WEBVTT 1 00:00.000 --> 00:04.360 Alright, so we're going to jump into the lab and this is our topology. 2 00:04.360 --> 00:08.080 We just have a square of switches and this lab topology is going to be made 3 00:08.080 --> 00:08.560 available 4 00:08.560 --> 00:12.000 for you to log in and poke around with spanning tree. 5 00:12.000 --> 00:15.680 Make sure that we run some of those familiar spanning tree commands and we can 6 00:15.680 --> 00:16.520 maybe predict 7 00:16.520 --> 00:20.880 which switches are going to be placing which ports into the blocked state. 8 00:20.880 --> 00:23.830 And we also want to take a look at the fact that this is a per VLAN spanning 9 00:23.830 --> 00:24.640 tree environment 10 00:24.640 --> 00:28.960 meaning that we can expect topologies on a per VLAN basis. 11 00:28.960 --> 00:33.640 Now this topology is fairly blank from the perspective that we do have. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 11 00:28.960 --> 00:33.640 Now this topology is fairly blank from the perspective that we do have. 12 00:33.640 --> 00:37.800 Our interface is configured as trunks and our VLANs configured and that's it. 13 00:37.800 --> 00:41.720 I have not done any special spanning tree configuration. 14 00:41.720 --> 00:46.010 Every switch is using the default priority which on a Cisco switch is that 32K 15 00:46.010 --> 00:46.400 value 16 00:46.400 --> 00:50.630 and then we're just relying on may whichever switch has the best MAC address is 17 00:50.630 --> 00:51.320 going to 18 00:51.320 --> 00:52.880 be elected the route. 19 00:52.880 --> 00:58.410 So let's go ahead and log into the lab environment and take a look at what we 20 00:58.410 --> 00:59.240 've got. 21 00:59.240 --> 01:04.510 And so we're starting on switch one here and the classic command is going to be WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 21 00:59.240 --> 01:04.510 And so we're starting on switch one here and the classic command is going to be 22 01:04.510 --> 01:05.440 show spanning 23 01:05.440 --> 01:06.560 tree. 24 01:06.560 --> 01:07.880 So I do a show spanning tree. 25 01:07.880 --> 01:12.810 It's going to produce the information that tells me a little bit about first of 26 01:12.810 --> 01:13.200 all the 27 01:13.200 --> 01:14.400 switch itself. 28 01:14.400 --> 01:19.020 So pay attention to the fact that this is the bridge ID meaning that this is 29 01:19.020 --> 01:19.720 the local 30 01:19.720 --> 01:20.920 ID for switch one. 31 01:20.920 --> 01:22.120 I'm on switch one. 32 01:22.120 --> 01:23.840 This is its bridge ID. 33 01:23.840 --> 01:28.260 Meanwhile the route ID is the information carried in the BP DU that's coming 34 01:28.260 --> 01:28.760 from the 35 01:28.760 --> 01:33.870 route and we should all have the same route ID in the spanning tree topology WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 35 01:28.760 --> 01:33.870 route and we should all have the same route ID in the spanning tree topology 36 01:33.870 --> 01:34.600 because we 37 01:34.600 --> 01:37.040 should all agree on who is the route. 38 01:37.040 --> 01:41.830 So I can look at this and see that I have a local MAC address that does not 39 01:41.830 --> 01:42.600 match the 40 01:42.600 --> 01:46.810 route MAC address which means I don't actually or I'm not the route and I know 41 01:46.810 --> 01:47.440 I'm not the 42 01:47.440 --> 01:48.440 route. 43 01:48.440 --> 01:51.820 So another thing or a couple of things to keep in mind first of all we see that 44 01:51.820 --> 01:52.080 this 45 01:52.080 --> 01:54.480 is listed for VLAN one. 46 01:54.480 --> 01:58.830 If I hit space here to look at the next input or output I guess we'll see that 47 01:58.830 --> 01:59.720 it's VLAN 48 01:59.720 --> 02:04.510 10 listed with its own set of ports down here in different statuses then I can WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 48 01:59.720 --> 02:04.510 10 listed with its own set of ports down here in different statuses then I can 49 02:04.510 --> 02:05.200 see VLAN 50 02:05.200 --> 02:08.650 20 and VLAN 30 and those are the VLANs that we have configured so that's the 51 02:08.650 --> 02:09.040 end of the 52 02:09.040 --> 02:10.280 command. 53 02:10.280 --> 02:14.920 But as I just run that command again and look at VLAN one we see that a few 54 02:14.920 --> 02:15.720 things. 55 02:15.720 --> 02:20.950 First of all VLAN one has its own interface as enabled with its own forwarding 56 02:20.950 --> 02:21.720 and blocking 57 02:21.720 --> 02:22.720 statuses. 58 02:22.720 --> 02:26.110 So we've got the status we've got the role listed here whether it's a 59 02:26.110 --> 02:27.080 designated port 60 02:27.080 --> 02:31.270 or a route port and we can tell that at least on this switch we're not blocking WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 60 02:27.080 --> 02:31.270 or a route port and we can tell that at least on this switch we're not blocking 61 02:31.270 --> 02:31.880 any ports 62 02:31.880 --> 02:33.640 for VLAN one. 63 02:33.640 --> 02:38.290 But when I look at VLAN 10 it lists its own ports because it's running a 64 02:38.290 --> 02:39.720 different topology 65 02:39.720 --> 02:43.810 even if the ports are in the exact same status they're both in forwarding the 66 02:43.810 --> 02:44.720 gig zero zero 67 02:44.720 --> 02:50.740 is the designated gig zero two is the route and so we could look up here again 68 02:50.740 --> 02:51.440 at VLAN 69 02:51.440 --> 02:55.830 one and say well it's the exact same topology and yet that is what per VLAN 70 02:55.830 --> 02:56.800 spanning tree 71 02:56.800 --> 03:02.640 is giving us is the fact that we have a topology per VLAN even if the top WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 71 02:56.800 --> 03:02.640 is giving us is the fact that we have a topology per VLAN even if the top 72 03:02.640 --> 03:05.560 ologies are identical. 73 03:05.560 --> 03:07.160 A couple other things to note. 74 03:07.160 --> 03:11.710 First of all when it says spanning tree is enabled with protocol IEEE protocol 75 03:11.710 --> 03:12.320 IEEE is 76 03:12.320 --> 03:17.320 Cisco's way of saying we're running 802.1d per VLAN spanning tree. 77 03:17.320 --> 03:20.730 So we're not running rapid is basically what this means and we're also not 78 03:20.730 --> 03:22.080 running MST 79 03:22.080 --> 03:26.200 which is the industry standard way of creating multiple topologies. 80 03:26.200 --> 03:30.720 So we have three different options or running the basic 802.1d spanning tree WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 80 03:26.200 --> 03:30.720 So we have three different options or running the basic 802.1d spanning tree 81 03:30.720 --> 03:31.320 when we see 82 03:31.320 --> 03:36.310 this and lastly we can see that the priority values not just for my local 83 03:36.310 --> 03:37.360 bridge ID but 84 03:37.360 --> 03:43.020 also for the route ID notice that it looks a little off 3,2769 doesn't quite 85 03:43.020 --> 03:43.640 roll off 86 03:43.640 --> 03:47.400 the tongue like 3,2768. 87 03:47.400 --> 03:53.140 So the reason for that is because 3,2768 is a multiple of 4,096 and as we've 88 03:53.140 --> 03:53.840 said all 89 03:53.840 --> 03:58.100 priorities must be a multiple of that because that's well we haven't actually 90 03:58.100 --> 03:58.880 said why yet 91 03:58.880 --> 04:00.520 I suppose. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 91 03:58.880 --> 04:00.520 I suppose. 92 04:00.520 --> 04:05.390 Well here's the reason why because Cisco wants to take the VLAN ID and add it 93 04:05.390 --> 04:05.880 to the 94 04:05.880 --> 04:12.630 priority meaning that for VLAN 1 this extended ID concept says I'm going to add 95 04:12.630 --> 04:13.560 1 to the 96 04:13.560 --> 04:21.070 priority so instead of 3,2768 it's 3,2768 plus 1 for VLAN 1 and meanwhile if I 97 04:21.070 --> 04:21.800 look down 98 04:21.800 --> 04:29.810 here at VLAN 10 we should see that it's 32 not 768 but 778 because we took 3,27 99 04:29.810 --> 04:30.440 68 and WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 99 04:29.810 --> 04:30.440 68 and 100 04:30.440 --> 04:32.520 added the VLAN ID of 10. 101 04:32.520 --> 04:35.950 So this is why Cisco makes it so that on their switches we can only configure 102 04:35.950 --> 04:36.760 multiples of 103 04:36.760 --> 04:41.860 4,096 because by default we're going to add the VLAN ID to the priority value 104 04:41.860 --> 04:42.280 that we 105 04:42.280 --> 04:48.820 configure and so more or less all 4,096 values or the next 4,096 values are 106 04:48.820 --> 04:51.240 taken by a 32K 107 04:51.240 --> 04:55.140 configuration here's the priority configured and then adding the VLAN ID we 108 04:55.140 --> 04:55.720 have to be 109 04:55.720 --> 05:01.560 able to add for example VLAN 4000 which takes us into the 36,000 priority range WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 109 04:55.720 --> 05:01.560 able to add for example VLAN 4000 which takes us into the 36,000 priority range 110 05:01.560 --> 05:01.760 at least 111 05:01.760 --> 05:04.560 that's the priority that would show up over here. 112 05:04.560 --> 05:09.190 So we see that effect on the root as well the root is clearly doing the same 113 05:09.190 --> 05:09.880 thing it 114 05:09.880 --> 05:14.200 has added 10 to the priority value that's configured and so since we have the 115 05:14.200 --> 05:15.240 same priority we're 116 05:15.240 --> 05:18.960 going to go by MAC addresses and so if I look at this MAC address for my local 117 05:18.960 --> 05:20.080 bridge ID 118 05:20.080 --> 05:25.560 it should be worse than the root and what does worse mean? 119 05:25.560 --> 05:31.090 Well worse means higher a higher number so 0c they both start with 0c and then WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 119 05:25.560 --> 05:31.090 Well worse means higher a higher number so 0c they both start with 0c and then 120 05:31.090 --> 05:31.600 we'll 121 05:31.600 --> 05:37.820 do that and then this has an a which is hex for 10 this has a 4 so 4 is lower 122 05:37.820 --> 05:38.880 than 10 and 123 05:38.880 --> 05:44.530 therefore this MAC address is indeed lower and better than the MAC address here 124 05:44.530 --> 05:45.200 locally 125 05:45.200 --> 05:46.200 configured. 126 05:46.200 --> 05:49.200 Lastly we do see the timers that are being advertised a whole load time of 2 127 05:49.200 --> 05:49.680 seconds 128 05:49.680 --> 05:54.370 match 8 max age of 20 for a delay of 15 those are the default timers and I have 129 05:54.370 --> 05:55.120 not changed 130 05:55.120 --> 05:57.160 them in this environment. 131 05:57.160 --> 06:02.580 So a little bit of well maybe a lot of information in this particular command WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 131 05:57.160 --> 06:02.580 So a little bit of well maybe a lot of information in this particular command 132 06:02.580 --> 06:03.720 this is a really 133 06:03.720 --> 06:09.000 great command to run and then we can also specify the VLAN that I want to see 134 06:09.000 --> 06:09.440 so if 135 06:09.440 --> 06:15.090 I just want to see VLAN 20 I can specify that it will just give me VLAN 20's 136 06:15.090 --> 06:16.160 information. 137 06:16.160 --> 06:21.400 So let's go find our root bridge remember this is our topology and so we know 138 06:21.400 --> 06:22.000 that switch 139 06:22.000 --> 06:25.400 one is not the root here at this point. 140 06:25.400 --> 06:26.560 So where is the root? 141 06:26.560 --> 06:33.190 Well if I look here we see my root port is gig 0 2 so switch one thinks that WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 141 06:26.560 --> 06:33.190 Well if I look here we see my root port is gig 0 2 so switch one thinks that 142 06:33.190 --> 06:34.160 the root is 143 06:34.160 --> 06:38.390 out this direction the fact that all of these are gig interfaces and we have a 144 06:38.390 --> 06:39.000 messed with 145 06:39.000 --> 06:44.300 spanning tree costs probably tells me then that switch 2 is likely not the root 146 06:44.300 --> 06:44.920 because 147 06:44.920 --> 06:50.070 it would have gone this way as the root port if switch 2 was indeed the root 148 06:50.070 --> 06:51.200 but since this 149 06:51.200 --> 06:57.510 is the root port here then we're guessing it's switch 3 but it could also be 150 06:57.510 --> 06:59.080 switch 4 and 151 06:59.080 --> 07:04.640 maybe it's deciding to go this way to get to switch 4 rather than that way. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 151 06:59.080 --> 07:04.640 maybe it's deciding to go this way to get to switch 4 rather than that way. 152 07:04.640 --> 07:08.580 So let's go ahead and take a look at that by the way let's go and write down 153 07:08.580 --> 07:09.160 the MAC 154 07:09.160 --> 07:17.350 address so we know that switch 1 is 0 CA and so we'll write down 0 CA as the 155 07:17.350 --> 07:17.960 start of 156 07:17.960 --> 07:20.040 the MAC address. 157 07:20.040 --> 07:25.180 So back here we'll go ahead and click on switch 2 and we'll do a show spanning 158 07:25.180 --> 07:26.040 tree and it 159 07:26.040 --> 07:27.840 can abbreviate it like that. 160 07:27.840 --> 07:33.420 So here we have got let's see here so switch 2 is the switch in the upper right WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 160 07:27.840 --> 07:33.420 So here we have got let's see here so switch 2 is the switch in the upper right 161 07:33.420 --> 07:34.000 we see 162 07:34.000 --> 07:37.790 that we're still not the root here I've got a cost to the root that's a good 163 07:37.790 --> 07:38.440 indicator 164 07:38.440 --> 07:42.720 that I'm not the root it also would tell me if I'm the root and of course last 165 07:42.720 --> 07:43.320 ly these 166 07:43.320 --> 07:46.480 MAC addresses would match if I was indeed the root. 167 07:46.480 --> 07:52.640 So this switch is 0 CA which is higher of numbered and worse than 0 CA 4 so 168 07:52.640 --> 07:53.600 this would 169 07:53.600 --> 08:01.230 be 0 CA on switch 2 and we do see a block port here so in gig 0 0 I'm not going WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 169 07:53.600 --> 08:01.230 be 0 CA on switch 2 and we do see a block port here so in gig 0 0 I'm not going 170 08:01.230 --> 08:01.560 to mark 171 08:01.560 --> 08:06.740 that in my diagram yet because I want us to kind of take a look at the topology 172 08:06.740 --> 08:07.000 and try 173 08:07.000 --> 08:11.500 to figure that out ourselves but when we see switch 2 having a blocked port on 174 08:11.500 --> 08:12.080 gig 0 175 08:12.080 --> 08:16.830 notice that it is listed on a per VLAN basis but it is the same port that's 176 08:16.830 --> 08:17.920 being blocked 177 08:17.920 --> 08:21.690 and that's because unless we may change one of the topologies by default 178 08:21.690 --> 08:22.840 everybody's relying 179 08:22.840 --> 08:27.190 on the same MAC address for root election therefore the topologies will be the 180 08:27.190 --> 08:27.520 same 181 08:27.520 --> 08:31.200 between all of our different VLANs. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 181 08:27.520 --> 08:31.200 between all of our different VLANs. 182 08:31.200 --> 08:36.850 So let's keep running commands we'll go to switch 3 show spanning tree our MAC 183 08:36.850 --> 08:37.520 address 184 08:37.520 --> 08:41.110 here is 0 CA 4 which should sound familiar because that's what we said was the 185 08:41.110 --> 08:41.520 root and 186 08:41.520 --> 08:46.880 indeed look at that it actually says this bridge is the root other switches 187 08:46.880 --> 08:47.640 right here show 188 08:47.640 --> 08:52.950 the cost to the root and the port to the root but on switch 3 which is the root 189 08:52.950 --> 08:53.360 we don't 190 08:53.360 --> 08:58.000 list the cost or the port we just simply say that we are the root so we can 191 08:58.000 --> 08:58.760 just take a 192 08:58.760 --> 09:03.450 look at that and say okay which bridge is the root and you know kind of poke WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 192 08:58.760 --> 09:03.450 look at that and say okay which bridge is the root and you know kind of poke 193 09:03.450 --> 09:03.920 around on 194 09:03.920 --> 09:08.950 the different switches until we find it so that's very useful for us so this is 195 09:08.950 --> 09:09.440 0 CA 196 09:09.440 --> 09:17.420 4 on switch 3 so 0 CA 4 and let's go ahead and complete our MAC address 197 09:17.420 --> 09:18.160 exploration by 198 09:18.160 --> 09:23.420 looking at switch 4 and do a show spanning tree one thing you might wonder is 199 09:23.420 --> 09:24.080 does the 200 09:24.080 --> 09:29.740 MAC address change between VLANs and in our case we've got 0 CA 7 6 here if we 201 09:29.740 --> 09:30.440 look down WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 201 09:29.740 --> 09:30.440 look down 202 09:30.440 --> 09:37.120 here 0 CA 7 6 2 9 B 2 2 9 B 2 alright so the MAC address isn't changing this 203 09:37.120 --> 09:38.280 can depend 204 09:38.280 --> 09:42.310 on the platform for the most part I will say that Cisco devices tend to use the 205 09:42.310 --> 09:42.920 same system 206 09:42.920 --> 09:49.500 MAC on a per VLAN basis the MAC addresses must be unique within a VLAN and so 207 09:49.500 --> 09:49.920 if we're 208 09:49.920 --> 09:53.840 not about different VLANs we can use the same MAC address between the VLANs 209 09:53.840 --> 09:55.040 because it different 210 09:55.040 --> 10:00.020 layer two domains so rather than use a bunch of different MAC addresses for all WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 210 09:55.040 --> 10:00.020 layer two domains so rather than use a bunch of different MAC addresses for all 211 10:00.020 --> 10:00.800 of the different 212 10:00.800 --> 10:05.130 VLANs we might have 4000 VLANs on the switch and we don't want to have to use 213 10:05.130 --> 10:06.240 4000 MAC 214 10:06.240 --> 10:10.640 addresses let's just use the same MAC address on every single VLAN and most of 215 10:10.640 --> 10:11.240 the Cisco 216 10:11.240 --> 10:17.130 products that I've ever used are doing exactly this so this is switch 4 switch 217 10:17.130 --> 10:19.880 4 is 0 C 7 so 218 10:19.880 --> 10:24.740 when I complete our MAC address discovery here we're going to find that they 219 10:24.740 --> 10:25.440 all start with 220 10:25.440 --> 10:30.870 0 C and so really as a matter of which has the lower number in the third digit WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 220 10:25.440 --> 10:30.870 0 C and so really as a matter of which has the lower number in the third digit 221 10:30.870 --> 10:32.120 so again we see 222 10:32.120 --> 10:40.040 a being 10 then 8 7 down to 4 so 4 is the lowest MAC address and therefore it 223 10:40.040 --> 10:40.680 is going to be the 224 10:40.680 --> 10:47.280 root because all of the priority values are 32 K and if they're all the exact 225 10:47.280 --> 10:48.320 same priority 226 10:48.320 --> 10:53.470 value then we're going to have to take a look at the MAC address which is the 227 10:53.470 --> 10:54.680 second half of 228 10:54.680 --> 10:59.180 the bridge IDs and so we compare our bridge IDs and we find out switch 3 is the 229 10:59.180 --> 11:00.440 root thanks to WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 229 10:59.180 --> 11:00.440 root thanks to 230 11:00.440 --> 11:06.360 this MAC address so here's the question if we were to analyze this diagram 231 11:06.360 --> 11:07.960 which of these ports would 232 11:07.960 --> 11:12.730 be in the blocked state we know that switch 3 is the root and importantly we 233 11:12.730 --> 11:13.840 also know that all of 234 11:13.840 --> 11:18.110 these links are gig meaning they all have the exact same cost in fact we saw 235 11:18.110 --> 11:19.160 one of the switches 236 11:19.160 --> 11:22.720 saying hey I've got a cost of 8 and the reason for that is because each one of 237 11:22.720 --> 11:23.680 these links has a 238 11:23.680 --> 11:30.040 cost of 4 so I believe that was switch 2 saying hey in order for me to get to WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 238 11:23.680 --> 11:30.040 cost of 4 so I believe that was switch 2 saying hey in order for me to get to 239 11:30.040 --> 11:31.280 the root I'm going to 240 11:31.280 --> 11:35.620 have to go through two links of 4 which is a total spanning tree cost of 8 but 241 11:35.620 --> 11:36.560 since they're all 242 11:36.560 --> 11:42.060 the same cost we can start to think of it more of a hop count so which where in 243 11:42.060 --> 11:43.760 this network essentially 244 11:43.760 --> 11:49.120 will we find that we are going to block well this is going to come down to 245 11:49.120 --> 11:51.400 switch 2 because it's the 246 11:51.400 --> 11:56.180 furthest away switch 1 and switch 4 both have a cost of 4 to the root where 247 11:56.180 --> 11:57.240 switch 2 is the only 248 11:57.240 --> 12:01.300 one that has the lowest cost so when we look at things from switch 2's WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 248 11:57.240 --> 12:01.300 one that has the lowest cost so when we look at things from switch 2's 249 12:01.300 --> 12:02.600 perspective it's going to 250 12:02.600 --> 12:07.450 be receiving an advertisement from switch 1 saying hey I've got a cost of 4 and 251 12:07.450 --> 12:08.920 I'm going to add my 252 12:08.920 --> 12:12.830 own 4 to that and say okay so I've got a cost of 8 this direction and I'm also 253 12:12.830 --> 12:14.240 getting an advertisement 254 12:14.240 --> 12:19.090 here of a cost of 4 I'm going to add my 4 and say okay I've got an 8 here so I 255 12:19.090 --> 12:20.320 have two different 256 12:20.320 --> 12:25.420 equal upstream costs of 8 which one am I going to pick well now we have to 257 12:25.420 --> 12:26.720 think back to those tie 258 12:26.720 --> 12:30.870 breakers if I've got two equal costs to the root I have to decide which one is WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 258 12:26.720 --> 12:30.870 breakers if I've got two equal costs to the root I have to decide which one is 259 12:30.870 --> 12:31.920 going to be my root 260 12:31.920 --> 12:36.470 port and which one is another upstream port and remember I block all of my 261 12:36.470 --> 12:37.920 upstream ports that are 262 12:37.920 --> 12:42.630 not the root port and that's what prevents loops so we have to think through 263 12:42.630 --> 12:44.080 the tie breakers because 264 12:44.080 --> 12:47.870 what really ultimately matters is the cost we always want to choose the fastest 265 12:47.870 --> 12:48.720 path if we have two 266 12:48.720 --> 12:54.020 equal cost paths then it can be completely arbitrary so what do we base it on 267 12:54.020 --> 12:55.600 well the arbitrary 268 12:55.600 --> 13:02.380 decision is comparing the upstream bridge IDs so I'm going to be comparing 0ca WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 268 12:55.600 --> 13:02.380 decision is comparing the upstream bridge IDs so I'm going to be comparing 0ca 269 13:02.380 --> 13:04.080 with 0c7 again it's 270 13:04.080 --> 13:08.860 arbitrary and it can be arbitrary at this point we just need to pick a path to 271 13:08.860 --> 13:10.720 use and a path to block 272 13:10.720 --> 13:17.010 so when I compare those two which one is the worst one and the answer is 0ca is 273 13:17.010 --> 13:18.560 worse because it has 274 13:18.560 --> 13:25.900 a higher value meaning that because 0c7 is lower it is the preferred path and 275 13:25.900 --> 13:27.520 therefore gig 02 is 276 13:27.520 --> 13:33.050 going to be the root port if gig 02 is my root port that means gig 0 0 here is WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 276 13:27.520 --> 13:33.050 going to be the root port if gig 02 is my root port that means gig 0 0 here is 277 13:33.050 --> 13:34.400 going to be placed 278 13:34.400 --> 13:40.870 into a blocking state so now the moment of truth let's go ahead and flip over 279 13:40.870 --> 13:42.800 and make sure that 280 13:42.800 --> 13:48.290 that is indeed the case so switch to gig 0 0 should be blocking come to switch 281 13:48.290 --> 13:50.240 to and we see gig 0 0 282 13:50.240 --> 13:55.360 is indeed blocking so the status says block that means it is blocking the role 283 13:55.360 --> 13:56.640 says alternate that's 284 13:56.640 --> 14:00.400 what this means we'll talk about what that means coming up that's actually a WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 284 13:56.640 --> 14:00.400 what this means we'll talk about what that means coming up that's actually a 285 14:00.400 --> 14:01.440 rapid spanning tree 286 14:01.440 --> 14:06.120 term and yes even the traditional spanning tree on Cisco platforms we'll still 287 14:06.120 --> 14:07.280 call it an alternate 288 14:08.160 --> 14:12.080 an alternate path essentially it's saying that's an alternate path to the root 289 14:12.080 --> 14:12.800 and therefore we 290 14:12.800 --> 14:17.740 are going to block it so we see that gig 02 is indeed the root port so we have 291 14:17.740 --> 14:19.440 correctly predicted 292 14:19.440 --> 14:23.420 which port would be blocked and this is the fun part about having access to a 293 14:23.420 --> 14:24.880 lab is I can 294 14:24.880 --> 14:29.490 create my own topology and say okay well we'll give in maybe these priority 295 14:29.490 --> 14:30.560 values and these WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 295 14:29.490 --> 14:30.560 values and these 296 14:30.560 --> 14:35.220 spanning tree costs let's go ahead and configure that in the topology and make 297 14:35.220 --> 14:36.560 sure that it's blocking 298 14:36.560 --> 14:40.260 the ports that I think it's going to block it's a great way for us to get 299 14:40.260 --> 14:41.520 practice with spanning 300 14:41.520 --> 14:44.620 tree topologies and I encourage you to take the time to do that given that we 301 14:44.620 --> 14:45.680 have access to this 302 14:45.680 --> 14:53.100 lab now one last thing to look at here is if we go back to our topology let's 303 14:53.100 --> 14:54.560 now say that we want 304 14:54.560 --> 15:01.840 to talk about a different topology for VLAN 10 so everything in blue was VLAN 1 WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 304 14:54.560 --> 15:01.840 to talk about a different topology for VLAN 10 so everything in blue was VLAN 1 305 15:01.840 --> 15:04.240 but now we're 306 15:04.240 --> 15:08.810 going to talk about VLAN 10 and creating a different topology what if I want 307 15:08.810 --> 15:11.520 switch 2 to be the root 308 15:11.520 --> 15:17.120 for VLAN 10 and at that point it would be a different blocked port in the 309 15:17.120 --> 15:18.240 network probably 310 15:18.240 --> 15:24.080 switch 3 would have to find a block because it's furthest away from switch 2 so 311 15:24.080 --> 15:24.960 what am I going to 312 15:24.960 --> 15:29.090 do here well in order to turn switch 2 into the root I'm going to have to 313 15:29.090 --> 15:30.400 change the priority WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 313 15:29.090 --> 15:30.400 change the priority 314 15:30.400 --> 15:36.970 and so maybe I'll make the priority 4096 that should work fine for VLAN 10 and 315 15:36.970 --> 15:37.520 it should turn 316 15:37.520 --> 15:42.910 it into the root so we're going to go ahead and flip back to our topology here 317 15:42.910 --> 15:43.760 get on to switch 2 318 15:43.760 --> 15:48.960 do a config t and I'm going to use this command spanning tree and then I have 319 15:48.960 --> 15:50.160 to specify the VLAN 320 15:50.160 --> 15:54.720 VLAN 10 and then I can change the priority value now let's say I just say I'm 321 15:54.720 --> 15:55.440 going to make it 322 15:55.440 --> 16:00.210 priority 10 I had enter and this is where Cisco says wait a second nope all of WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 322 15:55.440 --> 16:00.210 priority 10 I had enter and this is where Cisco says wait a second nope all of 323 16:00.210 --> 16:00.960 our priorities 324 16:00.960 --> 16:05.010 must be in the increments of 4096 and here's the best part they give us all of 325 16:05.010 --> 16:05.840 those numbers 326 16:05.840 --> 16:12.380 so if I don't have the 45 000 value memorized which I certainly don't have that 327 16:12.380 --> 16:12.880 memorized 328 16:12.880 --> 16:18.640 it's written right there so I could make it 45 056 and it would take that but I 329 16:18.640 --> 16:19.040 do have to 330 16:19.040 --> 16:23.360 pick one of these again I said on the diagram that would make it 4096 so let's 331 16:23.360 --> 16:24.080 just do that 332 16:24.240 --> 16:32.140 and now if I do a show spanning tree for VLAN 10 we should see some interesting WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 332 16:24.240 --> 16:32.140 and now if I do a show spanning tree for VLAN 10 we should see some interesting 333 16:32.140 --> 16:32.960 things 334 16:32.960 --> 16:36.670 so first of all we do see that we're the root and the reason has nothing to do 335 16:36.670 --> 16:37.840 with the MAC address 336 16:37.840 --> 16:42.430 it has everything to do with the priority again this priority looks funny but 337 16:42.430 --> 16:44.400 it's 4096 plus 10 338 16:44.400 --> 16:49.550 and it says that right here for my bridge ID so that's where we get that from 339 16:49.550 --> 16:50.960 we also notice that 340 16:50.960 --> 16:55.720 we are going to transition our ports through listening and learning and from 341 16:55.720 --> 16:57.360 here we're going to also 342 16:57.360 --> 17:01.290 establish a new blocked port out on the network now we said if we look at our WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 342 16:57.360 --> 17:01.290 establish a new blocked port out on the network now we said if we look at our 343 17:01.290 --> 17:02.400 diagram it'll probably 344 17:02.400 --> 17:06.750 be something over here on switch three so let's go ahead and take a look at 345 17:06.750 --> 17:08.000 switch three and see 346 17:08.000 --> 17:15.130 what's transpiring over here show spanning tree VLAN 10 and we do see indeed 347 17:15.130 --> 17:16.960 that we are blocking 348 17:16.960 --> 17:22.120 gig 0 2 so let's go ahead and look at our diagram here so we're blocking gig 0 349 17:22.120 --> 17:23.040 2 why are we blocking 350 17:23.040 --> 17:29.410 gig 0 2 well for the same reason VLAN 1 chose to block the path to switch 1 351 17:29.410 --> 17:31.280 switch 1's MAC address WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 351 17:29.410 --> 17:31.280 switch 1's MAC address 352 17:31.280 --> 17:35.220 is going to be worse than switch 4's MAC address and so switch 3 has to make 353 17:35.220 --> 17:36.080 the same decision it 354 17:36.080 --> 17:39.740 says hey I've got a path to the root this way that's 8 I've got a path to the 355 17:39.740 --> 17:41.520 root this way that's 8 356 17:41.520 --> 17:47.420 and so I'm going to choose that arbitrary tiebreaker to suggest that switch 4 357 17:47.420 --> 17:48.560 is better off than switch 358 17:48.560 --> 17:52.830 1 thanks to the MAC address so switch 1 just keeps losing these tiebreaker 359 17:52.830 --> 17:54.240 battles and so this will 360 17:54.240 --> 17:59.380 be my blocked port on you know going to switch 1 this will be my root port and 361 17:59.380 --> 18:00.560 meanwhile we see WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 361 17:59.380 --> 18:00.560 meanwhile we see 362 18:00.560 --> 18:06.570 we'll see the BPDUs flow down through the network from switch 2 in the same way 363 18:06.570 --> 18:08.720 that BPDUs for VLAN 1 364 18:08.720 --> 18:13.070 would be flowing out of switch 3 so we see that we have two completely 365 18:13.070 --> 18:14.560 different topologies a 366 18:14.560 --> 18:19.620 different blocked port for VLAN 1 compared to VLAN 10 over here and we can do 367 18:19.620 --> 18:20.800 this on a per VLAN 368 18:20.800 --> 18:25.090 basis I could make switch 1 the root for VLAN 20 switch for the root for VLAN 369 18:25.090 --> 18:26.160 30 that's not 370 18:26.160 --> 18:30.120 usually how we do it in real world networks but at least we do have options WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 370 18:26.160 --> 18:30.120 usually how we do it in real world networks but at least we do have options 371 18:30.120 --> 18:31.440 available so let's go 372 18:31.440 --> 18:36.370 and flip over to lab a couple of nice commands just to see them in action here 373 18:36.370 --> 18:37.120 if I do show 374 18:37.120 --> 18:41.930 spanning tree and then the command blocked ports if I hit enter it will tell me 375 18:41.930 --> 18:42.960 all of the ports 376 18:42.960 --> 18:47.040 that are blocked on the switch so that's a really great way just to at a glance 377 18:47.040 --> 18:48.080 see which ports are 378 18:48.080 --> 18:52.800 blocked and on a per VLAN basis we'll notice here as well so if I go back to 379 18:52.800 --> 18:54.320 let's say switch 1 380 18:54.320 --> 19:00.980 uh not switch 1 switch 2 we should have some blocked ports here on the WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 380 18:54.320 --> 19:00.980 uh not switch 1 switch 2 we should have some blocked ports here on the 381 19:00.980 --> 19:02.160 different VLANs and we 382 19:02.160 --> 19:08.780 do so we see that with that original topology VLANs 1 20 and 30 are all blocked 383 19:08.780 --> 19:10.400 meanwhile I can also 384 19:10.400 --> 19:14.950 do a show spanning tree root and it will tell me information about the root for 385 19:14.950 --> 19:16.320 every single VLAN 386 19:16.320 --> 19:21.230 as well as importantly the root port how do I get to the root for each VLAN so 387 19:21.230 --> 19:22.000 these are two 388 19:22.000 --> 19:26.240 great commands that at a glance tell us a whole lot about the topology and it 389 19:26.240 --> 19:27.200 doesn't require us to 390 19:27.200 --> 19:31.730 the full show spanning tree command and try to absorb all of that information WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 390 19:27.200 --> 19:31.730 the full show spanning tree command and try to absorb all of that information 391 19:31.730 --> 19:33.440 so that is an overview 392 19:33.440 --> 19:38.800 of just getting into the lab running some spanning tree commands comparing the 393 19:38.800 --> 19:40.160 lab topology with our 394 19:40.160 --> 19:45.360 diagram lots in here and it's time for us to jump in the lab and have some of 395 19:45.360 --> 19:46.880 that fun ourselves