Implementing Cisco IP Routing (642-902 ROUTE) Tutorials
Basic OSPF / Lab 1 - Router IDs pt. 2
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Subtitles of the Movie (Implementing Cisco IP Routing (642-902 ROUTE) / Basic OSPF / Lab 1 - Router IDs pt. 2)
So we are exploring OSPF Router ID's. We went in and put in a new Loopback address, there's that Loopback 255 that we put in and since it is the highest IP'd loop back address we would expect that the Router ID would change, but it doesn't. The Router ID is 10.10.3.1 still. Now if you'll recall this is because OSPF only calculates this Router ID when the process starts so let's say you added this Loopback address there just for testing, just so you could make sure you could get to this router, you know your messing with some of the WAN interfaces or whatever. Even this Loopback address in there, you finish your work and you just leave it there, you forget to unconfigure the Loopback 255 interface. Two and a half years from now, when you finally decide to reboot this router to upgrade the IOS, when it comes back up, now this Loopback 255 address becomes the Router ID and because the Router ID has now changed, Tennessee and Georgia, which have this router configured as a static neighbor for example, now no longer function and given that it's probably two or three o'clock in the morning, you're not going to remember two and a half years ago, that you put this Loopback address in there and heck if it's like most IT shops, the person that put that Loopback 255 address in there two and a half years ago, has probably moved onto a better job and you're now having to clean up afterwards and so you're stuck there at two o'clock in the morning with an OSPF network that converge, why won't it converge? Well it's because your Router ID has changed. Again assume that you're not using Loopback addresses or Loopback interfaces and you go and you add another serial interface that happens to be higher numbered than your existing serial interfaces. Well the next time that router reboots or the next time clear OSPF, the Router ID changes and on and on and on. So how do we prevent this? Well, obviously you can specify the Router ID and it is a Cisco best practice to specify your Router IDs especially if you have statically configured neighbors. Specifying the Router ID is very simple and again if you're implementing an OSPF network, you probably want to document what routers have what Router IDs, just because you're going to be manually setting them. To set it, you go into Config Mode, you do Router OSPF and the OSPF process ID, in this case we used one for all of our routers. And then you just do Router ID and whatever the Router ID is. Now this can be the IP address that you want it to be, in this case, 172.16.1.6 or it could be any IP address you want, even it's not a valid network on this router or a valid network anywhere on your WAN. For example, we could say Router ID 98.76.54.32, which is a valid IP address, not valid in our lab here. Once you do this, you have to reload or use Clear IP OSPF process for this to take effect. So we'll exit out, we'll turn OSPF debugging on the Tennessee router, for example, we'll do Debug IP OSPF and we'll do adjacency. OSPF adjacency debugging is on. So we'll do Clear IP OSPF Process on this, so Clear IP OSPF Process, will that reset all the processes? Yes and if we go over here on Tennessee, we'll see that the serial link dies or the neighbor adjacency dies and now you'll see that it's sending the database requests with it's new Router ID. Again, if we go over here to Show IP OSPF Neighbors, you'll see the Neighbor ID is 98.76.54.32 and it's in a full state now. Even though it's IP address that it reaches is with is 192.168.1.1 and obviously on the corp router, that IP address is not even valid. Of course since you've reloaded the OSPF process on your corporate router, you're entire WAN went down for about 30, 45 seconds while it recalculated the entire Link State database, so obviously this is not something you'd do in the middle of the day. So suffice it to say, that you're Router ID is very, very important and knowing what your Router ID's are is very, very important to building your OSPF network and that's one of the keys that's used to determine which router is considered the Master and which router is considered the Slave in the Neighbor Negotiation Process as well as the DR and BDR Election Process. We'll go over that whole process here in the next series of videos but for the time being this concludes our discussion of Router ID's in OSPF.
Tutorial Information
| Course: | Implementing Cisco IP Routing (642-902 ROUTE) |
| Author: | Greg Dickinson |
| SKU: | 34291 |
| ISBN: | 978-1-61866-028-2 |
| Release Date: | 2011-12-28 |
| Duration: | 10 hrs / 105 lessons |
| Work Files: |
Yes |
| Captions: | No |
| Compatibility: |
Vista/XP/2000, OS X, Linux QuickTime 7, Flash 8 |
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