WEBVTT 0:00:03.040000 --> 0:00:07.140000 Hello and welcome to this video, which is a refresher for the CCNA 200 0:00:07.140000 --> 0:00:11.920000 -301 on Open Shortest Path First, OSPF. 0:00:11.920000 --> 0:00:18.100000 Let's talk about OSPF is an interior gateway routing protocol. 0:00:18.100000 --> 0:00:21.820000 So it was designed to operate within an autonomous system. 0:00:21.820000 --> 0:00:24.840000 Just think of that as being synonymous with within a company's network 0:00:24.840000 --> 0:00:27.460000 to exchange routes within that autonomous system. 0:00:27.460000 --> 0:00:31.760000 It is an open standard, so it was specified in RFC. 0:00:31.760000 --> 0:00:35.920000 It's always been open, so any vendor's equipment should be able to support 0:00:35.920000 --> 0:00:41.200000 OSPF. It uses the shortest path first algorithm, otherwise known as the 0:00:41.200000 --> 0:00:46.040000 Dijkstra algorithm to compute the shortest path to any destination. 0:00:46.040000 --> 0:00:50.240000 One thing that makes OSPF interesting when it comes to routing protocols 0:00:50.240000 --> 0:00:54.680000 is that routing protocols generally fall into two different categories. 0:00:54.680000 --> 0:00:59.920000 There are those categories where a router will learn of routes from its 0:00:59.920000 --> 0:01:03.660000 directly connected neighbor, so from another router that's directly physically 0:01:03.660000 --> 0:01:06.000000 connected to it across a cable. 0:01:06.000000 --> 0:01:09.680000 And so a learner of routes, and it'll use that neighbor as the next hop. 0:01:09.680000 --> 0:01:14.460000 But it won't really know what the topology looks like behind that neighbor. 0:01:14.460000 --> 0:01:17.860000 It'd be sort of like me telling you, hey, you don't know what's on the 0:01:17.860000 --> 0:01:19.900000 other side of this curtain, what's on the other side of this wall right 0:01:19.900000 --> 0:01:25.860000 here. But if you go through me, there's like 50 different rooms back there. 0:01:25.860000 --> 0:01:28.680000 Well, you don't know what the size and shape of those rooms are. 0:01:28.680000 --> 0:01:29.880000 You don't know what the layout is. 0:01:29.880000 --> 0:01:34.580000 You just know that if you ever need to send a message to any of those 0:01:34.580000 --> 0:01:38.640000 rooms, just give that message to me, and I'll send it back there. 0:01:38.640000 --> 0:01:41.120000 So what's behind that curtain is pretty much invisible to you. 0:01:41.120000 --> 0:01:44.600000 You just know there's 50 different rooms back there, maybe the names of 0:01:44.600000 --> 0:01:46.120000 each one of those rooms. 0:01:46.120000 --> 0:01:47.660000 That'd be synonymous to a route. 0:01:47.660000 --> 0:01:50.360000 Well, OSPF takes a different approach. 0:01:50.360000 --> 0:01:55.500000 OSPF says, not only do you know about me and this connected network that 0:01:55.500000 --> 0:01:59.240000 we have, you know exactly what everything looks like behind me. 0:01:59.240000 --> 0:02:03.940000 You've got a full topology of what this building looks like, how the rooms 0:02:03.940000 --> 0:02:08.120000 are interconnected, their sizes, their shapes, where their doors are. 0:02:08.120000 --> 0:02:09.700000 That's like what OSPF is. 0:02:09.700000 --> 0:02:14.520000 OSPF router learns of what the whole topology looks like, and then decides 0:02:14.520000 --> 0:02:21.500000 for itself what the best neighbor is to use to get to a particular route. 0:02:21.500000 --> 0:02:24.380000 So OSPF forms neighbor relationships. 0:02:24.380000 --> 0:02:30.120000 Before any information, before any prefix or topology information is exchanged 0:02:30.120000 --> 0:02:34.980000 between a pair of routers, they have to form what's called an OSPF adjacency, 0:02:34.980000 --> 0:02:37.860000 what we sometimes call an OSPF neighbor relationship. 0:02:37.860000 --> 0:02:41.800000 And they do that by exchanging special type of OSPF messages called hello 0:02:41.800000 --> 0:02:47.040000 packets. As you can see here on the vast majority of network segments, 0:02:47.040000 --> 0:02:51.380000 hello packets go out every 10 seconds and they have a hold timer of 40 0:02:51.380000 --> 0:02:55.720000 seconds. And yes, you will want to memorize those timers, especially if 0:02:55.720000 --> 0:02:57.380000 you're taking any exam. 0:02:57.380000 --> 0:03:03.020000 Now, it's more than just a simple one to exchange of hellos for routers 0:03:03.020000 --> 0:03:06.040000 to say, oh, okay, let me share my information with you. 0:03:06.040000 --> 0:03:10.280000 In some protocols like EIGRP, it is about that simple. 0:03:10.280000 --> 0:03:14.440000 But in OSPF, there's about five or six different steps a router has to 0:03:14.440000 --> 0:03:18.380000 go through before it can become fully synchronized with another neighbor 0:03:18.380000 --> 0:03:20.720000 and have all the information. 0:03:20.720000 --> 0:03:26.720000 OSPF works based on an area hierarchy which minimizes LSA flooding. 0:03:26.720000 --> 0:03:28.000000 What is that talking about? 0:03:28.000000 --> 0:03:35.180000 All right. So in the world of OSPF, when I generate information that I'm 0:03:35.180000 --> 0:03:40.000000 going to give to you, let's say you are my OSPF neighbor, my primary objective 0:03:40.000000 --> 0:03:45.280000 in generating this packet called a link state advertisement in LSA is 0:03:45.280000 --> 0:03:47.600000 not to give you routing information. 0:03:47.600000 --> 0:03:49.400000 That's not the primary purpose of it. 0:03:49.400000 --> 0:03:53.240000 The primary purpose of it is to give you descriptive information about 0:03:53.240000 --> 0:03:58.560000 me, you know, my name, what links I have, what the cost is of those links, 0:03:58.560000 --> 0:03:59.700000 what type of links they are. 0:03:59.700000 --> 0:04:00.720000 Are they point to point? 0:04:00.720000 --> 0:04:02.720000 Are they something else? 0:04:02.720000 --> 0:04:06.840000 Because the goal is I want to give you enough information that you can 0:04:06.840000 --> 0:04:11.960000 plug me into your map that you know about me, you know who I'm connected 0:04:11.960000 --> 0:04:14.200000 to because I'm telling you that. 0:04:14.200000 --> 0:04:18.600000 And so all these routers will create link state advertisements and flood 0:04:18.600000 --> 0:04:22.600000 them. So the link state advertisement I give to you, you will put it in 0:04:22.600000 --> 0:04:26.340000 your database, you'll store it, but you'll also copy it and keep it going 0:04:26.340000 --> 0:04:29.680000 to the neighbor behind you and the neighbor behind him. 0:04:29.680000 --> 0:04:35.880000 So within OSPF area, everybody is going to learn about everything. 0:04:35.880000 --> 0:04:37.900000 Now, what's this concept of areas? 0:04:37.900000 --> 0:04:39.960000 Let's just do a quick review of that. 0:04:39.960000 --> 0:04:46.520000 So one of the rules of OSPF is that there are several different kinds 0:04:46.520000 --> 0:04:52.360000 of LSA's. And at the CCNA level, you don't have to know the names and 0:04:52.360000 --> 0:04:55.920000 numbers and functionality of every single kind of LSA. 0:04:55.920000 --> 0:04:59.440000 But the fundamental one you should know about, which I've mentioned in 0:04:59.440000 --> 0:05:05.880000 other videos, is called the type one router LSA. 0:05:05.880000 --> 0:05:08.680000 This is like what I was just describing. 0:05:08.680000 --> 0:05:12.460000 The main purpose of the router LSA is so that all the routers can learn 0:05:12.460000 --> 0:05:17.580000 about me, they can learn my links, they can learn who I'm connected to, 0:05:17.580000 --> 0:05:21.300000 and then by using that, and the LSA's they get from other routers, they 0:05:21.300000 --> 0:05:23.560000 can piece together a jigsaw puzzle. 0:05:23.560000 --> 0:05:27.160000 Well, one of the rules of OSPF says, let's say that your jigsaw puzzle 0:05:27.160000 --> 0:05:31.080000 is complete. You know the full topology, you've figured out the full map 0:05:31.080000 --> 0:05:34.260000 or what we call the tree of where everybody is and how they're connected 0:05:34.260000 --> 0:05:37.420000 to in your CPU in your mind. 0:05:37.420000 --> 0:05:40.940000 And then all of a sudden, you get a new router LSA. 0:05:40.940000 --> 0:05:45.060000 That topology has been nice and stable for the last day or week or month, 0:05:45.060000 --> 0:05:47.760000 and all of a sudden, now a new LSA comes in. 0:05:47.760000 --> 0:05:51.460000 Maybe about a brand new router, maybe about an existing router whose link 0:05:51.460000 --> 0:05:57.080000 has gone down. Well, the rules of OSPF say, if I get a new type one router 0:05:57.080000 --> 0:06:02.280000 LSA, I have to break apart my tree, which took a lot of memory and CPU 0:06:02.280000 --> 0:06:04.960000 resources to build in the first place. 0:06:04.960000 --> 0:06:07.500000 Now think about a jigsaw puzzle, it's not easy to build those things, 0:06:07.500000 --> 0:06:12.740000 right? You got to tear that thing apart and then rebuild it again, factoring 0:06:12.740000 --> 0:06:14.980000 in that new LSA that you just got. 0:06:14.980000 --> 0:06:20.200000 So the result of this is, if there's any instability in your network, 0:06:20.200000 --> 0:06:23.460000 if there's any link or link somewhere that are flapping going down, going 0:06:23.460000 --> 0:06:29.240000 up, going down, going up, that's going to be intrusive on all the OSPF 0:06:29.240000 --> 0:06:32.800000 routers, because every time it goes down, a new LSA goes out. 0:06:32.800000 --> 0:06:35.320000 So the router that had that link that just went down, he has to create 0:06:35.320000 --> 0:06:38.200000 a new router LSA saying, hey, everybody remember me? 0:06:38.200000 --> 0:06:42.520000 This link I told you about, it's gone, tear apart your tree, rebuild it, 0:06:42.520000 --> 0:06:43.860000 factoring that information. 0:06:43.860000 --> 0:06:45.780000 Link comes up. Hey, everybody remember about me? 0:06:45.780000 --> 0:06:49.880000 That link just came up and you can see that would be very intrusive. 0:06:49.880000 --> 0:06:53.980000 So the developers of OSPF who created this, they said, we're going to 0:06:53.980000 --> 0:06:56.040000 give people some options right here. 0:06:56.040000 --> 0:07:01.060000 So what we're going to do is we're going to say that when you configure 0:07:01.060000 --> 0:07:07.320000 OSPF that a link, which is basically a cable and network interface, is 0:07:07.320000 --> 0:07:10.160000 considered to be in an area. 0:07:10.160000 --> 0:07:15.740000 Now all the routers, go ahead and draw this right here, you could, if 0:07:15.740000 --> 0:07:20.480000 you wanted to, design your network in such a way that all the routers 0:07:20.480000 --> 0:07:27.120000 and all their links like this right here, we're all in one big area. 0:07:27.120000 --> 0:07:28.720000 You could do that. 0:07:28.720000 --> 0:07:30.600000 That's perfectly fine. 0:07:30.600000 --> 0:07:33.760000 But OSPF does require that you select an area number. 0:07:33.760000 --> 0:07:37.460000 So if you just have one area, that can be any area you want. 0:07:37.460000 --> 0:07:40.200000 I could make that one area area 17. 0:07:40.200000 --> 0:07:45.100000 Now it's probably a good design practice to make it area zero. 0:07:45.100000 --> 0:07:47.400000 And I'll talk about why that isn't just a second. 0:07:47.400000 --> 0:07:50.980000 But the takeaway from here is you could put all your links in one area 0:07:50.980000 --> 0:07:54.240000 if you wanted to, and that area number could be the same. 0:07:54.240000 --> 0:07:57.360000 So one thing that's very critical is that when two routers are exchanging 0:07:57.360000 --> 0:08:01.920000 LSA's, they're going to tell each other what area they believe their link 0:08:01.920000 --> 0:08:07.100000 is in. If they don't agree on that, they will not become OSPF neighbors. 0:08:07.100000 --> 0:08:09.900000 They have to agree that that link is in a common area. 0:08:09.900000 --> 0:08:12.560000 Now here's the downside. 0:08:12.560000 --> 0:08:15.880000 If I put all my routers in one area like this, and hey, if I've only got 0:08:15.880000 --> 0:08:18.720000 five or six routers like this, go for it. 0:08:18.720000 --> 0:08:20.040000 It's not going to hurt anything. 0:08:20.040000 --> 0:08:24.760000 But imagine if I had 100 routers, well, now we've got a greater likelihood 0:08:24.760000 --> 0:08:28.720000 of instability. The more routers, the more links you have, the greater 0:08:28.720000 --> 0:08:32.460000 likelihood that somewhere, some link is going to flap and go down, which 0:08:32.460000 --> 0:08:34.940000 is going to cause a problem for everybody. 0:08:34.940000 --> 0:08:39.660000 So the developers of OSPF said, hey, why don't you do this instead? 0:08:39.660000 --> 0:08:44.280000 If you want to, and if you think there might be some instability, instead 0:08:44.280000 --> 0:08:50.060000 of putting all your routers in one big area, why don't you subdivide it 0:08:50.060000 --> 0:08:52.580000 into two or more areas? 0:08:52.580000 --> 0:08:53.900000 So why don't you do something like this? 0:08:53.900000 --> 0:08:58.960000 Why don't we say that these links are going to be in one area, and we 0:08:58.960000 --> 0:09:04.640000 can say that these other links right here are in a different area. 0:09:04.640000 --> 0:09:08.940000 So now, if there's a difference between the two any instability in this 0:09:08.940000 --> 0:09:13.020000 area here, this blue area, it's only going to affect those four routers. 0:09:13.020000 --> 0:09:16.680000 It will not affect the two routers on the left because those type one 0:09:16.680000 --> 0:09:20.680000 router LSAs I talked to you about that's used to build the topology, those 0:09:20.680000 --> 0:09:25.020000 are only allowed to be flooded within an area. 0:09:25.020000 --> 0:09:26.660000 They can go no further. 0:09:26.660000 --> 0:09:28.580000 So they're going to be flooded around through here. 0:09:28.580000 --> 0:09:34.720000 But once they get to this guy who's bordering two areas, they stop at 0:09:34.720000 --> 0:09:38.920000 him. So if there's any instability, it will only affect the CPUs of the 0:09:38.920000 --> 0:09:40.740000 routers in that blue area. 0:09:40.740000 --> 0:09:43.100000 It will not affect the routers in the purple area. 0:09:43.100000 --> 0:09:46.000000 Now, you might be wondering, well, wait a second, if those LSAs only get 0:09:46.000000 --> 0:09:49.320000 to that guy and they stop, how is the purple area ever going to learn 0:09:49.320000 --> 0:09:52.660000 about the routes in the blue area? 0:09:52.660000 --> 0:09:56.420000 And vice versa. Well, there's a different kind of an LSA that is doing 0:09:56.420000 --> 0:10:00.140000 that. There's a special kind of an LSA that this guy right here creates 0:10:00.140000 --> 0:10:04.500000 to advertise routes from one area into another area. 0:10:04.500000 --> 0:10:08.280000 But that LSA does not affect the tree. 0:10:08.280000 --> 0:10:11.580000 Those types of LSAs can come and go all day long. 0:10:11.580000 --> 0:10:14.440000 And if your tree is nice and stable, it won't affect those. 0:10:14.440000 --> 0:10:17.400000 It does not. So when you receive those LSAs, you don't have to break apart 0:10:17.400000 --> 0:10:19.560000 your tree to account for those. 0:10:19.560000 --> 0:10:23.300000 Only for these type one router LSAs do you have to take that drastic action. 0:10:23.300000 --> 0:10:29.220000 Now, when you do have two or more areas, now we do have a restriction. 0:10:29.220000 --> 0:10:32.820000 One of those areas has to be the backbone area, which is what we call 0:10:32.820000 --> 0:10:38.380000 area zero. The other area, it can be whatever you want. 0:10:38.380000 --> 0:10:43.900000 Area one. So usually what I tell people is, if you're going to design 0:10:43.900000 --> 0:10:48.680000 an OSPA network that has two or more areas, you should think of your design 0:10:48.680000 --> 0:10:53.660000 as sort of representing like a flower, like this. 0:10:53.660000 --> 0:11:00.620000 Okay, where this is area zero, and these are all different areas, you 0:11:00.620000 --> 0:11:04.080000 can make them whatever numbers you want. 0:11:04.080000 --> 0:11:10.900000 And you have these routers right here that are connecting those areas. 0:11:10.900000 --> 0:11:14.960000 You could have one router connecting both like that. 0:11:14.960000 --> 0:11:21.260000 And these routers, a router that has a link in area zero and a link in 0:11:21.260000 --> 0:11:24.880000 something else, we call those area border routers. 0:11:24.880000 --> 0:11:29.120000 They're performing that special job of creating these special kinds of 0:11:29.120000 --> 0:11:32.500000 LSAs that advertise routes from one area into the other. 0:11:32.500000 --> 0:11:34.920000 Only area border routers can do that. 0:11:34.920000 --> 0:11:39.360000 Now, you see, if I forgot about that rule, and I had another area out 0:11:39.360000 --> 0:11:44.460000 here, like area 55, and here was a router right there with a link in area 0:11:44.460000 --> 0:11:48.640000 55 and a link in area 18, that would not be an area border router. 0:11:48.640000 --> 0:11:51.540000 He would know that he's not an area border router because he would say, 0:11:51.540000 --> 0:11:53.620000 I don't have any links in area zero. 0:11:53.620000 --> 0:11:57.540000 So he would learn about the routes in area 55, but he would not be allowed 0:11:57.540000 --> 0:12:01.000000 to forward those routes into area 18. 0:12:01.000000 --> 0:12:03.520000 He couldn't do that because it's not his job. 0:12:03.520000 --> 0:12:08.520000 So that's why it's a better idea when designing an OSPF network to design 0:12:08.520000 --> 0:12:12.620000 it in like this flower type of shape here, where each pedal is connecting 0:12:12.620000 --> 0:12:14.860000 into area zero in the center. 0:12:14.860000 --> 0:12:22.760000 So as this slide mentions here, minimizes LSA flooding. 0:12:22.760000 --> 0:12:26.240000 And this is one of the big benefits of dividing your network into multiple 0:12:26.240000 --> 0:12:30.940000 areas, because if there's any instability in one area, the LSAs that are 0:12:30.940000 --> 0:12:36.120000 flooded that carry that instability are not propagated into other areas. 0:12:36.120000 --> 0:12:45.080000 So the metric that OSPF uses, so when an OSPF router learns via LSAs, 0:12:45.080000 --> 0:12:50.220000 two or more paths to the exact same network, it has to use a metric to 0:12:50.220000 --> 0:12:51.240000 figure out the best route. 0:12:51.240000 --> 0:12:55.140000 And we know that in all routing protocols, the lower the metric, the better 0:12:55.140000 --> 0:12:59.220000 the route. Well, the metric that OSPF uses is cost. 0:12:59.220000 --> 0:13:00.060000 That's the name of it. 0:13:00.060000 --> 0:13:04.440000 And like spanning tree, OSPF cost is based on interface bandwidth. 0:13:04.440000 --> 0:13:09.100000 The greater the bandwidth or the faster the interface, the lower the cost. 0:13:09.100000 --> 0:13:15.760000 So the best path to any given network is the path with the lowest cumulative 0:13:15.760000 --> 0:13:19.840000 cost, just like spanning tree decides its best path to get to the root 0:13:19.840000 --> 0:13:22.580000 bridge. Now the formulas are different. 0:13:22.580000 --> 0:13:26.540000 Actually, the formula for OSPF is actually very easy to remember. 0:13:26.540000 --> 0:13:35.660000 The formula for OSPF is you just take 100 million and divide it by the 0:13:35.660000 --> 0:13:42.160000 interface bandwidth in bits per second. 0:13:42.160000 --> 0:13:49.340000 So for example, if I had a fast ethernet interface, well, fast ethernet 0:13:49.340000 --> 0:13:52.100000 is 100 million bits per second. 0:13:52.100000 --> 0:13:53.260000 That's fast ethernet. 0:13:53.260000 --> 0:13:58.200000 So we would end up having 100 million divided by our bandwidth, which 0:13:58.200000 --> 0:14:02.940000 is also 100 million, which would give us a cost of one. 0:14:02.940000 --> 0:14:05.560000 That's the cost of a fast ethernet interface. 0:14:05.560000 --> 0:14:14.780000 Slower interfaces would have bigger costs. 0:14:14.780000 --> 0:14:18.860000 And costs can be configured explicitly or by affecting or by changing 0:14:18.860000 --> 0:14:20.800000 the interface bandwidth. 0:14:20.800000 --> 0:14:27.140000 So you can go, I'll go ahead and show you right here, go on to a router. 0:14:27.140000 --> 0:14:33.260000 Now this guy is not running OSPF yet, but if I go on to an interface, 0:14:33.260000 --> 0:14:38.540000 for example, if I do show interface gigabit zero slash one. 0:14:38.540000 --> 0:14:43.840000 Okay, see how this interface has a bandwidth? 0:14:43.840000 --> 0:14:46.480000 Now this is in kilobits per second. 0:14:46.480000 --> 0:14:51.120000 So we'd have to add three zeros to this to get the actual bandwidth in 0:14:51.120000 --> 0:14:52.460000 bits per second. 0:14:52.460000 --> 0:14:59.580000 But this number with three extra zeros, as the, I always forget as the 0:14:59.580000 --> 0:15:02.780000 denominator, the one on the bottom, or the numerator, the one, anyway, 0:15:02.780000 --> 0:15:05.980000 in a division, the one on the bottom, that's where the bandwidth would 0:15:05.980000 --> 0:15:10.700000 go. So if we have 100 million divided by this, that's what your OSPF cost 0:15:10.700000 --> 0:15:14.420000 would be. As a matter of fact, let's just go ahead and do that. 0:15:14.420000 --> 0:15:16.960000 Interface gig zero slash one. 0:15:16.960000 --> 0:15:19.620000 Do we have an IP address on there? 0:15:19.620000 --> 0:15:21.620000 I don't think so. 0:15:21.620000 --> 0:15:29.240000 So let's just put one IP address. 0:15:29.240000 --> 0:15:32.380000 All right, IP OSPF one area zero. 0:15:32.380000 --> 0:15:34.640000 We'll get to the commands for this in just a second. 0:15:34.640000 --> 0:15:37.740000 I don't want you to focus on the command just yet. 0:15:37.740000 --> 0:15:39.560000 But this is what I want you to see. 0:15:39.560000 --> 0:15:50.400000 So when I do the commands show IP OSPF interface for that interface, it 0:15:50.400000 --> 0:15:54.100000 shows me right here the cost is one. 0:15:54.100000 --> 0:15:57.100000 Now you said, but wait a second, Keith, you said a fast ethernet was one. 0:15:57.100000 --> 0:15:58.040000 This is gigabit. 0:15:58.040000 --> 0:15:59.740000 Well, here's the problem. 0:15:59.740000 --> 0:16:02.220000 Fast ethernet is one, right? 0:16:02.220000 --> 0:16:04.200000 Fast ethernet is 100 million bits per second. 0:16:04.200000 --> 0:16:09.780000 And actually, everything from fast ethernet on up, gigabit, 10 gigabit, 0:16:09.780000 --> 0:16:13.520000 40 gigabit, they're all going to come up with a cost of one as far as 0:16:13.520000 --> 0:16:15.240000 OSPF is concerned. 0:16:15.240000 --> 0:16:19.140000 Now there is a command you can use to change that, but just know that 0:16:19.140000 --> 0:16:20.900000 that's the default behavior. 0:16:20.900000 --> 0:16:22.360000 So the cost is one. 0:16:22.360000 --> 0:16:24.560000 Now watch how I can change this. 0:16:24.560000 --> 0:16:27.680000 I can go back onto that interface. 0:16:27.680000 --> 0:16:30.340000 One thing I could do is I could change the bandwidth. 0:16:30.340000 --> 0:16:34.460000 So let's give it some really low bandwidth. 0:16:34.460000 --> 0:16:38.860000 How about 128 kilobits per second? 0:16:38.860000 --> 0:16:41.580000 It's almost like a dial up line at this point. 0:16:41.580000 --> 0:16:48.520000 Now if I do show IP OSPF interface gigabit zero slash one, you need to 0:16:48.520000 --> 0:16:50.280000 proceed that with a do. 0:16:50.280000 --> 0:16:58.280000 Look at that. Now it increased the cost to 781 because I made the bandwidth 0:16:58.280000 --> 0:17:01.600000 a lot slower. It really bumped up the cost. 0:17:01.600000 --> 0:17:07.900000 So this interface is not as good from the perspective of OSPF anymore. 0:17:07.900000 --> 0:17:14.800000 Or I could actually use this command, IP OSPF cost and then explicitly 0:17:14.800000 --> 0:17:17.940000 give it a cost. How about 27? 0:17:17.940000 --> 0:17:22.200000 So now he's no longer looking at the bandwidth at all. 0:17:22.200000 --> 0:17:30.380000 Now he's looking directly at what I did. 0:17:30.380000 --> 0:17:33.560000 So that's what that is talking about there at the bottom that you can 0:17:33.560000 --> 0:17:38.200000 configure explicitly or or modify the bandwidth command to have that cost 0:17:38.200000 --> 0:17:42.620000 changed. All right. 0:17:42.620000 --> 0:17:46.040000 There are several different OSPF packet types that OSPF uses to accomplish 0:17:46.040000 --> 0:17:49.460000 its goals. We talked about the OSPF hello packet. 0:17:49.460000 --> 0:17:54.600000 So this is actually also showing the packets are exchanged when we're 0:17:54.600000 --> 0:17:56.020000 building a neighbor relationship. 0:17:56.020000 --> 0:17:58.440000 So let's just walk through this process. 0:17:58.440000 --> 0:18:02.480000 So first thing that happens is you and I, let's just say you're a router 0:18:02.480000 --> 0:18:05.760000 and I'm a router and we're directly connected across a link. 0:18:05.760000 --> 0:18:09.120000 First thing is we have to exchange hello packets and there's some parameters 0:18:09.120000 --> 0:18:11.720000 in those hello packets that have to match. 0:18:11.720000 --> 0:18:14.260000 And we'll talk about that in just a moment. 0:18:14.260000 --> 0:18:18.400000 Assuming those parameters match, then we exchange these things called 0:18:18.400000 --> 0:18:20.240000 database descriptor packets. 0:18:20.240000 --> 0:18:22.360000 What is that and why do we do it? 0:18:22.360000 --> 0:18:28.400000 Well, our objective here, our objective is that if you and I are in the 0:18:28.400000 --> 0:18:32.320000 same area, which we need to be otherwise we're not going to be neighbors, 0:18:32.320000 --> 0:18:37.680000 ultimately, whatever LSAs you have in your database have to match mine 0:18:37.680000 --> 0:18:41.280000 and vice versa. We call that synchronization. 0:18:41.280000 --> 0:18:45.300000 You and I need to be synchronized as far as our LSA databases are concerned. 0:18:45.300000 --> 0:18:48.500000 Well, at the beginning, we don't know what each other has and we don't 0:18:48.500000 --> 0:18:50.560000 know for missing some stuff. 0:18:50.560000 --> 0:18:54.420000 So the purpose of the database descriptor exchange and that's a type of 0:18:54.420000 --> 0:18:59.440000 OSPF packet is for me to give you like a high level overview of the LSA 0:18:59.440000 --> 0:19:00.340000 as I've got in my database. 0:19:00.340000 --> 0:19:04.240000 So in my database descriptor, I might say, hey, I have an OSPF LSA from 0:19:04.240000 --> 0:19:06.780000 John with sequence number two. 0:19:06.780000 --> 0:19:11.280000 I got an OSPF LSA from Sally with sequence number 17 and so on and so 0:19:11.280000 --> 0:19:14.360000 forth. And then you'll give me your database descriptors and by the time 0:19:14.360000 --> 0:19:18.380000 we're done exchanging database descriptors, we will know if we're not 0:19:18.380000 --> 0:19:21.140000 synchronized, if we're missing something, something. 0:19:21.140000 --> 0:19:24.100000 We probably are, we're probably not synchronized. 0:19:24.100000 --> 0:19:27.440000 So then we're going to go into link state requests, link state updates 0:19:27.440000 --> 0:19:29.540000 and link state acknowledgments. 0:19:29.540000 --> 0:19:32.700000 So I will request from you, I'll say, hey, you gave me a database descriptor 0:19:32.700000 --> 0:19:37.240000 and you said you had an LSA from Juan with a sequence number of 52. 0:19:37.240000 --> 0:19:38.580000 I'm requesting that. 0:19:38.580000 --> 0:19:40.300000 Can you please give that to me? 0:19:40.300000 --> 0:19:44.120000 Then you'll give it to me in the form of a link state update. 0:19:44.120000 --> 0:19:48.260000 So an OSPF link state update is the actual name of the OSPF packet type 0:19:48.260000 --> 0:19:52.320000 that has LSAs inside of it. 0:19:52.320000 --> 0:19:55.860000 People who are first learning about OSPF, a lot of times they make the 0:19:55.860000 --> 0:20:00.620000 common mistake that they think that an OSPF LSA is the name of a packet 0:20:00.620000 --> 0:20:02.920000 that, oh, we exchange LSAs. 0:20:02.920000 --> 0:20:08.640000 No, we exchange link state updates and in the body of the link state update, 0:20:08.640000 --> 0:20:12.580000 it has LSA information inside there. 0:20:12.580000 --> 0:20:16.500000 For every link state update that I send you, you have to acknowledge it 0:20:16.500000 --> 0:20:18.800000 and we use link state acknowledgments for that. 0:20:18.800000 --> 0:20:23.100000 So OSPF does not use UDP. 0:20:23.100000 --> 0:20:24.760000 It does not use TCP. 0:20:24.760000 --> 0:20:27.140000 It is its own IP protocol. 0:20:27.140000 --> 0:20:30.300000 So just like you could see that the IP header and then normally after 0:20:30.300000 --> 0:20:33.860000 the IP header, you'd have a TCP header or a UDP header. 0:20:33.860000 --> 0:20:37.440000 Well, if it's carrying OSPF, right after the IP header, you've got your 0:20:37.440000 --> 0:20:39.960000 OSPF header and your OSPF data. 0:20:39.960000 --> 0:20:44.240000 And yet because OSPF has link state acknowledgments, it has some connection 0:20:44.240000 --> 0:20:48.260000 oriented properties similar to what TCP has. 0:20:48.260000 --> 0:20:53.140000 So eventually, after we finish doing our link state updates and link state 0:20:53.140000 --> 0:20:57.280000 acknowledgments, we'll reach this place of Nirvana where we say, hey, 0:20:57.280000 --> 0:20:58.700000 we are synchronized. 0:20:58.700000 --> 0:21:02.540000 You've got the exact same sets of LSAs that I have and now our neighbor 0:21:02.540000 --> 0:21:06.480000 relationship will be complete. 0:21:06.480000 --> 0:21:12.500000 Most OSPF packets are sent as multicasts and you need to know what multicasted 0:21:12.500000 --> 0:21:15.200000 dresses are reserved for OSPF. 0:21:15.200000 --> 0:21:26.860000 So in the world of IP version four, most OSPF packets go to 224.0.0.5. 0:21:26.860000 --> 0:21:31.620000 Some of them go to 224.0.0.6. 0:21:31.620000 --> 0:21:36.480000 So those two addresses, those class D multicast addresses are reserved 0:21:36.480000 --> 0:21:41.960000 for OSPF. Now there is an equivalent of OSPF for IPV6. 0:21:41.960000 --> 0:21:48.580000 For IPV4, it's actually OSPF version two. 0:21:48.580000 --> 0:21:52.800000 For IPV6, it's OSPF version three. 0:21:52.800000 --> 0:21:55.980000 OSPF version three uses FF. 0:21:55.980000 --> 0:21:59.400000 Remember FF is multicast in the world of IPV6. 0:21:59.400000 --> 0:22:10.880000 FF02 colon colon five and FF02 colon six. 0:22:10.880000 --> 0:22:11.020000 So you can see that there are also some things that are in the world of 0:22:11.020000 --> 0:22:12.380000 IPV6. So this is the one that you can see between the addresses. 0:22:12.380000 --> 0:22:21.820000 Network types. All right, this is also something that's unique to OSPF. 0:22:21.820000 --> 0:22:27.480000 So OSPF classifies links based on the layer two encapsulation. 0:22:27.480000 --> 0:22:32.920000 So the moment you tell OSPF via a command, hey, this fast ethernet link, 0:22:32.920000 --> 0:22:34.420000 I want you to start working on it. 0:22:34.420000 --> 0:22:36.880000 This gigabit link, I want you to start working on it. 0:22:36.880000 --> 0:22:39.680000 The first thing OSPF is going to do, it's going to look at the layer two 0:22:39.680000 --> 0:22:41.880000 encapsulation on that link. 0:22:41.880000 --> 0:22:43.680000 So it's going to say, oh, you want me to operate there. 0:22:43.680000 --> 0:22:44.460000 All right, what are you doing? 0:22:44.460000 --> 0:22:50.660000 Are you doing PPP, HDLC, Ethernet, frame relay, what have you got on that 0:22:50.660000 --> 0:22:52.740000 interface as far as encapsulation? 0:22:52.740000 --> 0:22:57.560000 And by getting the answer to that question, OSPF categorizes that link 0:22:57.560000 --> 0:23:01.640000 as one of about four or five different network types, which we're going 0:23:01.640000 --> 0:23:03.280000 to look at here in the next slide. 0:23:03.280000 --> 0:23:04.460000 Now, why does it do that? 0:23:04.460000 --> 0:23:07.360000 Why is it care what the network type is? 0:23:07.360000 --> 0:23:11.780000 Well, because OSPF operates a little bit differently depending on what 0:23:11.780000 --> 0:23:14.440000 network type it's connected to. 0:23:14.440000 --> 0:23:18.540000 As it says things, as it says here, the network type will determine things 0:23:18.540000 --> 0:23:21.800000 like, can I dynamically discover a neighbor across that link? 0:23:21.800000 --> 0:23:25.640000 If the answer is no, based on that particular type of network, that means 0:23:25.640000 --> 0:23:27.820000 you have to statically configure your neighbor. 0:23:27.820000 --> 0:23:35.820000 You can see here, what's subnet mask is used to identify the link. 0:23:35.820000 --> 0:23:39.140000 On some types of links, OSPF will just pick up whatever the subnet mask 0:23:39.140000 --> 0:23:41.020000 is that you've configured on the interface. 0:23:41.020000 --> 0:23:47.160000 If you configured 1111 slash 24, it'll advertise that link as slash 24. 0:23:47.160000 --> 0:23:51.600000 But other types of links like loopback interfaces, it doesn't matter what 0:23:51.600000 --> 0:23:53.100000 subnet mask you select. 0:23:53.100000 --> 0:23:55.840000 It'll just always advertise that as a slash 32. 0:23:55.840000 --> 0:23:58.680000 So that's another reason why it needs to identify the link type. 0:23:58.680000 --> 0:24:02.740000 Is there a need for a designated router and a backup designated router 0:24:02.740000 --> 0:24:07.400000 election? Some link types need that, some links types don't. 0:24:07.400000 --> 0:24:12.200000 And the link type can be manually changed on most interface types. 0:24:12.200000 --> 0:24:18.580000 So for example, if I go back to here, notice that when I enabled OSPF 0:24:18.580000 --> 0:24:22.600000 on gig 01, it says, all right, based on the fact that this is running 0:24:22.600000 --> 0:24:27.300000 ethernet, gigabit ethernet, I'm going to categorize this as a network 0:24:27.300000 --> 0:24:29.440000 type of broadcast. 0:24:29.440000 --> 0:24:30.960000 But I can change that. 0:24:30.960000 --> 0:24:37.600000 I can tell OSPF, no, I want you to look at it a little bit differently. 0:24:37.600000 --> 0:24:42.560000 IP OSPF network, I can choose from these right here. 0:24:42.560000 --> 0:24:48.140000 Why don't you look at that as a point-to-point network instead? 0:24:48.140000 --> 0:24:51.220000 And now he's no longer looking at the encapsulation type. 0:24:51.220000 --> 0:24:56.320000 He's looking exactly what I told him and actually sees it as point-to 0:24:56.320000 --> 0:25:02.080000 -point. Now, at the CCNA level, you don't really need to know that command. 0:25:02.080000 --> 0:25:06.140000 You don't really need to know why you would change the network type. 0:25:06.140000 --> 0:25:10.880000 Just know that OSPF categorizes links based on their network type. 0:25:10.880000 --> 0:25:14.360000 And that is something you can change via configuration should you want 0:25:14.360000 --> 0:25:24.560000 to do that. At the CCNP level, you get formally learn that command. 0:25:24.560000 --> 0:25:28.880000 All right, these are the network types. 0:25:28.880000 --> 0:25:33.520000 And like I said, it's primarily based off the type of encapsulation that 0:25:33.520000 --> 0:25:39.120000 is on there. So for the purposes of any Cisco exam or maybe other person, 0:25:39.120000 --> 0:25:43.280000 other companies exam, you'll want to memorize what's on this slide right 0:25:43.280000 --> 0:25:49.120000 here. Notice that point-to-multipoint is the only type of network that 0:25:49.120000 --> 0:25:52.620000 OSPF cannot dynamically determine. 0:25:52.620000 --> 0:25:54.780000 That's not based on any encapsulation type. 0:25:54.780000 --> 0:25:57.460000 That's something you'd have to manually configure. 0:25:57.460000 --> 0:26:02.340000 So once you, if you Google OSPF-point -to-multipoint and you start learning 0:26:02.340000 --> 0:26:05.540000 about the benefits of it, and you decide, oh, yeah, that's actually something 0:26:05.540000 --> 0:26:07.600000 I could use. I want to do that. 0:26:07.600000 --> 0:26:11.760000 That would require a configuration command on an interface to tell OSPF 0:26:11.760000 --> 0:26:14.040000 that that's what you wanted to do. 0:26:14.040000 --> 0:26:17.600000 And loopback, well, that's clearly for loopback interfaces. 0:26:17.600000 --> 0:26:22.980000 All right, I mentioned that you and I exchanging hellos is the beginning 0:26:22.980000 --> 0:26:26.800000 of the process. I said there are certain things within those hellos that 0:26:26.800000 --> 0:26:30.900000 have to match between us if we're going to take the next step and exchange 0:26:30.900000 --> 0:26:33.220000 those database descriptor packets. 0:26:33.220000 --> 0:26:34.720000 What are those things? 0:26:34.720000 --> 0:26:36.260000 Well, that's what we have right here. 0:26:36.260000 --> 0:26:38.780000 So our timers have to be the same. 0:26:38.780000 --> 0:26:43.400000 Like I said, usually those timers are going to be 10 seconds for the hello 0:26:43.400000 --> 0:26:46.640000 and 40 seconds for the dead interval. 0:26:46.640000 --> 0:26:51.080000 If I change those on my interface, whatever I change them to has to match 0:26:51.080000 --> 0:26:52.800000 on your interface. 0:26:52.800000 --> 0:26:55.380000 Otherwise, we will not become neighbors. 0:26:55.380000 --> 0:26:58.460000 OSPF does have the ability to use authentication. 0:26:58.460000 --> 0:27:03.120000 You know, this is a way to prevent rogue unauthorized routers from connecting 0:27:03.120000 --> 0:27:07.620000 to your network and doing funky stuff with your routing tables. 0:27:07.620000 --> 0:27:09.320000 Configure authentication. 0:27:09.320000 --> 0:27:13.040000 Those rogue routers will not know what the password is. 0:27:13.040000 --> 0:27:14.860000 And so we'll just ignore them. 0:27:14.860000 --> 0:27:19.140000 So the authentication type and the authentication password have to match. 0:27:19.140000 --> 0:27:19.980000 That's optional. 0:27:19.980000 --> 0:27:21.700000 You don't have to do it. 0:27:21.700000 --> 0:27:23.520000 I mentioned the area ID. 0:27:23.520000 --> 0:27:26.600000 If you and I are directly connected across a link, we have to agree what 0:27:26.600000 --> 0:27:27.980000 area this link is in. 0:27:27.980000 --> 0:27:30.640000 The prefix length. 0:27:30.640000 --> 0:27:35.340000 So what that's talking about is, you know, maybe here's me. 0:27:35.340000 --> 0:27:39.300000 Here's you. We're connected across a link. 0:27:39.300000 --> 0:27:43.600000 We both agree that this link is an area one, let's say. 0:27:43.600000 --> 0:27:48.380000 I'm configured as 1.1.1.1.1.1.1.2. 0:27:48.380000 --> 0:27:51.100000 But somebody screwed up. 0:27:51.100000 --> 0:27:54.860000 And on my side, I'm configured with this subnet mask. 0:27:54.860000 --> 0:27:59.800000 On your side, you're configured with this subnet mask. 0:27:59.800000 --> 0:28:02.540000 That is actually carried inside the hello packet. 0:28:02.540000 --> 0:28:06.280000 And we will see that our subnet masks, otherwise known here as prefix 0:28:06.280000 --> 0:28:08.480000 length, are not the same. 0:28:08.480000 --> 0:28:11.680000 And if they're not the same, we can't be neighbors. 0:28:11.680000 --> 0:28:13.940000 That's clearly a configuration error, though. 0:28:13.940000 --> 0:28:16.260000 You'd want to fix that. 0:28:16.260000 --> 0:28:18.720000 And the stub area flag. 0:28:18.720000 --> 0:28:22.520000 At the CCI level, you don't really learn what stub areas are. 0:28:22.520000 --> 0:28:27.500000 Stub areas are special types of areas that say certain types of LSA's 0:28:27.500000 --> 0:28:29.120000 are not allowed in here. 0:28:29.120000 --> 0:28:32.960000 We're going to allow some LSAs, but other special types of LSAs keep those 0:28:32.960000 --> 0:28:37.520000 out. So if you configure an area as a stub area, all the routers in that 0:28:37.520000 --> 0:28:40.860000 area have to know that, and they have to agree to that. 0:28:40.860000 --> 0:28:47.740000 All right, so we've already talked about this. 0:28:47.740000 --> 0:28:49.980000 There's nothing new here. 0:28:49.980000 --> 0:28:57.000000 What this is referring to is, you know, once again, like I just said a 0:28:57.000000 --> 0:29:02.340000 couple minutes ago, once I enable OSPF on myself on at least one interface, 0:29:02.340000 --> 0:29:06.800000 I'm going to create a link state advertisement describing myself and that 0:29:06.800000 --> 0:29:10.060000 interface and any neighbors I have on the interface. 0:29:10.060000 --> 0:29:12.540000 All right, I own that LSA. 0:29:12.540000 --> 0:29:13.720000 I send it out there. 0:29:13.720000 --> 0:29:17.600000 It gets flooded to all the routers in the same area as me. 0:29:17.600000 --> 0:29:20.340000 That LSA is not good forever. 0:29:20.340000 --> 0:29:21.700000 It has a lifetime. 0:29:21.700000 --> 0:29:26.260000 The lifetime of LSAs by default is one hour, 60 minutes. 0:29:26.260000 --> 0:29:30.680000 So if you never see that LSA from me again, after 60 minutes, you will 0:29:30.680000 --> 0:29:35.040000 purge it. And whatever routes were generated as a result of that LSA, 0:29:35.040000 --> 0:29:37.500000 you'll strip those routes out of your routing table. 0:29:37.500000 --> 0:29:42.460000 So the originator of the LSA, which is me, I'm going to refresh that every 0:29:42.460000 --> 0:29:45.660000 30 minutes. So even if nothing changes and nothing's happened with this 0:29:45.660000 --> 0:29:49.360000 link every 30 minutes, I will increment my sequence number in that LSA 0:29:49.360000 --> 0:29:56.840000 by one, and I will send it out and flood it again. 0:29:56.840000 --> 0:30:00.000000 We already talked about area zero being the backbone. 0:30:00.000000 --> 0:30:05.000000 We already talked about what an area border router is, just as a review. 0:30:05.000000 --> 0:30:09.680000 He's got one, at least one, link connected to area zero, and at least 0:30:09.680000 --> 0:30:14.280000 one link connected to any other non-zero area. 0:30:14.280000 --> 0:30:20.760000 And if I have a router that is doing something other than OSPF, like maybe 0:30:20.760000 --> 0:30:27.020000 he's doing RIP or EIGR Pities, learned about a bunch of non-OSPF routes, 0:30:27.020000 --> 0:30:31.780000 and then he wants to convert those into OSPF LSA's and inject them into 0:30:31.780000 --> 0:30:33.440000 the OSPF domain. 0:30:33.440000 --> 0:30:35.960000 We call that an autonomous system border router. 0:30:35.960000 --> 0:30:39.720000 Depending on the paper or document you read, sometimes they'll call that 0:30:39.720000 --> 0:30:44.420000 an autonomous system boundary router, but everybody knows what ASBR means, 0:30:44.420000 --> 0:30:45.820000 whether it's border or boundary. 0:30:45.820000 --> 0:30:51.520000 It's a router that's injecting non-OSPF stuff into OSPF. 0:30:51.520000 --> 0:30:56.960000 All right, there's nothing new here. 0:30:56.960000 --> 0:30:59.980000 We've already talked about what link state advertisements are that they 0:30:59.980000 --> 0:31:03.000000 age out after one hour. 0:31:03.000000 --> 0:31:09.580000 OSPF fully supports variable links subnet masking and classless inter 0:31:09.580000 --> 0:31:16.960000 -domain routing. It does support summarization, but at the CCI level, you're 0:31:16.960000 --> 0:31:20.300000 not expected to know how to do summarization or how to troubleshoot it. 0:31:20.300000 --> 0:31:24.120000 If you're not familiar with that term summarization, all that means is, 0:31:24.120000 --> 0:31:29.140000 imagine that you and I, once again, are neighbors, you and I are in area 0:31:29.140000 --> 0:31:34.000000 one. Now, imagine that behind me, I have another link to another neighbor 0:31:34.000000 --> 0:31:37.480000 who's I am an area border router. 0:31:37.480000 --> 0:31:39.340000 I'm straddling two areas. 0:31:39.340000 --> 0:31:44.960000 Now, imagine area zero, I learned about 200 different routes that all 0:31:44.960000 --> 0:31:52.560000 start with the number 20, 20.1.1 slash 24, 20.56.90, all sorts of routes 0:31:52.560000 --> 0:31:53.720000 that begin with 20. 0:31:53.720000 --> 0:31:58.420000 Well, the normal behavior is, I would send you all those hundreds of routes. 0:31:58.420000 --> 0:32:02.280000 Well, summarization means, hey, I'm your only way out. 0:32:02.280000 --> 0:32:05.260000 You can't get to those networks except through me. 0:32:05.260000 --> 0:32:07.140000 They all start with the number 20. 0:32:07.140000 --> 0:32:11.060000 So, while I just summarize them down and just send you one LSA saying, 0:32:11.060000 --> 0:32:15.940000 hey, any route, any packet you get that needs to go to 20.anything, go 0:32:15.940000 --> 0:32:19.640000 through me. So, I'm not going to tell you about those specific routes. 0:32:19.640000 --> 0:32:21.400000 They all have something in common. 0:32:21.400000 --> 0:32:23.220000 I'll tell you about what they have in common. 0:32:23.220000 --> 0:32:27.120000 That's route summarization. 0:32:27.120000 --> 0:32:33.300000 We already talked about how 224, 005 and 006 are reserved for OSPF. 0:32:33.300000 --> 0:32:36.340000 And we'll talk about designated routers and backup designated routers 0:32:36.340000 --> 0:32:38.560000 in a subsequent video. 0:32:38.560000 --> 0:32:42.980000 And I talked about how OSPF supports special area types, such as stub, 0:32:42.980000 --> 0:32:45.180000 totally stub, and not so stubby area. 0:32:45.180000 --> 0:32:49.040000 As I mentioned, the main thing you just need to know about those is that, 0:32:49.040000 --> 0:32:52.640000 well, I'll just give you a real high level, real high level over you of 0:32:52.640000 --> 0:32:59.740000 that. So, imagine that we have something like this. 0:32:59.740000 --> 0:33:03.480000 All right. This is area zero. 0:33:03.480000 --> 0:33:05.140000 This is area 15. 0:33:05.140000 --> 0:33:07.160000 And this is area 27. 0:33:07.160000 --> 0:33:11.800000 All right. And we have a couple of ABRs. 0:33:11.800000 --> 0:33:18.540000 Here's ABR2. Here's ABR1. 0:33:18.540000 --> 0:33:27.020000 Okay. And then on area 15, right here, we have an ASBR. 0:33:27.020000 --> 0:33:35.660000 So, this guy is learning some non-OSPF routes, maybe RIP, maybe EIGRP. 0:33:35.660000 --> 0:33:37.280000 Who knows what they are. 0:33:37.280000 --> 0:33:41.740000 And what he's doing is he's injecting those into OSPF. 0:33:41.740000 --> 0:33:46.680000 Now, he's going to use a special type of LSA, not the type one router 0:33:46.680000 --> 0:33:50.540000 LSA. He's going to use a different type of LSA to inject those. 0:33:50.540000 --> 0:33:55.900000 And one thing that's very unique about that LSA is that LSA actually floods 0:33:55.900000 --> 0:33:58.660000 throughout the entire OSPF domain. 0:33:58.660000 --> 0:34:01.960000 So, that would come in here in the area zero. 0:34:01.960000 --> 0:34:05.120000 And it would come in here in the area 27. 0:34:05.120000 --> 0:34:10.280000 Well, sometimes you might say, hey, you know what, in area 27, ABR2 is 0:34:10.280000 --> 0:34:12.520000 our only way in and out. 0:34:12.520000 --> 0:34:17.440000 That's it. So, in area 27, we don't need all that stuff. 0:34:17.440000 --> 0:34:20.020000 As a matter of fact, why don't we just, you know, it would really reduce 0:34:20.020000 --> 0:34:23.540000 the size of our memory, reduce the size of our routing tables and our 0:34:23.540000 --> 0:34:28.140000 database if instead of getting those routes, which, you know, we need 0:34:28.140000 --> 0:34:29.620000 reachability to that. 0:34:29.620000 --> 0:34:31.380000 But what if we did this instead? 0:34:31.380000 --> 0:34:37.560000 What if we just told this guy to send one LSA that had a default route? 0:34:37.560000 --> 0:34:40.040000 And not to send all that other stuff. 0:34:40.040000 --> 0:34:42.640000 That's called a stub area. 0:34:42.640000 --> 0:34:45.760000 Now, it gets more complicated than that, but that's the essence of a stub 0:34:45.760000 --> 0:34:50.400000 area that says, hey, keep those redistributed routes out of here. 0:34:50.400000 --> 0:34:54.240000 I don't want to know those, and I'll just use a single default route if 0:34:54.240000 --> 0:34:57.740000 I need to send packets to an unknown network. 0:34:57.740000 --> 0:34:59.600000 That's a stub area. 0:34:59.600000 --> 0:35:08.660000 All right. And the last thing I want to review here are the various tables 0:35:08.660000 --> 0:35:11.620000 and data structures that OSPF uses. 0:35:11.620000 --> 0:35:17.300000 You should be aware that OSPF maintains or feeds into all of these. 0:35:17.300000 --> 0:35:20.620000 So OSPF maintains a neighbor table. 0:35:20.620000 --> 0:35:33.540000 And you can view that with show IP OSPF neighbor to see if you have any 0:35:33.540000 --> 0:35:37.560000 OSPF neighbors. Those are maintained in the table. 0:35:37.560000 --> 0:35:42.840000 All the LSA is that you've either created yourself or LSA's that you've 0:35:42.840000 --> 0:35:47.740000 learned from other people are stored in your link state database. 0:35:47.740000 --> 0:35:56.740000 And you can view the contents of that with show IP OSPF database. 0:35:56.740000 --> 0:36:04.740000 Database. Forget my, forgive my sloppy writing right there. 0:36:04.740000 --> 0:36:10.500000 The SPF tree is like your jigsaw puzzle where you finally put together 0:36:10.500000 --> 0:36:14.120000 everything into a coherent map, coherent picture. 0:36:14.120000 --> 0:36:17.300000 I have searched and I have not been able to find. 0:36:17.300000 --> 0:36:18.680000 I don't know if it exists. 0:36:18.680000 --> 0:36:22.320000 I don't know of any commands that you can issue to actually see the SPF 0:36:22.320000 --> 0:36:25.920000 tree. I think that's just a bunch of memory cells linked together and 0:36:25.920000 --> 0:36:29.720000 dram memory or something that the CPU of the router can access. 0:36:29.720000 --> 0:36:33.340000 But I don't think we have visibility to that ourselves. 0:36:33.340000 --> 0:36:38.020000 And then, of course, the routes are extracted from the SPF tree. 0:36:38.020000 --> 0:36:40.920000 And the best routes are put into the IP routing table. 0:36:40.920000 --> 0:36:45.940000 And we can certainly do show IP route. 0:36:45.940000 --> 0:36:50.460000 But if you just want to look at OSPF routes, not show IP routes, show 0:36:50.460000 --> 0:36:58.300000 IP route, you can tack onto that show IP route OSPF to just view the OSPF 0:36:58.300000 --> 0:37:08.120000 learned routes. So that concludes this video refresher on OSPF. 0:37:08.120000 --> 0:37:09.380000 I hope you enjoyed it.