, you're now in the Full State. And in the Full State, life is happy and grand. I've got a Neighbor, he's sending me packets, I'm sending him packets, we're all happy and grand and this is the state that is normal for an OSPF Router. If the router is stuck in any other state it's an indication that there's some kind of problem forming the Adjacency and the only exception to this is the 2-Way State which is normal in a broadcast network. Routers will achieve the Full State with their Designated Routers and Backup Designated Routers. Neighbors always see each other as 2-Way in a broadcast network. However on a Point-to-Point network, which is most of what we'll be simulating in our lab, everything will be in a Full State pretty much all the time. Wasn't that fun? Troubleshooting the OSPF Neighbor Adjacency, like I've said, is, is some, sometimes the hardest part of running OSPF but that's the overview and that concludes our discussion of OSPF Neighbors.
|Course:||Implementing Cisco IP Routing (642-902 ROUTE)|
|Duration:||10 hrs / 105 lessons|
|Captions:||Available on CD and Online University|
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