WEBVTT 1 00:00.000 --> 00:04.760 So at some point we might ask, "Hey, all of these timers are great, but where 2 00:04.760 --> 00:04.960 did they 3 00:04.960 --> 00:06.800 come from?" 4 00:06.800 --> 00:11.810 We've got the "Hello Timer" set at 2 seconds, "Ford Delay" set at 15 seconds, " 5 00:11.810 --> 00:12.320 Max Age" 6 00:12.320 --> 00:14.200 set at 20 seconds. 7 00:14.200 --> 00:17.930 And the reason I need to ask this question is because if I'm going to tune any 8 00:17.930 --> 00:18.360 of these 9 00:18.360 --> 00:24.320 parameters, I need to make sure that we're not causing loops in the network. 10 00:24.320 --> 00:29.750 One of the reality is that these two timers specifically were developed or were 11 00:29.750 --> 00:30.320 decided WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 11 00:29.750 --> 00:30.320 decided 12 00:30.320 --> 00:34.200 upon mathematically based on two parameters. 13 00:34.200 --> 00:39.020 First of all, based on the "Hello Timer" set, and second of all, something that 14 00:39.020 --> 00:39.880 we call 15 00:39.880 --> 00:44.600 the "NetworkDiameter". 16 00:44.600 --> 00:46.240 What is the network diameter? 17 00:46.240 --> 00:49.870 Well, we should probably answer that before we get too much further into this 18 00:49.870 --> 00:50.800 conversation. 19 00:50.800 --> 00:57.380 The network diameter is going to lay out for us how many switches in a row we 20 00:57.380 --> 00:58.120 can have 21 00:58.120 --> 01:02.880 in our network space, including the switch that we start with. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 21 00:58.120 --> 01:02.880 in our network space, including the switch that we start with. 22 01:02.880 --> 01:07.100 So if I'm going to go hop-by-hop through this network, I'm going to find that 23 01:07.100 --> 01:07.760 my network 24 01:07.760 --> 01:10.000 diameter is 4. 25 01:10.000 --> 01:11.880 Again, we include that first one. 26 01:11.880 --> 01:15.320 So 1, 2, 3, and 4. 27 01:15.320 --> 01:23.720 Meanwhile, if I have a square topology like this, then we might say, "Oh, well, 28 01:23.720 --> 01:24.000 my 29 01:24.000 --> 01:28.440 topology or my network diameter is maybe less than that except if I start at 30 01:28.440 --> 01:29.000 any one of 31 01:29.000 --> 01:36.880 these switches, 1, 2, well, starting with this one, 1, 2, 3, and then 4." WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 31 01:29.000 --> 01:36.880 these switches, 1, 2, well, starting with this one, 1, 2, 3, and then 4." 32 01:36.880 --> 01:39.400 So even this topology is still 4. 33 01:39.400 --> 01:42.290 And the best way to envision this is why it happens if one of these redundant 34 01:42.290 --> 01:42.840 links goes 35 01:42.840 --> 01:47.910 down, at that point I can tell that I'm going to have to go the same span, the 36 01:47.910 --> 01:49.800 same diameter. 37 01:49.800 --> 01:56.880 Now what about a situation like this, where now I've got a link in the middle? 38 01:56.880 --> 02:02.690 Well, we could say, "Oh, well, this is good because now I can maybe have a WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 38 01:56.880 --> 02:02.690 Well, we could say, "Oh, well, this is good because now I can maybe have a 39 02:02.690 --> 02:03.840 lower diameter 40 02:03.840 --> 02:08.080 in some capacity, but the problem is that we have to assume a worst-case 41 02:08.080 --> 02:08.960 scenario. 42 02:08.960 --> 02:14.020 Worst-case scenario is we lose our redundant links and we're right back to 1, 2 43 02:14.020 --> 02:15.120 , 3, 4." 44 02:15.120 --> 02:17.960 So kind of an interesting situation here. 45 02:17.960 --> 02:21.580 We more or less have to remove all the loops that could possibly happen in the 46 02:21.580 --> 02:22.400 network, 47 02:22.400 --> 02:26.360 remove all of the redundant links, and at that point figure out what the worst- 48 02:26.360 --> 02:27.160 case scenario 49 02:27.160 --> 02:31.320 is what this true diameter ends up being. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 49 02:27.160 --> 02:31.320 is what this true diameter ends up being. 50 02:31.320 --> 02:33.600 Now notice what else I didn't say in all of this. 51 02:33.600 --> 02:36.160 I did not say that we're starting from the root. 52 02:36.160 --> 02:39.610 In fact, very rarely do we start from the root because the root's actually kind 53 02:39.610 --> 02:40.000 of right 54 02:40.000 --> 02:42.400 in the middle of everything in theory. 55 02:42.400 --> 02:48.620 And so if I've got a topology that's like this, with the root up there in the 56 02:48.620 --> 02:49.400 middle, 57 02:49.400 --> 02:53.650 we might say, "Hey, you know, I've removed the redundant links here, then maybe 58 02:53.650 --> 02:54.360 my diameter 59 02:54.360 --> 02:56.880 is only 3, 1, 2, 3." 60 02:56.880 --> 03:00.680 And it's 3 this way as well, except again, we're not measuring from the root. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 60 02:56.880 --> 03:00.680 And it's 3 this way as well, except again, we're not measuring from the root. 61 03:00.680 --> 03:04.940 We're measuring the maximum length of switches from anyone switch to any other 62 03:04.940 --> 03:05.560 switch, and 63 03:05.560 --> 03:08.160 usually that's an edge switch to another edge switch. 64 03:08.160 --> 03:11.440 In this case, the diameter would be 5. 65 03:11.440 --> 03:14.880 So that is the definition of the network diameter. 66 03:14.880 --> 03:17.320 Back to our conversation about timers. 67 03:17.320 --> 03:22.720 We said this is based on the hello timer as well as the network diameter. 68 03:22.720 --> 03:27.750 So specifically, these values, 15 and 20 seconds, are going to be based on a 2 69 03:27.750 --> 03:30.920 second hello timer WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 69 03:27.750 --> 03:30.920 second hello timer 70 03:30.920 --> 03:38.920 and based on a diameter, believe it or not, of 7 switches. 71 03:38.920 --> 03:44.960 This is a very large diameter, and that creates relatively large timers. 72 03:44.960 --> 03:46.440 So that's kind of interesting. 73 03:46.440 --> 03:49.240 If I look at my network topology down here and say, "Hey, wait a second. 74 03:49.240 --> 03:54.760 I've got a diameter of 5, then maybe I could tune my timers down a little bit." 75 03:54.760 --> 04:02.010 Furthermore, if I change the hello timer, that's also going to change my max WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 75 03:54.760 --> 04:02.010 Furthermore, if I change the hello timer, that's also going to change my max 76 04:02.010 --> 04:02.720 age and 77 04:02.720 --> 04:05.600 my forwarding delay timers as well. 78 04:05.600 --> 04:07.760 Now we might look at that and say, "Okay, that's great." 79 04:07.760 --> 04:12.370 And I'll accept there was a lot of math involved with taking these two 80 04:12.370 --> 04:13.800 parameters, or really 81 04:13.800 --> 04:17.400 these two parameters here, and working our way back. 82 04:17.400 --> 04:20.730 How am I as a network engineer supposed to run all of that math if it's not a 83 04:20.730 --> 04:21.160 simple 84 04:21.160 --> 04:22.160 formula? 85 04:22.160 --> 04:26.660 Regardless of whether it is or is not a simple formula, fortunately for us, 86 04:26.660 --> 04:27.720 Cisco has yet 87 04:27.720 --> 04:28.720 another macro. 88 04:28.720 --> 04:30.760 In fact, it's the same macro. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 88 04:28.720 --> 04:30.760 In fact, it's the same macro. 89 04:30.760 --> 04:34.720 Remember that root or primary command? 90 04:34.720 --> 04:38.160 And for that matter, secondary, either way. 91 04:38.160 --> 04:42.720 As part of this macro, I can now specify the two different parameters. 92 04:42.720 --> 04:50.160 I can specify a diameter and I can specify a hello timer. 93 04:50.160 --> 04:55.520 Now what if my diameter is 5 and I want to change my hello time to 1 second? 94 04:55.520 --> 04:59.600 Well this will drastically improve or reduce, I should say, the timer. 95 04:59.600 --> 05:01.120 It's going to lower the timer. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 95 04:59.600 --> 05:01.120 It's going to lower the timer. 96 05:01.120 --> 05:03.600 So I can enter this end using this macro. 97 05:03.600 --> 05:08.810 Again, this is a macro and the result is it will change for me the hello time, 98 05:08.810 --> 05:09.200 which 99 05:09.200 --> 05:11.440 is nice based on this parameter. 100 05:11.440 --> 05:13.120 It will also calculate. 101 05:13.120 --> 05:23.340 It will do the math for me to figure out the forward delay as well as the max 102 05:23.340 --> 05:24.480 age. 103 05:24.480 --> 05:28.170 Generally speaking, if we are changing the timers, if we want to know what we 104 05:28.170 --> 05:28.920 should change 105 05:28.920 --> 05:34.820 our timers to, we should never or very rarely just change the timers to WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 105 05:28.920 --> 05:34.820 our timers to, we should never or very rarely just change the timers to 106 05:34.820 --> 05:36.240 something that we 107 05:36.240 --> 05:37.720 think might work better. 108 05:37.720 --> 05:42.610 Instead, we should really use this macro to figure out what these values should 109 05:42.610 --> 05:43.080 be. 110 05:43.080 --> 05:48.370 Now I might be in an environment where I've got my root switch set and it's set 111 05:48.370 --> 05:48.960 at 4K 112 05:48.960 --> 05:50.520 and I don't want to run these macros. 113 05:50.520 --> 05:54.720 I don't want to change my backup root configuration whenever the situation is. 114 05:54.720 --> 05:55.840 Well that's fine. 115 05:55.840 --> 06:04.610 I could actually go to a non-root switch, run the macros and see what the WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 115 05:55.840 --> 06:04.610 I could actually go to a non-root switch, run the macros and see what the 116 06:04.610 --> 06:06.280 timers are going 117 06:06.280 --> 06:07.720 to be. 118 06:07.720 --> 06:12.000 I can then take those timers and manually configure those on the root, which 119 06:12.000 --> 06:12.680 will then 120 06:12.680 --> 06:15.840 push the timers to the rest of the network. 121 06:15.840 --> 06:18.400 So I don't have to run this macro on the root switch. 122 06:18.400 --> 06:23.700 Instead, I can run it on any switch, get the values and then use those values 123 06:23.700 --> 06:24.640 on my actual 124 06:24.640 --> 06:26.640 root. 125 06:26.640 --> 06:32.120 So let's take a look in the lab as far as what exactly this looks like. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 125 06:26.640 --> 06:32.120 So let's take a look in the lab as far as what exactly this looks like. 126 06:32.120 --> 06:33.320 So I'm logged in here. 127 06:33.320 --> 06:34.320 I'm on switch one. 128 06:34.320 --> 06:38.000 We see switch one is the root for VLAN one. 129 06:38.000 --> 06:41.960 It's running my hello time of four seconds in my max age of 20 and for delay of 130 06:41.960 --> 06:42.520 15. 131 06:42.520 --> 06:46.800 So if I'm looking at this saying I want to make this as efficient as possible, 132 06:46.800 --> 06:47.260 which 133 06:47.260 --> 06:48.640 timer should I use? 134 06:48.640 --> 06:53.490 Well, we can go to any other switch in the environment like switch two here and 135 06:53.490 --> 06:54.000 run one 136 06:54.000 --> 06:55.000 of those macros. 137 06:55.000 --> 06:59.440 So I can say spanning tree VLAN one doesn't even have to be VLAN one really. 138 06:59.440 --> 07:02.910 And then I can run a question mark and then we see that I've got that root WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 138 06:59.440 --> 07:02.910 And then I can run a question mark and then we see that I've got that root 139 07:02.910 --> 07:03.480 macro. 140 07:03.480 --> 07:04.920 So then I run question mark. 141 07:04.920 --> 07:07.250 Let's just say I'm going to run the secondary command because I don't want to 142 07:07.250 --> 07:07.920 be disruptive. 143 07:07.920 --> 07:10.280 I don't want to actually take over as the root. 144 07:10.280 --> 07:12.120 And now we see that diameter command. 145 07:12.120 --> 07:17.970 So I can say diameter and this network is let's say diameter five question mark 146 07:17.970 --> 07:18.540 again. 147 07:18.540 --> 07:21.680 We can see that could specify the hello time. 148 07:21.680 --> 07:24.400 And let's just go ahead and say it's two seconds. 149 07:24.400 --> 07:29.760 So now what I've done is I have changed my local timer values. 150 07:29.760 --> 07:34.150 So if I do show spanning tree VLAN one, we'll see that my local value is two WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 150 07:29.760 --> 07:34.150 So if I do show spanning tree VLAN one, we'll see that my local value is two 151 07:34.150 --> 07:34.920 seconds for 152 07:34.920 --> 07:39.440 hello and notice the max age has shrunk and the four delay has shrunk. 153 07:39.440 --> 07:43.800 Now the network is still using whatever the root says, but I now have my values 154 07:43.800 --> 07:44.240 that I 155 07:44.240 --> 07:47.960 can configure on the root if I want to tune this environment. 156 07:47.960 --> 07:50.560 So two, 16 and 12. 157 07:50.560 --> 07:55.640 So I come back to switch one config T spanning tree VLAN one. 158 07:55.640 --> 07:57.240 I see that I've got my options here. 159 07:57.240 --> 08:01.240 So I'm going to set a hello time of two seconds. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 159 07:57.240 --> 08:01.240 So I'm going to set a hello time of two seconds. 160 08:01.240 --> 08:05.680 I'm going to set a max age of 16 seconds. 161 08:05.680 --> 08:12.320 And then I will set my forward time to it was 12 seconds, I believe, right? 162 08:12.320 --> 08:13.320 Right. 163 08:13.320 --> 08:14.320 Okay. 164 08:14.320 --> 08:15.320 So here we go. 165 08:15.320 --> 08:18.070 I have configured and tuned my network using the macro, even though I didn't 166 08:18.070 --> 08:18.640 use the macro 167 08:18.640 --> 08:19.680 on the root switch. 168 08:19.680 --> 08:24.080 So I come back here to switch to I do a show spanning tree VLAN one. 169 08:24.080 --> 08:28.080 And now we do see that the root is propagating the very values that I 170 08:28.080 --> 08:29.360 calculated here by 171 08:29.360 --> 08:30.520 using the macro. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 171 08:29.360 --> 08:30.520 using the macro. 172 08:30.520 --> 08:33.280 So the macro is very useful for us for tuning timers. 173 08:33.280 --> 08:36.560 And if we don't want to use it for setting the root priority, it can absolutely 174 08:36.560 --> 08:36.840 give 175 08:36.840 --> 08:40.240 us what we need when it comes to figuring out the most efficient way to deploy 176 08:40.240 --> 08:40.680 spanning 177 08:40.680 --> 08:42.320 tree timers into our environment.