WEBVTT 1 00:00.000 --> 00:04.880 When we're configuring our spanning tree environment, we are going to want to 2 00:04.880 --> 00:05.320 be very 3 00:05.320 --> 00:09.760 intentional about which of our switches is the root. 4 00:09.760 --> 00:12.160 And keep in mind, this is on a per VLAN basis. 5 00:12.160 --> 00:17.570 So in a Cisco environment, we might choose VLANs 10, 20, and 30 for the root on 6 00:17.570 --> 00:18.440 the left. 7 00:18.440 --> 00:24.690 And then on the right, we might choose this to be the root for 15 and 25 and 35 8 00:24.690 --> 00:25.720 , something 9 00:25.720 --> 00:26.720 along those lines. 10 00:26.720 --> 00:30.020 I say Cisco environment, because at this point, we're still running per VLAN WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 10 00:26.720 --> 00:30.020 I say Cisco environment, because at this point, we're still running per VLAN 11 00:30.020 --> 00:30.440 spanning 12 00:30.440 --> 00:34.240 tree, we have the capability of still doing this with industry standard MST, 13 00:34.240 --> 00:34.960 which we'll 14 00:34.960 --> 00:36.880 talk about later in the course. 15 00:36.880 --> 00:40.660 But for right now, we're running PVST plus, and we want to make sure that we 16 00:40.660 --> 00:41.600 are, again, 17 00:41.600 --> 00:44.080 very intentional about the root configuration. 18 00:44.080 --> 00:49.540 Well, the root configuration is going to require us to configure a root 19 00:49.540 --> 00:52.080 priority on a per VLAN 20 00:52.080 --> 00:53.080 basis. 21 00:53.080 --> 00:56.040 This is going to be very important for us. 22 00:56.040 --> 00:57.960 And so we've got that command that we looked at. 23 00:57.960 --> 01:03.320 We can set the spanning tree priority on a per VLAN basis. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 23 00:57.960 --> 01:03.320 We can set the spanning tree priority on a per VLAN basis. 24 01:03.320 --> 01:08.200 And so I could set this to be, let's say, 4,0, 40, 96. 25 01:08.200 --> 01:14.200 I could set the backup root to be 8K for these VLANs 10, 20, and 30. 26 01:14.200 --> 01:19.040 Meanwhile, it is the root for these VLANs at 4K and the backup root over here 27 01:19.040 --> 01:19.840 at 8K. 28 01:19.840 --> 01:23.590 Those are the kinds of configurations that are very common in the real world, 29 01:23.590 --> 01:23.800 where I've 30 01:23.800 --> 01:28.800 got a priority that makes me the root for certain VLANs and then a priority 31 01:28.800 --> 01:29.440 that makes 32 01:29.440 --> 01:34.360 me the backup root for the other VLANs and vice versa on the other switch. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 32 01:29.440 --> 01:34.360 me the backup root for the other VLANs and vice versa on the other switch. 33 01:34.360 --> 01:38.370 We'll especially see this in a campus environment where we have distribution 34 01:38.370 --> 01:39.360 switches because 35 01:39.360 --> 01:43.540 it's pretty common for us to have a pair of distribution switches like this 36 01:43.540 --> 01:43.720 that go 37 01:43.720 --> 01:46.840 down to our access layer. 38 01:46.840 --> 01:50.720 So it's well and good that I can go out here and I can use my configuration 39 01:50.720 --> 01:51.280 command 40 01:51.280 --> 01:54.800 to set specific priority values. 41 01:54.800 --> 01:59.000 However, Cisco also gives us these macros. 42 01:59.000 --> 02:02.480 And they expect us to understand how these macros work. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 42 01:59.000 --> 02:02.480 And they expect us to understand how these macros work. 43 02:02.480 --> 02:06.410 It's something that shows up in a lot of Cisco materials and for that matter, 44 02:06.410 --> 02:07.080 it's something 45 02:07.080 --> 02:09.920 that we can be using in the real world. 46 02:09.920 --> 02:11.800 Now keep in mind what a macro is. 47 02:11.800 --> 02:18.280 A macro is really not a command so much as background commands. 48 02:18.280 --> 02:25.120 It's going to run commands in the background after we run the macro and these 49 02:25.120 --> 02:26.000 commands 50 02:26.000 --> 02:29.280 go into our configuration. 51 02:29.280 --> 02:32.880 Once I run a macro, I'm never going to see the macro inside of the config. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 51 02:29.280 --> 02:32.880 Once I run a macro, I'm never going to see the macro inside of the config. 52 02:32.880 --> 02:35.440 If I do a show run, the macro won't show up. 53 02:35.440 --> 02:40.890 It's these commands that end up running behind the scenes that get put into our 54 02:40.890 --> 02:42.480 configuration. 55 02:42.480 --> 02:45.640 But these are pretty handy because they do more than just run background 56 02:45.640 --> 02:46.280 commands for 57 02:46.280 --> 02:50.280 us, they also perform an evaluation of the current environment. 58 02:50.280 --> 02:54.280 Specifically, we have two primary macros we're going to talk about. 59 02:54.280 --> 02:58.800 The first one is our root primary macro. 60 02:58.800 --> 03:06.960 Root primary macros intention is to set the switch as the root. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 60 02:58.800 --> 03:06.960 Root primary macros intention is to set the switch as the root. 61 03:06.960 --> 03:09.280 This is what we're trying to accomplish. 62 03:09.280 --> 03:14.800 However, keep in mind that we don't know what priorities are out there right 63 03:14.800 --> 03:15.160 now. 64 03:15.160 --> 03:19.030 There could be a current root that's configured with a priority of 20k or 65 03:19.030 --> 03:19.960 something along 66 03:19.960 --> 03:21.560 those lines. 67 03:21.560 --> 03:24.760 This is why this macro is going to go out. 68 03:24.760 --> 03:30.600 It's going to discover the current root and it's going to figure out what its WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 68 03:24.760 --> 03:30.600 It's going to discover the current root and it's going to figure out what its 69 03:30.600 --> 03:31.440 priority 70 03:31.440 --> 03:35.120 is and that's the important part. 71 03:35.120 --> 03:39.720 Then from there, what it's going to do is it's going to set a priority that 72 03:39.720 --> 03:40.560 makes it 73 03:40.560 --> 03:42.760 the root. 74 03:42.760 --> 03:44.240 This is a little bit of a conversation. 75 03:44.240 --> 03:48.600 What priority is going to be required to set this switch as the root? 76 03:48.600 --> 03:53.070 What's interesting about this is if I have a lower MAC address, then that means 77 03:53.070 --> 03:53.440 all I 78 03:53.440 --> 03:58.000 have to do is to tie the current priority. 79 03:58.000 --> 04:04.850 If the current switch has a value of 12k, then I can just set myself to 12k and WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 79 03:58.000 --> 04:04.850 If the current switch has a value of 12k, then I can just set myself to 12k and 80 04:04.850 --> 04:05.200 I'm 81 04:05.200 --> 04:06.280 good. 82 04:06.280 --> 04:10.830 If I have a higher MAC address, well that means that I would lose the tie 83 04:10.830 --> 04:11.680 breaker. 84 04:11.680 --> 04:21.240 I'm going to go 4000 less than the current root priority. 85 04:21.240 --> 04:24.280 Because of this, the mileage may vary a little bit. 86 04:24.280 --> 04:29.090 I might run this command and it sets it to one value on one switch but a 87 04:29.090 --> 04:30.240 different value WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 87 04:29.090 --> 04:30.240 different value 88 04:30.240 --> 04:31.920 on a different switch. 89 04:31.920 --> 04:35.850 But importantly, notice that in either one of these situations, it's going to 90 04:35.850 --> 04:36.240 go the 91 04:36.240 --> 04:38.040 minimum distance. 92 04:38.040 --> 04:42.970 It's going to go down as few priority slots as needed in order to make us the 93 04:42.970 --> 04:43.960 priority. 94 04:43.960 --> 04:46.560 It's not just going to drop all the way down. 95 04:46.560 --> 04:49.640 It's going to go the minimum amount. 96 04:49.640 --> 04:54.960 The other caveat in this is that it will not pick zero as an option. 97 04:54.960 --> 05:01.440 So if the current root is 4000, 4096, and it has a better MAC address than me, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 97 04:54.960 --> 05:01.440 So if the current root is 4000, 4096, and it has a better MAC address than me, 98 05:01.440 --> 05:01.960 I can't 99 05:01.960 --> 05:03.560 actually become the root. 100 05:03.560 --> 05:05.640 I can go as low as 4000. 101 05:05.640 --> 05:09.530 And at that point, I can't go below the current root because zero isn't an 102 05:09.530 --> 05:10.200 option. 103 05:10.200 --> 05:13.650 It might say, well, it'd be nice if zero is an option except if the current 104 05:13.650 --> 05:14.120 root had 105 05:14.120 --> 05:16.400 a priority of zero, it would have the same problem. 106 05:16.400 --> 05:21.700 So either way, we're going to hit the limit for this command if there are pre- 107 05:21.700 --> 05:22.520 configured 108 05:22.520 --> 05:24.560 priorities out there. 109 05:24.560 --> 05:30.350 Usually, generally speaking, we want to use this command in a default WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 109 05:24.560 --> 05:30.350 Usually, generally speaking, we want to use this command in a default 110 05:30.350 --> 05:31.840 environment. 111 05:31.840 --> 05:36.130 If there are other switches out there that have configured priority values, 112 05:36.130 --> 05:36.960 then again, 113 05:36.960 --> 05:41.100 we've got some things in place to make this work still, but it isn't exactly 114 05:41.100 --> 05:41.920 necessarily 115 05:41.920 --> 05:44.040 going to do what we expect. 116 05:44.040 --> 05:48.220 But it is very important for us to understand what it is that this command is 117 05:48.220 --> 05:50.040 actually doing. 118 05:50.040 --> 05:57.120 Now there is another priority macro that we can use, which is known as root 119 05:57.120 --> 05:58.560 secondary. 120 05:58.560 --> 06:06.170 Root secondary is intended to set the switch, as we might imagine, as a backup WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 120 05:58.560 --> 06:06.170 Root secondary is intended to set the switch, as we might imagine, as a backup 121 06:06.170 --> 06:07.080 root. 122 06:07.080 --> 06:09.680 Now what is a backup root? 123 06:09.680 --> 06:13.840 Well, from a spanning tree perspective, it's a big nothing. 124 06:13.840 --> 06:18.360 I mean, there is no concept of a backup root in spanning tree. 125 06:18.360 --> 06:23.720 But what there is is the idea that if my primary root dies, then the switch 126 06:23.720 --> 06:24.200 with the 127 06:24.200 --> 06:26.880 next highest priority will take over. 128 06:26.880 --> 06:30.550 And so that's what we're trying to configure here, back up to this conversation WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 128 06:26.880 --> 06:30.550 And so that's what we're trying to configure here, back up to this conversation 129 06:30.550 --> 06:30.960 in a real 130 06:30.960 --> 06:35.100 world with distribution switches, we probably want our other distribution 131 06:35.100 --> 06:35.960 switch to take 132 06:35.960 --> 06:36.960 over. 133 06:36.960 --> 06:40.670 And so that's why we said we do 4,000 for one set of VLANs, but I'm going to do 134 06:40.670 --> 06:41.600 8,000 135 06:41.600 --> 06:42.600 for the other set. 136 06:42.600 --> 06:46.690 So if I really look at it from these VLANs perspective, I have a 4,000 for the 137 06:46.690 --> 06:47.280 root, but 138 06:47.280 --> 06:49.760 then the backup root is set for 8,000. 139 06:49.760 --> 06:53.840 And the assumption here is all these other switches are set to the default of 140 06:53.840 --> 06:54.600 32K. 141 06:54.600 --> 06:59.200 So is this switch doing anything for VLANs 10, 20, and 30? 142 06:59.200 --> 07:01.440 And the answer is absolutely not. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 142 06:59.200 --> 07:01.440 And the answer is absolutely not. 143 07:01.440 --> 07:06.160 It's just participating in the topology, just like any other switch would. 144 07:06.160 --> 07:11.940 But if I do lose my root, then this switch will take over as the new primary 145 07:11.940 --> 07:12.920 until that 146 07:12.920 --> 07:14.800 root comes back online. 147 07:14.800 --> 07:19.440 And so always keep that in mind that the root is extremely dynamic in spanning 148 07:19.440 --> 07:20.040 tree. 149 07:20.040 --> 07:23.470 And that can be good when I want to take over as root, but it can also be 150 07:23.470 --> 07:24.240 really bad. 151 07:24.240 --> 07:28.200 And we're going to see later on how we can protect our topology against a new 152 07:28.200 --> 07:28.920 root showing 153 07:28.920 --> 07:29.920 up. 154 07:29.920 --> 07:33.600 And we didn't actually want that root to take over as the root spanning tree WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 154 07:29.920 --> 07:33.600 And we didn't actually want that root to take over as the root spanning tree 155 07:33.600 --> 07:34.360 doesn't natively 156 07:34.360 --> 07:35.920 protect us against that. 157 07:35.920 --> 07:38.800 But Cisco does give us some protection mechanisms. 158 07:38.800 --> 07:42.600 So back to the conversation at hand, I want to be able to configure essentially 159 07:42.600 --> 07:44.000 this concept 160 07:44.000 --> 07:51.090 where the switch I'm on is going to take over as root for the primary when it 161 07:51.090 --> 07:52.520 goes down. 162 07:52.520 --> 07:58.180 So what I want to do is have a value that is better than the default value of 163 07:58.180 --> 07:59.080 32K, but 164 07:59.080 --> 08:01.960 not as good as what's configured with this macro. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 164 07:59.080 --> 08:01.960 not as good as what's configured with this macro. 165 08:01.960 --> 08:04.760 These macros are intended to be used together. 166 08:04.760 --> 08:09.120 Well we mentioned that this macro will go as low as not zero. 167 08:09.120 --> 08:10.840 So as low as 4K. 168 08:10.840 --> 08:14.520 So the range here, let's change colors back to the original blue. 169 08:14.520 --> 08:20.640 So the range for the primary, we know starts at 4K because it can't go lower 170 08:20.640 --> 08:21.920 than that. 171 08:21.920 --> 08:30.530 But it also has a maximum value of 24K, meaning it will not choose 28 and 32 is WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 171 08:21.920 --> 08:30.530 But it also has a maximum value of 24K, meaning it will not choose 28 and 32 is 172 08:30.530 --> 08:31.960 the default. 173 08:31.960 --> 08:34.040 So we wouldn't want to choose that either. 174 08:34.040 --> 08:36.800 Well that leaves that value right in the middle. 175 08:36.800 --> 08:46.360 So the secondary is going to simply set the priority to 28K. 176 08:46.360 --> 08:51.440 Now notice this isn't a particularly sophisticated macro. 177 08:51.440 --> 08:55.960 We just said that the primary macros can go out and perform this discovery. 178 08:55.960 --> 08:58.560 The secondary macro doesn't do that at all. 179 08:58.560 --> 09:03.190 It truly is just intended to work with the root primary in a default WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 179 08:58.560 --> 09:03.190 It truly is just intended to work with the root primary in a default 180 09:03.190 --> 09:04.400 environment. 181 09:04.400 --> 09:07.840 In a default environment, all of the switches are 32K. 182 09:07.840 --> 09:13.140 The primary is going to be somewhere in this range and therefore 28K is exactly 183 09:13.140 --> 09:13.280 what I 184 09:13.280 --> 09:17.920 would need in order to set myself up to be the backup root. 185 09:17.920 --> 09:19.240 So that's all we're doing. 186 09:19.240 --> 09:21.400 We're setting it at 28K. 187 09:21.400 --> 09:27.280 And even if this command doesn't make me the secondary, we don't really have a 188 09:27.280 --> 09:27.960 great way 189 09:27.960 --> 09:28.960 of knowing that. 190 09:28.960 --> 09:32.080 I mean keep in mind this might be a very large network. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 190 09:28.960 --> 09:32.080 I mean keep in mind this might be a very large network. 191 09:32.080 --> 09:35.680 And so I might get onto this switch and tell it, "Hey, you're the secondary." 192 09:35.680 --> 09:38.880 The primary is up here at let's say 24K. 193 09:38.880 --> 09:44.200 And I have no idea that this switch over here is also at 24K just with a worse 194 09:44.200 --> 09:45.320 MAC address. 195 09:45.320 --> 09:50.230 I have no way of knowing that because the BPDUs are flowing downstream this way 196 09:50.230 --> 09:50.560 . 197 09:50.560 --> 09:54.920 I would never have any idea that that switch is out there. 198 09:54.920 --> 09:56.400 Now this scenario, the primary goes down. 199 09:56.400 --> 09:58.440 My network is split in half. 200 09:58.440 --> 10:03.200 But the point is simply I don't see the BPDUs from all of the other switches. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 200 09:58.440 --> 10:03.200 But the point is simply I don't see the BPDUs from all of the other switches. 201 10:03.200 --> 10:07.670 I can only see what the root is, which is why I can perform this discovery for 202 10:07.670 --> 10:08.400 the current 203 10:08.400 --> 10:09.400 root. 204 10:09.400 --> 10:13.600 For the secondary, I'm just going to have to trust that this value will work. 205 10:13.600 --> 10:17.890 And I'm not going to know for sure just by entering the macro that this did 206 10:17.890 --> 10:18.640 indeed make 207 10:18.640 --> 10:21.640 me the secondary root. 208 10:21.640 --> 10:25.760 Now let's just take one quick moment and talk about the real world. 209 10:25.760 --> 10:30.680 In the real world, we typically don't use these macros a whole lot. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 209 10:25.760 --> 10:30.680 In the real world, we typically don't use these macros a whole lot. 210 10:30.680 --> 10:36.940 They are convenient, sure, but for the most part we want to be very intentional 211 10:36.940 --> 10:37.200 with how 212 10:37.200 --> 10:39.880 we're configuring our environment. 213 10:39.880 --> 10:46.140 And I will say that in my configurations, designs, everything I've ever done, I 214 10:46.140 --> 10:46.360 like 215 10:46.360 --> 10:51.600 to use something like 4K for the primary root, 8K for the backup root. 216 10:51.600 --> 10:54.360 And the macros don't really tie very well into this. 217 10:54.360 --> 10:58.160 But this is the best way to protect ourselves from other switches getting 218 10:58.160 --> 10:59.040 plugged in that 219 10:59.040 --> 11:00.880 maybe have lower priority values. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 219 10:59.040 --> 11:00.880 maybe have lower priority values. 220 11:00.880 --> 11:05.820 I wouldn't necessarily want to rely on 24K and 32K when somebody could plug a 221 11:05.820 --> 11:06.280 switch 222 11:06.280 --> 11:09.840 into the network that is configured for 4K. 223 11:09.840 --> 11:13.120 And so we don't want to have to worry about that. 224 11:13.120 --> 11:16.530 We just want to stick with something that's very intentional, that protects as 225 11:16.530 --> 11:16.920 well as 226 11:16.920 --> 11:18.560 we can. 227 11:18.560 --> 11:21.360 And for the most part, the macros aren't going to provide that. 228 11:21.360 --> 11:25.940 But that said, they are absolutely within the scope of this exam, within the 229 11:25.940 --> 11:26.640 scope of 230 11:26.640 --> 11:29.120 the blueprint and within the scope of this course. 231 11:29.120 --> 11:32.750 So we want to make sure we understand how these macros actually work under the WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 231 11:29.120 --> 11:32.750 So we want to make sure we understand how these macros actually work under the 232 11:32.750 --> 11:33.160 hood, 233 11:33.160 --> 11:35.560 what values they could possibly assign. 234 11:35.560 --> 11:38.880 And hey, playing around with it in the lab is a really good way to make sure 235 11:38.880 --> 11:39.280 that we 236 11:39.280 --> 11:42.280 cement that knowledge.