WEBVTT 1 00:00.000 --> 00:02.600 In the previous video, we configured IBGP 2 00:02.600 --> 00:07.800 neighborships between R11, R12, and R13 as shown here. 3 00:07.800 --> 00:09.800 And in this video, we're gonna walk through 4 00:09.800 --> 00:12.680 configuring external BGP neighborships 5 00:12.680 --> 00:17.680 between R11 and R21 and R11 and R31. 6 00:17.680 --> 00:19.800 Now the configuration for setting up 7 00:19.800 --> 00:22.800 an external BGP neighborship is quite shocking 8 00:22.800 --> 00:24.360 when you compare it to what it looks like 9 00:24.360 --> 00:26.120 for an internal BGP neighborship. 10 00:26.120 --> 00:26.960 And here's the syntax. 11 00:26.960 --> 00:28.320 We're gonna configuration mode, 12 00:28.320 --> 00:31.200 we'll type in router, space, BGP, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 12 00:28.320 --> 00:31.200 we'll type in router, space, BGP, 13 00:31.200 --> 00:33.360 we'll specify our local autonomous system number. 14 00:33.360 --> 00:34.440 And then now that we're in router, 15 00:34.440 --> 00:37.280 BGP configuration mode, we're gonna use the command neighbor, 16 00:37.280 --> 00:40.840 space, the IP address of the reachable neighbor, 17 00:40.840 --> 00:44.740 space, the keyword remote dash AS, and the AS number. 18 00:44.740 --> 00:49.040 So if we were configuring a BGP neighborship 19 00:49.040 --> 00:52.720 between R11 and R31, we would specify that the target, 20 00:52.720 --> 00:55.400 the destination is 23 to 10 to 30 dot. 21 00:55.400 --> 00:58.480 And each router is last octet ends with its number. 22 00:58.480 --> 01:00.720 So the interface here, four slash one, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 22 00:58.480 --> 01:00.720 So the interface here, four slash one, 23 01:00.720 --> 01:04.800 interface addresses is 23 to 10 to 30 dot 31. 24 01:04.800 --> 01:06.160 So that'd be this address here. 25 01:06.160 --> 01:09.060 And then the remote AS that this router is in is 30. 26 01:09.060 --> 01:11.720 So the only difference between an internal 27 01:11.720 --> 01:14.400 and an external BGP neighborship and setting it up, 28 01:14.400 --> 01:16.760 initially is that we're pointing to an AS, 29 01:16.760 --> 01:18.080 this different than ours. 30 01:18.080 --> 01:22.200 So R11 is an AS 10, R31 is an AS 30. 31 01:22.200 --> 01:24.240 And as a result, once we set it up on both sides, 32 01:24.240 --> 01:26.700 we'll have an external BGP neighborship. 33 01:26.700 --> 01:28.040 So that's the syntax. 34 01:28.040 --> 01:31.440 Now with external BGP, it's expecting the peer WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 34 01:28.040 --> 01:31.440 Now with external BGP, it's expecting the peer 35 01:31.440 --> 01:34.240 is on a common link that's like no more than one hop away. 36 01:34.240 --> 01:38.040 So in the event that you have to do appearing to loopbacks 37 01:38.040 --> 01:40.560 or non-directly connected networks, 38 01:40.560 --> 01:43.960 there is some additional configuration for external BGP. 39 01:43.960 --> 01:45.880 And I'll point one of those options out as we go through, 40 01:45.880 --> 01:47.280 however, in our topology, 41 01:47.280 --> 01:49.520 because the external neighbors are directly connected 42 01:49.520 --> 01:52.400 on the same link, we won't actually need to use them. 43 01:52.400 --> 01:54.520 But I will show you what the syntax is for that as well. 44 01:54.520 --> 01:57.000 So using the syntax, we're gonna go to R11 45 01:57.000 --> 02:00.440 and configure an external BGP neighborship for R31, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 45 01:57.000 --> 02:00.440 and configure an external BGP neighborship for R31, 46 02:00.440 --> 02:02.920 and we'll also configure an external BGP neighborship 47 02:02.920 --> 02:04.160 for R21. 48 02:04.160 --> 02:07.400 Now both those routers, R21 and R31, 49 02:07.400 --> 02:09.080 they are already pre-configured 50 02:09.080 --> 02:12.160 as are the routers here down in AS 40. 51 02:12.160 --> 02:15.640 So all we need to do is set up the external BGP neighborship 52 02:15.640 --> 02:18.200 from R11 to those two routers. 53 02:18.200 --> 02:19.200 And if we do so correctly, 54 02:19.200 --> 02:22.040 we should be able to build our external BGP neighborships 55 02:22.040 --> 02:24.480 between R11 and those respective routers. 56 02:24.480 --> 02:26.640 So let's go over to R11 and get to work. 57 02:26.640 --> 02:28.800 All right, so back in the lab environment here, 58 02:28.800 --> 02:30.760 we are gonna go to R11, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 58 02:28.800 --> 02:30.760 we are gonna go to R11, 59 02:30.760 --> 02:34.120 and let's first set up the appearing over to R21. 60 02:34.120 --> 02:38.040 Now R21 is at 23.10.20.21, 61 02:38.040 --> 02:39.560 we can confirm that real quick, 62 02:39.560 --> 02:42.960 just by going to R21 and doing a show IP interface brief, 63 02:42.960 --> 02:44.560 just to confirm its address. 64 02:44.560 --> 02:48.080 And indeed, it is 23.10.20.21. 65 02:48.080 --> 02:49.640 I also use the middle two octets 66 02:49.640 --> 02:53.720 to indicate the two AS numbers it was going between. 67 02:53.720 --> 02:55.240 So if you see 10.20, 68 02:55.240 --> 02:57.600 it's gonna be the network between a Thomas system 10 69 02:57.600 --> 02:59.120 and a Thomas system 20. 70 02:59.120 --> 03:01.320 If you see the 2040, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 70 02:59.120 --> 03:01.320 If you see the 2040, 71 03:01.320 --> 03:03.800 that would be the link between a Thomas system 20 72 03:03.800 --> 03:05.000 and a Thomas system 40. 73 03:05.000 --> 03:06.720 So I'm gonna go ahead and simply highlight that 74 03:06.720 --> 03:08.720 to copy that and put it in my buffer. 75 03:08.720 --> 03:13.720 And let's go over to R11 and configure 23.10.20.21, 76 03:13.720 --> 03:15.840 which is R21 as appear. 77 03:15.840 --> 03:17.640 So we'll go back to R11. 78 03:17.640 --> 03:21.240 We'll go into configuration mode for router, BGP10. 79 03:21.240 --> 03:22.360 And we'll say neighbor. 80 03:22.360 --> 03:26.160 Now I'll put in the neighbors address of 23.10.20.21, 81 03:26.160 --> 03:30.320 which is R21's address on its FA4/0 interface. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 81 03:26.160 --> 03:30.320 which is R21's address on its FA4/0 interface. 82 03:30.320 --> 03:32.800 Space will type in remote dash AS. 83 03:32.800 --> 03:35.720 And R21 is in AS20. 84 03:35.720 --> 03:39.680 So boom, we've just configured an external BGP neighborship. 85 03:39.680 --> 03:41.920 And because R21 is already configured, 86 03:41.920 --> 03:43.800 we should here in a moment have a console message 87 03:43.800 --> 03:45.840 indicating that we have that neighborship. 88 03:45.840 --> 03:47.880 So let's do the same treatment, but this time, 89 03:47.880 --> 03:51.360 let's go ahead and do it for our external BGP neighborship 90 03:51.360 --> 03:55.360 between R11 and down to R31 that's in AS30. 91 03:55.360 --> 03:58.360 So let's just check R31's interface address 92 03:58.360 --> 04:00.120 on its FA4/1. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 92 03:58.360 --> 04:00.120 on its FA4/1. 93 04:00.120 --> 04:01.640 So if we go to R31, 94 04:01.640 --> 04:04.400 there's an output of show IP interface brief right here. 95 04:04.400 --> 04:08.920 And its FA4/1 address is this 23.10.30.31. 96 04:08.920 --> 04:10.160 So I'll copy that. 97 04:10.160 --> 04:12.640 We'll go back to R11 and we'll type in neighbor. 98 04:12.640 --> 04:16.040 One more time here in router BGP10 configuration mode. 99 04:16.040 --> 04:17.200 It's both type in neighbor. 100 04:17.200 --> 04:19.480 I'll paste in the IP address that's reachable there 101 04:19.480 --> 04:22.440 on R31 and I'll say remote AS. 102 04:22.440 --> 04:24.160 And it's in the tone system 30. 103 04:24.160 --> 04:25.680 Let's just check our diagram real quick. 104 04:25.680 --> 04:28.280 So sure enough, R31 is in AS30. 105 04:28.280 --> 04:30.440 So we'll put a 30 there, press enter. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 105 04:28.280 --> 04:30.440 So we'll put a 30 there, press enter. 106 04:30.440 --> 04:34.160 And because R31 is already configured here in a moment 107 04:34.160 --> 04:36.120 as they negotiate their session, 108 04:36.120 --> 04:37.880 we should have a BGP neighborship. 109 04:37.880 --> 04:42.880 So now if we did a show IP BGP summary and press enter, 110 04:42.880 --> 04:44.800 it'll show us our four neighbors. 111 04:44.800 --> 04:46.600 We have two internal neighbors. 112 04:46.600 --> 04:51.240 That's there are two IGP sessions over to R12 and R13. 113 04:51.240 --> 04:53.600 And the second two are both external 114 04:53.600 --> 04:55.640 because we're going to a top system 20. 115 04:55.640 --> 05:00.560 R21 is and also to a top system 30 where R31 is. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 115 04:55.640 --> 05:00.560 R21 is and also to a top system 30 where R31 is. 116 05:00.560 --> 05:02.400 And that's shown right here in this column 117 05:02.400 --> 05:05.000 referring to the top system that peers in. 118 05:05.000 --> 05:06.640 Also, we have learned some routes. 119 05:06.640 --> 05:07.480 Check this out. 120 05:07.480 --> 05:09.520 This is showing us that we have learned three prefixes. 121 05:09.520 --> 05:15.440 That's a fancy way of saying routes in the world of BGP and routing. 122 05:15.440 --> 05:19.880 But anyway, we learn three routes from the peer at .21, 123 05:19.880 --> 05:22.080 which is R21 up in AS20. 124 05:22.080 --> 05:26.480 We've also learned three routes from R31, 125 05:26.480 --> 05:28.280 which is in a time system 30. 126 05:28.280 --> 05:29.880 So if we want to see those routes that we've learned 127 05:29.880 --> 05:33.080 or should be six of them, we can do a show IP BGP. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 127 05:29.880 --> 05:33.080 or should be six of them, we can do a show IP BGP. 128 05:33.080 --> 05:36.720 And that's going to show us the routing information base for BGP. 129 05:36.720 --> 05:37.760 What it's learned about. 130 05:37.760 --> 05:40.320 So press enter and there are the six routes. 131 05:40.320 --> 05:42.840 So we've learned about the 2222 network, 132 05:42.840 --> 05:45.920 the 3232 network and the 4343 network. 133 05:45.920 --> 05:50.600 Twice, once from the next top here of .31, which is R31. 134 05:50.600 --> 05:54.240 And also from the next top here of .21, which is R21. 135 05:54.240 --> 05:56.440 And that is true for those other two routes as well. 136 05:56.440 --> 05:59.520 And if we take a look at these two right here, 137 05:59.520 --> 06:02.160 this little greater than symbol here represents WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 137 05:59.520 --> 06:02.160 this little greater than symbol here represents 138 06:02.160 --> 06:03.400 is the best route. 139 06:03.400 --> 06:06.520 Now why in the world did R11 choose this route 140 06:06.520 --> 06:10.800 with the next top of 2310, 2021, which is R21? 141 06:10.800 --> 06:14.600 Why did it choose that as the best route over that same route 142 06:14.600 --> 06:19.120 that was learned from 2310, 3031, which is R31? 143 06:19.120 --> 06:22.960 And we talked in the overview about the path selection process. 144 06:22.960 --> 06:25.840 And one of those properties that we looked at is the AS path. 145 06:25.840 --> 06:27.280 So check it out the path here. 146 06:27.280 --> 06:30.840 The path for this route is going through autonomous system 20. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 146 06:27.280 --> 06:30.840 The path for this route is going through autonomous system 20. 147 06:30.840 --> 06:35.160 And the route that we learned from .31, who's an AS 30, 148 06:35.160 --> 06:38.440 that route had to go all the way through autonomous system 20, 149 06:38.440 --> 06:40.440 and then 40, and then 30. 150 06:40.440 --> 06:42.240 And based on the shortest AS path 151 06:42.240 --> 06:44.960 and the other previous qualities all being equal, 152 06:44.960 --> 06:45.960 including weight. 153 06:45.960 --> 06:48.560 That's why R11 chose this route, 154 06:48.560 --> 06:50.480 the one with the icon right here of the best path, 155 06:50.480 --> 06:54.840 using the next top of 23.10.20.21, and not the other one. 156 06:54.840 --> 06:57.160 And in this case, it's because of the shorter AS path. 157 06:57.160 --> 06:59.760 So if we were to do a show AP route, this route, 158 06:59.760 --> 07:03.640 a 22222222, with the next top of .21, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 158 06:59.760 --> 07:03.640 a 22222222, with the next top of .21, 159 07:03.640 --> 07:05.240 should be in the routing table. 160 07:05.240 --> 07:06.600 So we'll do a show IP route. 161 07:06.600 --> 07:09.160 Back down into a show IP route for just BGP learned routes 162 07:09.160 --> 07:10.560 to be very easy to see that way. 163 07:10.560 --> 07:12.520 And here we have our BGP learned routes. 164 07:12.520 --> 07:15.640 So that next top is 23.10.20.21. 165 07:15.640 --> 07:18.000 And that's because up here, that is the next top 166 07:18.000 --> 07:19.640 for its path selection process, 167 07:19.640 --> 07:21.440 would it consider to be the best route, 168 07:21.440 --> 07:24.280 which once again, boil down to the lowest AS path, 169 07:24.280 --> 07:27.160 with 20 being less than 30, 40, and 20. 170 07:27.160 --> 07:30.080 Now, I would like to show you one other really fascinating thing WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 170 07:27.160 --> 07:30.080 Now, I would like to show you one other really fascinating thing 171 07:30.080 --> 07:32.040 about this route that our 11 is learned about 172 07:32.040 --> 07:33.240 for the 22 network. 173 07:33.240 --> 07:37.760 And that is, it has shared that route with R13 and R12. 174 07:37.760 --> 07:40.000 And if we were to take a look at R12, 175 07:40.000 --> 07:42.240 and see what its opinion is of that route, 176 07:42.240 --> 07:43.440 it's gonna be enlightening. 177 07:43.440 --> 07:45.600 So let's go ahead and go to R12. 178 07:45.600 --> 07:48.520 And on R12, we'll do a show IP BGP. 179 07:48.520 --> 07:50.720 Notice there's a route here for that 22 network. 180 07:50.720 --> 07:54.680 However, it doesn't have this little symbol next to any one of them. 181 07:54.680 --> 07:58.240 It's not considering any of these routes to be best. 182 07:58.240 --> 08:00.960 And as a result, R12 is not putting those routes, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 182 07:58.240 --> 08:00.960 And as a result, R12 is not putting those routes, 183 08:00.960 --> 08:03.200 any of those routes in the routing table. 184 08:03.200 --> 08:06.560 So in the next video, I'd like to walk you through why here on R12, 185 08:06.560 --> 08:09.480 it's not using that route to 22222, 186 08:09.480 --> 08:12.480 and also share with you how you can fix it. 187 08:12.480 --> 08:14.720 So I'll see you in the next video for exactly that.