Hi, and welcome to this CertificationKits CCNA training video on designing OSPF. We’re going to be talking about what an OSPF area is and what that does, the benefits of using areas with OSPF, Area 0, the backbone area; what an Area Border Router is, Route Summarization and Stub areas. All terms you’re going to have to be familiar with for the CCNA exam. You don't have to be an expert at designing an OSPF network, but you’re going to have to be comfortable with why it's important to design an OSPF network properly.
An area is simply a section of your network with OSPF. You got a bunch of routers; let's say we got a ton of routers over here, every circle I am drawing up here is a router by the way and these are WAN links in between them. What we can do is we can chop this network up in to areas. This section right here might be what's called the backbone area – Area 0. It's always important to start with Area 0. The way OSPF is laid out, all information about the routes and routing needs to pass through one central area which would be Area 0. This could be another area right here. It's just the section of our network and these routers could be another area. Now, the router that borders these areas, so one interface is in Area 0, and the other three interfaces on this router are in area let's called this Area 1. This could be Area 2. And, each of these interfaces right here is in Area 2. This interface is in Area 1, and these are called Area Borders Routers. And what that does is that router keeps detailed information about every area it's a part of. You can call it ABR for short. Now the whole purpose of this area is to minimize the amount of information a router has to have in its topology database.
So with smaller topology databases; what that means is this router right here with the big smile, he has detailed information about all the connections within Area 1. However, he doesn’t have detailed information about Area 0 or Area 2. If this link goes down, he is not going to know about it. All he knows is when he needed to send something over there somewhere, he sends it to this router right here; the Area Border Router. This guy doesn’t even know that that link goes down. But he knows anything going over to this area and as he goes to this router right here, he would know whether or not that particular link was down and forward traffic appropriately. If this link's down, he would drop the traffic. This guy wouldn’t be able to send the information over. So, what it does, it allows them to keep smaller topology databases. Remember a topology database is where they keep the information about how the network is laid out. They run their SPF algorithm on the topology database to determine their best cost path to a particular subnet. So it's also these areas are going to minimize the update traffic. So if a change takes place here, all the routers in the area get updated. But it doesn’t have to send update traffic to this router or these routers over here which reduces the overhead on the router. In a couple of ways it reduces the traffic and it reduces the amount of time it takes to calculate the information and the topology databases to find out what the best cost path is. For this to work appropriately, it needs to fall in line with the IP addressing scheme that the areas are using. So you have to use an appropriate IP addressing scheme to get this to work.
Let me show you an example of what I mean by that. I have brought up a different CCNA slide here with a little bit better drawing than what I was doing by hand. So I’ve got a couple of routers here; a bunch of routers actually. Router A and router B are the two center routers and got some subnets going off in this direction and some additional routers over here. So what we could do is we could chop this network up into multiple areas. Each one of these interfaces on router B as well as all of these routers right here would be in one area and then we could have the interfaces between router A and B. B in another area and then interfaces off to this side be in a separate areas. We’ve got three areas, we’ll have area be Area 0, this area will be Area 1 and this area will be Area 2. Now, what is the purpose of this is? It is to minimize the topology tables. So what we’re going to be doing is we’re going to take a look at what would happen if this link were to go down and how that would affect the other routers and how appropriate IP addressing comes into play.
I cleaned up the CCNA slide a little bit and I specified a network address. This entire network here is network 20.0.0.8. Everything over here and including behind these routers that might throw some extra routers in there some other subnets — I'm just throwing the number out there of 45 subnets total and they would be numbered all like this in succession and there’s just a bunch more that you can't see. On this side of the router, there’s a bunch more routers and there are already 100 subnets total in this area. And then over here between this router is just one subnet. To get the IP scheme to work appropriately; what I'm going to do is make an easy addressing scheme here of 20.1.0.0. I will represent this subnet right here with the /16 subnet mask and what I would do here is 20.2.0.0 with /16 subnet mask will represent this entire area here, Area 2 of 45 subnets. So, Area 2 will be represented by 20.2.0.0 for all 45 subnets and what I would do over here is 20.3.0.0 would represent all 100 subnets coming off of router A over here.
Now, what I'm going to do is since there’s a hundred subnets over here, I will do what’s called variable length subnet masks. I will change this /16 subnet masks to a subnet mask that will allow me to have a hundred sub areas of the 20.3.0.0 subnet. So, I've got a subnet basically and that will be using variable lengths subnet masks. Let’s take a look real quick at how I'm going to come up with the appropriate subnet masks for each one of these different areas and implement this VLSM to get this OSPF design to function appropriately because IP is an intimate part of how OSPF areas are designed. What we’re going to do first is forget that I used the word intimate and we’re going to go and check out how I'm going to come up with these new subnet masks. So, we need a hundred subnets for the 20.3 area and we need 45 subnets for the 20.2 area.
So, it’s a little bit review of CCNA subnetting here. So I'm going to change this subnet mask again. What I’ll do is multiply some twos together until I get a number bigger than the number of subnets I needed. So, we have 64 here. So that’s bigger than the 45, so it takes 1, 2, 3, 4, 5, 6 twos so what that tells me to do is extend my subnet mask out six places and my new subnet mask for area 2 is going to be 255.255.252.0, all zeros in last step. So, that’s all the way up through the number 4 there which makes it 252, my first subnet is going to be 20.2.4.0 with a /20 subnet mask. And if this is totally foreign to you, go back and check out that CCNA subnetting video. You need some practice. To get a hundred I'm just going to keep going with this and that’s 128 so that it takes 7 binary places so I'll extend the subnet mask out, 7 spaces. So, my new subnet mask is going to be 255.255.254. So my first subnet will be turning this on right here, 20.3.2.0 with the /21 subnet mask and my math totally stinks. Maybe I need to go back and view that CCNA video. This is going to be a /22 subnet mask, sorry about that. The /23 subnet mask for this one. I’ m going to pause this video for a moment and go back and review some subnetting myself.
Now, again this is — 23 represents 255.255.254.0 and this represents 255.255.252.0, so we are going to go back to our diagram and throw these subnets in there. Then we'll take a look at what the OSPF process is going to do with appropriate IP configuration. So, bring this up and now here is Area 2, our 45 subnets. So this is subnet one, subnet two, subnet three, four, five so on. So, we set the subnets for going to be 20.2.4.0 and there’s just going to go increment to two seconds subnet would be .8.0 third subnet would be .12.0, four subnet would be .16.0, just increments of four I can keep adding four all they 20.0 on hopefully I won’t make any mistakes so, that’s the 45 subnets we could just list them all out in increments of four with the new subnet mask of /22. Now over here with the our subnets would go 20.3.2.0 to .4.0.6.0 now I just list out and label every one of those subnet in increments of two, first, second, third subnet, again this would be 2, 4, 6 for each of the subnets there and that would be that using that new subnet mask of /23 also known as 255.255.254.0. We can leave this one alone Area 0, 20.1.0.0; we'll just leave that alone and leave this /16. This'll be .0.1.0.2 — so put .1.2 for those for those IP’s. Now how does that affect the routing or the routing tables? What happens is all of the routers within this area know of all the subnets. They are all configured with the /22 mask so they know all about every subnet 8, 12, 16. So of this subnet, let's say the subnet is 32 and this subnets goes down or this link goes down. All the other routers know about that and they all have individual entries in the routing table for each one of these subnets here.
What would happen is the Area Border Router would keep all this detailed information as well. But when router B passed information on to router A; it would lump all these subnets together and would just tell router A; hey if you need to get anything to an IP address that’s starts with a 20 and a 2 in the first two octets…its just going throw that out there and a /16 mask send it over to me. I don’t want you to worry about the .4 subnet, .8 subnet, .12 subnet, .16. We have designed the network appropriately, so everything that starts with the 20 and a 2 in the first couple of octets it’s going to be over here by me. So you don’t have to keep all that excess information in your routing table. Just roll this entry in your routing table and we'll be fine. 20.2.0.0./16 that’s all you need to know and I will take care of the rest. These routers will have the detailed information but router A over here and all the routers over here on this side will simply have one entry for all 45 subnets. And the same thing for this network over here on the other side of the router A. What it'll do is all these routers, router A included and the routers on the left side a router A will have all of the subnets the .2 subnet, 4, 6, 8, 10, 12 keep going with the /23 subnet mask.
Now they all know detailed information if the .6 subnet goes down. Every router over here including this router is going to know about that. But what router A is going to tell Router B and all of these guys over here? He is going to say, hey, if you have anything that needs to get to the network that starts with an IP address of 20.3 I don’t care what is in last two octets; send it over to me. So this is what it's going to be sent over to router B and all the routers over here. So everything; all 100 subnets are going to be lumped together into one routing table entry of 20.3.0.0. So you can see, maybe you can't see, with all this mess up here hopefully you can see if router A and all the routers over there only have one entry for all 45 subnet of 20.2.0.0 that's their entry. If this network goes down right here and this link goes down; it does not affect these routers and the information about that link going down doesn't have to go to these routers because it doesn’t affect this entry. Still for router A anything destined for 20.2 still needs to be sent over to router B. So they will leave it alone at that and if this route goes down, traffic might not be a able to hit this router. But only router B has got to know that. So router A would still forward it along and router B would end up dumping that information or send to router A “hey, I can't get there, that link's down”, as he will tell it at that point. But router A won't know specifically that subnet's down unless maybe he is trying to send the information to it. And router B tells them that he can't get the information there. It says sorry I can't get to that IP address right now, that's the only way router A would know that maybe the 16 subnet's down. I had to clean up that CCNA slide real quick and let me recap all that craziness I had on the CCNA slide before.
So what we do is with appropriate IP design and area design; we'd have this be Area 0, Area 2, Area 3. All areas you need to connect directly to area 0 or we need to pretend their connected Area 0 with what's call a virtual link. So over here there might be a hundred subnets. One subnet here, 45 subnets over there and again we would just throw in simple IPs 20.1.0.0, 20.2.0.0. with the /16 address who represent this entire area too and 20.3.0.0 with the /16 address represent the entire Area 3. Then what we would do when we actually configure the IPs for the routers in here; they would all be using a /22 mask and .4, .8, .0 subnets .12.0, over here. They'd be using a /23 subnet mask with the .2 .4 .6 subnets and so the routers within the area would have detailed information. But as far as router A goes, his routing table will have this one entry in there — 20.2.0.0 /16 mask. Anything that need to go 20.2 send it out, my serial 0 interface because that's going to go to router B then router B would have the detailed information for the appropriate IPs. Maybe he is trying to get to IP address of 20.2.36.9, maybe he is trying to get to that specific IP address some computer over here. Router A gets a packet destined for 20.2.36.9 and he look at his routing table and said hey, let's start with the 20.2; anything with 20.2 let me send it router B. He sends it to router B then router B goes okay 20.2.36.9, let me check in my routing table for 20.2.36.0. He is the one that will have 36.0 subnet and go oh, to get there I've got to go to this next top router because the .36 subnets on the other side of that guy so he will have the detailed information. And again, if this goes down, router A will still forward it on because of the 22 thing there. But router B will try to get it there over there. This router says hey that's down. Router B will tell router A, hey, I can't get it there right now.
So again it allows the router to keep a smaller amount of information in their topology table which reduces a lot of the overhead on OSPF and a lot of a traffic that needs to go back and forth and that reduction of information there is called Route Summarization. It's summarizing the routing table information that cut down on the size of those routing tables. There’s one other concept that I want to talk about real quick and that is called a Stub Area. I want to bring up a blank CCNA slide here. So a Stub area, we've got a router with all the stuff going on. Connecting the other routers this is Area 0 right here. There's a real busy network going on, a bunch of subnets all over the place, abunch of craziness over there. Over here it's real simple. He has got one router, he got a switch and some machines. So they are just hanging out nothing like going on over here. It's like small town. So what would happen is this could be considered an area right here. This area — this router only has one way out. It's only going to send everything to this guy. If he doesn’t send to that guy, it's not got to go anywhere.
So this Area Borders Router right here. ABR is what I'm writing as we would designate that this was a Stub network. We would tell it that's a Stub area, so it says hey this is Stub area here. The reason we would tell the routers that this is a Stub area is to minimize the amount of information that goes to this router. It's basically kept in a dark, all this told us hey anything you want to get out send it to me I'm not going to give you much more information than that. As far as the CCNA goes, you are not really going to worry about Stub area too much. But you might see that term on the CCNA exam, so a Stub area is an area that has simply one way out so it is configured to minimize the amount of traffic that travels into that area. So we have talked about what an area is, benefits of area and how it helps OSPF reduce some of that overhead. Area 0 – the backbone area, Area Border Routers, Route Summarization and eliminating all that stress in that topology in routing table and what a Stub area is. So I hope you have enjoyed this CertificationKits CCNA training video on designing with OSPF.