So we're going to talk about OSPF Summarization. Now as we've built this lab up, we notice that we have a stinking huge Routing Table up here on the Chicago Router and this is just three areas with a couple of routers in each area and four Loopbacks on each of these routers. Imagine if you had 200 areas or 200 Subnets all advertising these LSAs, all back into Area 0, imagine how big the Routing Table would be up here in the Core network, the Chicago and the West and the East Routers. And while internal Route Tables are not nearly as big as BGP Route Tables as, as we'll discuss when we talk about BGP, obviously you want to introduce the least amount of complexity into your network as possible and as a result, if you've built your network properly so that you can summarize routes at your Area Border Routers, then you can actually summarize those routes as they're advertised back into Area 0 and therefore shrink the size of this Routing Table up here in your Core network. Now as you'll recall when we talked about Summarization with EIGRP. With EIGRP, you can summarize anywhere in the network you want. If you've got a router and it has fifteen networks behind, you can go on that router and say, hey advertise a summary, starting with you. And it will advertise a summary going through that router. If you'll recall with OSPF however, the only places you can advertise summary routes are on Area Border Routers. In this case, West, East and East Podunk because recall we have that Virtual Link going through Area 2 to dump all of East Podunk's routes back into the, the core network. I just like saying East Podunk. So let's step through the commands to advertise the summary routes back into Area 0 and let's look at the effect on the Routing Table in each of our areas. So we'll start up here on the Chicago Router actually and again we'll verify that we have this huge Routing Table, we're getting re-distributed routes from San Diego, from the RIP process that's running on this router as well. We've got all these routes, all these WAN links, et cetera, et cetera. Now we're not going to worry about summarizing these WAN links that are inside these areas, just for the sake of complexity or the sake of simplicity I should say, but obviously you could summarize them as well, because I built the network to do this and if you're running this lab at home, you can try setting up the Summarization yourself on those. So we'll go over to West, now that we've verified that Chicago still has all the routes, nothing up my sleeve. We'll go over to the West Router and if we were to look in the Routing Table, Show IP route, we'll notice that all the routes inside Area 1 start with 10.1. You'll notice that any route that is not a 10.1 route is flagged with IA saying it's an Inter Area route and obviously the routes from Area 0 are 10.0. As all the routes from Area 1 are 10.1, Area 2 are 10.2, you, you see the pattern I've got here. So what we'll do is we'll go onto the West Router, go into Configure Mode, Router OSPF oop, Router OSPF 1, and we will enter the Command Area, in this case we'll say Area 1, hit the Question Mark here and the command we're going to use is the Range. Summarize routes matching the address mask, Border Routers only. If you try to enter this on a Non Area Border Router, it'll just tell you, hey you can't do that here, if, if it even exposes the command. Some versions of the IOS won't even expose the command unless it realizes that it's an Area Border Router. So we'll do Area 1, Range, it wants to know the IP address to match, in this case 10.1.0.0 and the IP mask, in this case we'll do 0.0.255.255. So we're saying any networks that start with 10.1 I want you to summarize that route into Area 0. If we hit Question Mark we'll see that we can tell it to advertise the range or not to advertise the range. Now I don't know why you'd set up a range and not advertise it, I'm. I'm sure there's a reason why, it just escapes me right now. Or you can also specify a user specified Metric for this range. Now that'll make a little more sense when we talk about Redistribution when you advertise custom metrics and custom costs into your Redistribution Configuration. In this case, we'll just take the default which is to advertise this entire range of networks, so we'll hit enter and it barks an error at me, isn't that nice? That's because even though it says that it needs the IP mask, I put in the bit mask, in this case I have to reverse this. So let's go in here and do 10.1.0.0 255.255.0.0 and now we're advertising the summary route back into Area 0. So we'll Control Z and give the route some time to build up here on Chicago. Now if we do Show IP Route up here on Chicago, we notice that all the 10.1 networks are gone from our Route Table and replaced with this single / 16 Summary route. And so basically the West Router is telling Area 0, anything that starts with 10.1, send my way because I know where they're at. You notice that the Route Tables shrunk a little bit already. So let's go over on the East Router and do that, we'll do that in the next video because we're going to summarize routes going both directions into Area 2.
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