WEBVTT 0:00:03.260000 --> 0:00:07.560000 Welcome to this video, which is a quick refresher on Cisco's dynamic trunking 0:00:07.560000 --> 0:00:11.960000 protocol. So what is DTP? 0:00:11.960000 --> 0:00:16.880000 Well, when you're configuring a trunk, a VLAN trunk on a Cisco switch, 0:00:16.880000 --> 0:00:20.620000 I'm presuming you're going to be using 802.1Q, you have two ways you could 0:00:20.620000 --> 0:00:26.080000 possibly do it. You can configure as on. 0:00:26.080000 --> 0:00:29.180000 Now you don't actually say on any keywords, but that's another way of 0:00:29.180000 --> 0:00:32.400000 saying that you're statically configuring the trunk. 0:00:32.400000 --> 0:00:40.620000 Just as a refresher, that's when you go into the interface and you say 0:00:40.620000 --> 0:00:48.680000 switch port mode trunk. 0:00:48.680000 --> 0:00:53.980000 Alternatively, you can make use of Cisco's dynamic trunking protocol, 0:00:53.980000 --> 0:01:01.940000 which is a Cisco trunk, and maintain the trunk because DTP is also used 0:01:01.940000 --> 0:01:03.520000 as a keep alive mechanism. 0:01:03.520000 --> 0:01:07.420000 As long as there's a bidirectional flow of DTP every minute or so, that 0:01:07.420000 --> 0:01:09.220000 trunk will stay up. 0:01:09.220000 --> 0:01:13.520000 And that would be using either on or desirable modes, two different modes 0:01:13.520000 --> 0:01:18.120000 for doing DTP. You do want to be familiar with the differences between 0:01:18.120000 --> 0:01:20.620000 those two modes. 0:01:20.620000 --> 0:01:22.840000 So desirable initiates the trunk. 0:01:22.840000 --> 0:01:27.720000 So when you type switch port mode dynamic desirable, that means that interface 0:01:27.720000 --> 0:01:31.000000 will start sending out DTP messages and saying, hey, I don't know who's 0:01:31.000000 --> 0:01:33.720000 at the other end of this link, but if you can trunk with me, that would 0:01:33.720000 --> 0:01:38.100000 be great. And if it receives DTP back, now we've got that bidirectional 0:01:38.100000 --> 0:01:40.580000 flow of DTP the trunk can form. 0:01:40.580000 --> 0:01:45.880000 Alternatively, you can tell you switch port mode dynamic auto, and auto 0:01:45.880000 --> 0:01:48.280000 means you're sitting there passively listening. 0:01:48.280000 --> 0:01:52.600000 If DTP comes in, you will respond to it, but you will not initiate the 0:01:52.600000 --> 0:01:58.160000 exchange. So as you can see there, there's our command switch port mode 0:01:58.160000 --> 0:02:04.000000 dynamic, and then you simply choose between desirable and auto. 0:02:04.000000 --> 0:02:07.820000 Alternatively, you could completely disable DTP. 0:02:07.820000 --> 0:02:12.440000 For example, let's say you were trunking to something that was that didn't 0:02:12.440000 --> 0:02:17.660000 support DTP, like a Cisco router as an example, because only Cisco switches 0:02:17.660000 --> 0:02:22.340000 support DTP. Even Cisco routers and firewalls don't understand what it 0:02:22.340000 --> 0:02:25.800000 is. So if you want to trunk to one of those guys, or you want to trunk 0:02:25.800000 --> 0:02:29.900000 to a server or maybe a non-Cisco device, then you're not going to be doing 0:02:29.900000 --> 0:02:33.920000 dynamic. So in that particular case, on your Cisco switch, you'll say 0:02:33.920000 --> 0:02:41.260000 switch port mode trunk. 0:02:41.260000 --> 0:02:46.000000 But when you type switch port mode trunk, the default behavior is that 0:02:46.000000 --> 0:02:51.440000 trunk interface will still transmit DTP messages as like a courtesy for 0:02:51.440000 --> 0:02:53.400000 the other side to pick up on it. 0:02:53.400000 --> 0:02:56.780000 Now switch port mode trunk does not rely on DTP himself. 0:02:56.780000 --> 0:02:58.720000 He's just sending it out as a courtesy. 0:02:58.720000 --> 0:03:02.020000 Well, if I'm connecting to a non-Cisco switch or a server or something, 0:03:02.020000 --> 0:03:03.800000 I don't need to send DTP. 0:03:03.800000 --> 0:03:07.040000 Now DTP goes out very, very infrequently. 0:03:07.040000 --> 0:03:09.780000 It's not going to consume hardly any bandwidth. 0:03:09.780000 --> 0:03:12.040000 But if you are one of those types of people that says, you know what? 0:03:12.040000 --> 0:03:15.080000 I want to trim off everything I possibly can. 0:03:15.080000 --> 0:03:17.020000 Maybe for security reasons. 0:03:17.020000 --> 0:03:17.580000 Maybe say, you know what? 0:03:17.580000 --> 0:03:21.260000 If I'm sending out DTP and that's captured in a stiffer trace, people 0:03:21.260000 --> 0:03:24.540000 are going to see something about my switch that I don't want them to see. 0:03:24.540000 --> 0:03:25.880000 That's fair enough. 0:03:25.880000 --> 0:03:29.240000 Then you would just say switch port no negotiate. 0:03:29.240000 --> 0:03:35.060000 And that will completely disable DTP on your switch port. 0:03:35.060000 --> 0:03:40.360000 And here we can do show interface trunk or show interface switch port 0:03:40.360000 --> 0:03:54.960000 to view that. For example, show interface trunk. 0:03:54.960000 --> 0:04:05.020000 You can see here that this particular switch has several interfaces that 0:04:05.020000 --> 0:04:06.300000 are currently trunking. 0:04:06.300000 --> 0:04:09.800000 All of them are doing 802.1Q. 0:04:09.800000 --> 0:04:13.820000 These first few are in the desirable state. 0:04:13.820000 --> 0:04:16.640000 This last one here is auto. 0:04:16.640000 --> 0:04:20.760000 So whatever is connected to the other side of this interface must be transmitting 0:04:20.760000 --> 0:04:24.640000 DTP messages because we did successfully form a trunk. 0:04:24.640000 --> 0:04:29.460000 The end in front of the end dash, you know, 802.1Q, that end stands for 0:04:29.460000 --> 0:04:36.220000 dynamically negotiated and negotiated to that. 0:04:36.220000 --> 0:04:41.060000 So that concludes this session on the dynamic trunking protocol and a 0:04:41.060000 --> 0:04:43.260000 review of its operations and configuration.