WEBVTT 0:00:03.160000 --> 0:00:08.340000 Hello and welcome this video titled, Influencing OSPF Path Selection. 0:00:08.340000 --> 0:00:16.220000 So we're going to look at when OSPF has multiple paths to the same destination, 0:00:16.220000 --> 0:00:21.360000 we're going to answer questions like, is there a way to control how many 0:00:21.360000 --> 0:00:24.360000 multiple paths it can install in the routing table? 0:00:24.360000 --> 0:00:28.540000 Is there a way to control which path it selects as the best path? 0:00:28.540000 --> 0:00:32.480000 If I don't like the one it's selecting as the best path by itself, can 0:00:32.480000 --> 0:00:36.560000 I change that? We're going to look at that in this section. 0:00:36.560000 --> 0:00:41.800000 Okay, most routing protocols, BGP is the exception. 0:00:41.800000 --> 0:00:47.080000 Most routing protocols, if they learn multiple paths to the same destination, 0:00:47.080000 --> 0:00:51.380000 and if those paths are equal, the metric is the same, we call that equal 0:00:51.380000 --> 0:00:55.040000 cost routing. And most routing protocols will say, okay, if those two 0:00:55.040000 --> 0:00:59.440000 or three paths are all the same to network 50, 50, I'll install them all 0:00:59.440000 --> 0:01:03.000000 in my routing table, then just load bounds between them. 0:01:03.000000 --> 0:01:10.700000 So most routing protocols allow up to four equal cost paths into your 0:01:10.700000 --> 0:01:15.680000 routing table. You can use the maximum dash paths command to increase 0:01:15.680000 --> 0:01:19.420000 or decrease that amount if you so desire. 0:01:19.420000 --> 0:01:23.980000 All right, another thing. 0:01:23.980000 --> 0:01:33.480000 We know that OSPF uses as its formula as their review, 100 million divided 0:01:33.480000 --> 0:01:43.080000 by the interface bandwidth in bits per second, which means if we have 0:01:43.080000 --> 0:01:48.540000 a fast ethernet interface, a gig interface, a 10 gig interface, or anything 0:01:48.540000 --> 0:01:52.900000 even higher, they will all equal to one. 0:01:52.900000 --> 0:01:58.400000 They'll all have the exact same cost, which leads us to this next bullet 0:01:58.400000 --> 0:02:01.440000 point. For networks with gigabit ethernet and above, this can lead to 0:02:01.440000 --> 0:02:04.560000 inferior path selection. 0:02:04.560000 --> 0:02:10.920000 So we can use this command right here to change that formula. 0:02:10.920000 --> 0:02:15.940000 Maybe I instead want 1 billion divided by the bandwidth in bits per second 0:02:15.940000 --> 0:02:19.460000 or 100 billion divided by the bandwidth in bits per second. 0:02:19.460000 --> 0:02:23.760000 So by using the auto dash cost reference dash bandwidth, we can change 0:02:23.760000 --> 0:02:26.700000 that initial number on the top of the formula. 0:02:26.700000 --> 0:02:29.740000 Like this says here on the bottom, if you're going to do this, you're 0:02:29.740000 --> 0:02:32.920000 changing the way OSPF comes up with its metric. 0:02:32.920000 --> 0:02:36.380000 So all the routers should be using the exact same formula or they will 0:02:36.380000 --> 0:02:42.100000 have an inconsistent view of what the costs are to reach particular destinations. 0:02:42.100000 --> 0:02:51.600000 Okay, I showed you in a previous video that we can actually modify cost 0:02:51.600000 --> 0:02:53.360000 in one of two ways. 0:02:53.360000 --> 0:02:58.000000 We can modify it by changing the bandwidth on an interface, artificially 0:02:58.000000 --> 0:03:02.640000 setting a bandwidth number we want, or by changing the cost value itself 0:03:02.640000 --> 0:03:04.240000 on the interface. 0:03:04.240000 --> 0:03:08.880000 Either way, will influence OSPF cost, either to go up or to go down and 0:03:08.880000 --> 0:03:13.080000 make that path either less or more preferable than it previously was. 0:03:13.080000 --> 0:03:19.860000 And the last thing I want to talk about is how to originate default routes 0:03:19.860000 --> 0:03:28.940000 in OSPF. So OSPF is quite a bit different in how you get to input to inject 0:03:28.940000 --> 0:03:33.700000 a default route than other routing protocols like RIP or EIGRP. 0:03:33.700000 --> 0:03:37.600000 So with OSPF, it actually has a special command as you can see here just 0:03:37.600000 --> 0:03:42.400000 for this purpose, the default dash information originate command. 0:03:42.400000 --> 0:03:44.340000 Let's just talk about this here for just a moment. 0:03:44.340000 --> 0:03:46.000000 Let's look at this example. 0:03:46.000000 --> 0:03:50.480000 So in this example, we have this router right here, and maybe he's doing 0:03:50.480000 --> 0:03:55.780000 BGP with an internet router. 0:03:55.780000 --> 0:03:59.340000 Now, with BGP, you can actually literally exchange hundreds of thousands 0:03:59.340000 --> 0:04:01.340000 of routes with a peering router. 0:04:01.340000 --> 0:04:05.240000 And as of this time of this recording, there are literally, I think, over 0:04:05.240000 --> 0:04:09.400000 800,000 routes consisting of the internet right now. 0:04:09.400000 --> 0:04:13.580000 That's how big your BGP table could be, especially if your router that's 0:04:13.580000 --> 0:04:18.120000 owned by AT&T, sprint, time-owner cables, something like that. 0:04:18.120000 --> 0:04:23.160000 Now, let's say that router X here is learning of a bunch of BGP routes. 0:04:23.160000 --> 0:04:28.380000 Now, router X, he is the only way to get out to the outside as far as 0:04:28.380000 --> 0:04:30.260000 the OSPF domain is concerned. 0:04:30.260000 --> 0:04:32.400000 Here's our OSPF domain. 0:04:32.400000 --> 0:04:38.240000 Or maybe this. Maybe router X isn't learning any BGP routes. 0:04:38.240000 --> 0:04:38.940000 He does like this. 0:04:38.940000 --> 0:04:40.460000 He does has a static route. 0:04:40.460000 --> 0:04:43.640000 He's got a static route saying, hey, to get to the default, to get to 0:04:43.640000 --> 0:04:47.600000 anything I don't know, send it to my internet gateway. 0:04:47.600000 --> 0:04:49.200000 Send to my ISP routers. 0:04:49.200000 --> 0:04:51.100000 That's probably more feasible. 0:04:51.100000 --> 0:04:55.460000 So now what we want to do is we want to have router X generate a link 0:04:55.460000 --> 0:05:03.420000 state update. And inside of it, we want him to generate an LSA for a default 0:05:03.420000 --> 0:05:10.040000 route. This is where we would use the default information originate command. 0:05:10.040000 --> 0:05:14.160000 Now notice, there's also a keyword at the end of here called always, which 0:05:14.160000 --> 0:05:16.960000 is optional. You don't have to type in always. 0:05:16.960000 --> 0:05:19.320000 When would you use that and when would you not? 0:05:19.320000 --> 0:05:24.140000 Well, if the router, like router X in this case, that's going to be advertising 0:05:24.140000 --> 0:05:26.160000 the default route into OSPF. 0:05:26.160000 --> 0:05:31.160000 If he already has a default route, learn via some other method, a default 0:05:31.160000 --> 0:05:34.800000 route via static route, like we see here, a default route that you learned 0:05:34.800000 --> 0:05:37.780000 via EIGRP or BGP. 0:05:37.780000 --> 0:05:41.900000 If he's already got a default route, then all we have to type in is default 0:05:41.900000 --> 0:05:44.200000 information originate. 0:05:44.200000 --> 0:05:47.600000 And then he'll go ahead and send out an LSA to the rest of the OSPF routers 0:05:47.600000 --> 0:05:49.140000 of a default route. 0:05:49.140000 --> 0:05:52.780000 Now, what if he doesn't have a default route himself? 0:05:52.780000 --> 0:05:57.200000 Like in my first example, when we didn't have the static route back when 0:05:57.200000 --> 0:06:01.080000 you said, hey, these two guys here are here, they're doing BGP. 0:06:01.080000 --> 0:06:08.400000 And this guy is sending 800 ,000 plus routes to router X. 0:06:08.400000 --> 0:06:13.340000 So router X, he doesn't need a default route because he knows everything. 0:06:13.340000 --> 0:06:15.540000 He knows all the routes that exist in the entire world. 0:06:15.540000 --> 0:06:17.780000 So he doesn't need a default route. 0:06:17.780000 --> 0:06:21.620000 So if on router X, if I did default information originate and then I hit 0:06:21.620000 --> 0:06:25.280000 my return key, you would see it doesn't work. 0:06:25.280000 --> 0:06:29.240000 Because that command only works if the router has a default route that 0:06:29.240000 --> 0:06:34.700000 you've learned via some other method, unless I add the keyword always. 0:06:34.700000 --> 0:06:38.400000 So you got to be real careful with this keyword because when you say default 0:06:38.400000 --> 0:06:43.400000 information originate always, that means you will send out a default route. 0:06:43.400000 --> 0:06:47.700000 You'll tell everybody, hey, come to me with your unknown traffic, even 0:06:47.700000 --> 0:06:51.120000 if you don't have a default route yourself. 0:06:51.120000 --> 0:06:54.520000 Now, like I said, if router X has knows about everything, he doesn't need 0:06:54.520000 --> 0:06:56.220000 a default route. 0:06:56.220000 --> 0:07:01.140000 But router X could potentially say default information originate always 0:07:01.140000 --> 0:07:05.800000 and send out a default route when he himself is missing information. 0:07:05.800000 --> 0:07:08.400000 And that would be problematic. 0:07:08.400000 --> 0:07:16.780000 So that concludes our refresher on OSPF theory and implementation. 0:07:16.780000 --> 0:07:17.420000 Thank you for watching.