Implementing Cisco IP Routing (642-902 ROUTE) Tutorials
Advanced OSPF / Lab 3 - Not So Stubby Areas pt. 2
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Subtitles of the Movie (Implementing Cisco IP Routing (642-902 ROUTE) / Advanced OSPF / Lab 3 - Not So Stubby Areas pt. 2)
So we're investigating NSSA, or Not So Stubby Area, areas, say that five times fast and we have configured Area 1 in our OSPF routing domain to be a Not So Stubby Area and to originate default information from the ABR up here, the West router. So now that we have this advertisement set up, let's look over here on the West router, check his Routing Table and see if he sees the Type seven LSA coming in from San Diego. So do Show IP Route, there's the 172.16 networks and there is the 172.20 networks. You'll notice that it has a Route Modifier or Route Identifier of N2, it shows that it's coming as an NSSA External Type two route signifying that it sees that, that's a Type seven LSA. I won't go through the entire Routing Table here. If we do Show IP OSPF Database, we can actually see all of the LSAs. So there is the Router and the Summary Links and if we scroll down to the very bottom of all this mess here, we will see the Type seven LSAs, there they are coming from 10.1.6.1 or more accurately, 10.1.1 down here, the 172.20 networks and it's also seeing the Type five External Links coming from the Chicago router as well as it seeing Type 5 LSAs coming from itself. Very interesting, so now let's go up to Chicago and see how it sees these routes. So if we do a Show IP route up here, we should see these routes as E2 Routes and we do. We see them as just straight out External Type two routes. It does not signify that they are NSSA External Routes because the west router converts that type seven LSA into a type five LSA before it advertises back into Area 0. Of course, once it's back in Area 0, it's all over the network. We could go over here to Boston for example, and do a Show IP Route over here, and we'll see the 172.20 networks advertised as external type two routes going back up through Chicago. Now that's all well and good but what if we don't want San Diego to have this huge Routing Table. I mean you've got all these inter area routes here again, you don't really need them, because you know, what, what's the only way you can get out? Alright, I keep going back to this diagram, I, I hope you all don't mind. So let's talk about how to cut that down. We can turn area one into a Not So Stubby Area Totally Stub Area. Again say that five times fast. And again if you'll go back to our discussion about OSPF Area Types, the NSSA Totally Stub Area acts just like a regular Totally Stub Area except it allows Type seven LSAs to flood throughout the area and the Area Border Router converts them to Type five LSAs as it advertises them back into the rest of the OSPF routing domain. So as with the Totally Stub Area you only have to configure it on the Area Border Router because that's the guy that's going to do all of the default route injection or all the stuff like that. So let's go up to the West router and we will go into Config T, we'll do not Interface, but Router OSPF 1. We will do Area 1 NSSA and we'll use the No Summary Keyword just we, as we would if this was a regular Stub Area. So we'll do No Summary, oop I got to do No S to get No Summary and once we do that, we'll exit back out and let the routers exchange Route Tables again. Go back over to San Diego, do a Show IP Route on this guy, you notice it knows about it's connected networks, it knows about a couple of OSPF networks coming from the West and everything else you're getting a default route, so this Route Table is back exactly as it was when it was just a Totally Stub Area except now we are able to advertise these 172.20 networks back into OSPF. To validate that we can do Show IP OSPF Database here and it sees very few LSAs. There's the Type seven LSAs that it's advertising, back into the area and that's it. If we go up to the West router and we do Show IP OSPF Database, we'll see the Type seven LSAs coming from San Diego as well as the Type five LSAs coming from itself that it's advertising back into OSPF. So there's the Type seven AS External Links in Area 1 and there's the Type five External Link States that it's advertising. There's the routes from Chicago and here's the routes from the West router and again we go up to Chicago, we show the Route Table up here, we still see those 172.20 networks going through the West router, because again that's how you get to that re-distributed network. So as you can see there's a lot of complexity that you can introduce in OSPF when you talk about the various different kinds of areas and where you want to flood routes and how you want to advertise these routes back into your areas and back into your routing domains and we've talked about all the areas and in the next video we're going to talk about Route Summarization. We built the network such that we could actually summarize all of these routes coming from Areas 1, 2 and 3 as they're advertised back into Area 0. And we can actually cut down on the size of the Routing Tables in all of these routers as well. It also cuts down on network traffic because you're not having to advertise those network LSAs back into Area 0, but we've got a whole video to go on that and for now this concludes our discussion of OSPF area types.
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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