1 00:00:00,000 --> 00:00:05,000 Now on links to devices that don’t support DTP 2 00:00:05,000 --> 00:00:08,000 the port will remain an access port 3 00:00:08,000 --> 00:00:11,000 so on this link between switch 2 and router 2 4 00:00:11,000 --> 00:00:18,000 the default mode is being used which is dynamic auto. 5 00:00:18,000 --> 00:00:21,000 so DTP is enabled but it's using dynamic auto. 6 00:00:21,000 --> 00:00:27,000 I’ll configure it to use desirable. 7 00:00:27,000 --> 00:00:35,000 sh int g0/2 switchport 8 00:00:35,000 --> 00:00:39,000 notice the administrative mode is dynamic desirable 9 00:00:39,000 --> 00:00:41,000 but the operational mode is static access. 10 00:00:41,000 --> 00:00:49,000 So it’s not trunking it's remained as an access port in the default VLAN of 1. 11 00:00:49,000 --> 00:00:53,000 and that’s because router 2 isn’t configured to use DTP 12 00:00:53,000 --> 00:00:57,000 this interfaces are routed interface 13 00:00:57,000 --> 00:01:00,000 so the switch port interface 14 00:01:00,000 --> 00:01:04,000 at the moment that interfaces is down, so let’s enable it 15 00:01:04,000 --> 00:01:07,000 and see if it makes any difference. 16 00:01:07,000 --> 00:01:16,000 But notice if we type the command sh switchport it’s not gonna work 17 00:01:16,000 --> 00:01:25,000 sh int f0/0 this is a routed port not a switchport interface. 18 00:01:25,000 --> 00:01:32,000 So it’s acting as a PC or traditional router on switch 2 19 00:01:32,000 --> 00:01:36,000 sh int g0/2 switchport 20 00:01:36,000 --> 00:01:38,000 notice the mode has remained a static access 21 00:01:38,000 --> 00:01:41,000 even though it’s configured as desirable 22 00:01:41,000 --> 00:01:46,000 that will be true for both dynamic auto and dynamic desirable. 23 00:01:46,000 --> 00:01:51,000 Unless both sides can negotiate to use DTP 24 00:01:51,000 --> 00:01:56,000 it will just remain an access port in the default VLAN which is VLAN 1. 25 00:01:56,000 --> 00:01:59,000 Now let’s look at some negotiation 26 00:01:59,000 --> 00:02:01,000 so let’s do some Wireshark captures 27 00:02:01,000 --> 00:02:08,000 so I start capturing traffic on the hub, in this case on port 2 28 00:02:08,000 --> 00:02:20,000 the switches are using gigabit 0/3, so sh int g0/3 switchport 29 00:02:20,000 --> 00:02:23,000 at the moment its configured to use dynamic auto 30 00:02:23,000 --> 00:02:26,000 in other words DTP but auto mode. 31 00:02:26,000 --> 00:02:32,000 The same is true on switch 2, notice dynamic auto. 32 00:02:32,000 --> 00:02:37,000 In the Wireshark captures we see Spanning Tree BPDUs 33 00:02:37,000 --> 00:02:43,000 we see some CDP messages and notice here we see dynamic trunk protocol. 34 00:02:43,000 --> 00:02:52,000 We can see a message from switch with MAC address ending in cc:03 35 00:02:52,000 --> 00:02:55,000 configured for auto access mode 36 00:02:55,000 --> 00:02:59,000 sh int g0/3 37 00:02:59,000 --> 00:03:09,000 notice this is the Mac address of switch 2 ending in cc:03 38 00:03:09,000 --> 00:03:12,000 so in the output, we can see the trunk status 39 00:03:12,000 --> 00:03:16,000 values configured for auto access 40 00:03:16,000 --> 00:03:26,000 it’s using ISL and the sender ID is switch 2. 41 00:03:26,000 --> 00:03:31,000 We can also see a message from switch 1 42 00:03:31,000 --> 00:03:38,000 notice MAC address ff0/3 sh int g0/3 43 00:03:38,000 --> 00:03:43,000 so there’s the command, notice the MAC address ends in ff0/3. 44 00:03:43,000 --> 00:03:53,000 This switch is also using DTP auto access mode scrolling down 45 00:03:53,000 --> 00:03:55,000 we can see it wants to negotiate to use ISL 46 00:03:55,000 --> 00:03:59,000 and there’s the MAC address of the switch. 47 00:03:59,000 --> 00:04:05,000 So at the moment, neither side is initiating trunking. 48 00:04:05,000 --> 00:04:10,000 So if we type sh int trunk 49 00:04:10,000 --> 00:04:14,000 you’ll notice gigabit 0/3 is not in the list of trunk ports. 50 00:04:14,000 --> 00:04:23,000 on either side sh int g0/3 switchport 51 00:04:23,000 --> 00:04:29,000 at the moment it’s operating as an access port. 52 00:04:29,000 --> 00:04:42,000 We can see that on both sides but if we change 1 side so int g0/3 53 00:04:42,000 --> 00:04:53,000 switchport mode dynamic desirable sh int trunk 54 00:04:53,000 --> 00:05:00,000 Notice that gigabit 0/3 is configured to use trunking 55 00:05:00,000 --> 00:05:05,000 with ISL encapsulation because this side is configured as desirable 56 00:05:05,000 --> 00:05:11,000 sh int trunk this side is configured as auto 57 00:05:11,000 --> 00:05:14,000 so they’ve negotiated to set up a trunk. 58 00:05:14,000 --> 00:05:19,000 In the Wireshark capture, I scroll down. 59 00:05:19,000 --> 00:05:25,000 Here we have a message from switch 2 60 00:05:25,000 --> 00:05:28,000 notice the mode is trunk desirable. 61 00:05:28,000 --> 00:05:38,000 So the switch is initiating the trunking using ISL with switch 1. 62 00:05:38,000 --> 00:05:41,000 Switch 1 is still using trunk auto 63 00:05:41,000 --> 00:05:47,000 but switch 2 is using dynamic desirable trunking 64 00:05:47,000 --> 00:05:49,000 so it’s initiating the trunking. 65 00:05:49,000 --> 00:05:53,000 What happens if we change the encapsulation? 66 00:05:53,000 --> 00:05:56,000 so int g0/3 67 00:05:56,000 --> 00:06:01,000 switchport trunk encapsulation dot1q 68 00:06:01,000 --> 00:06:05,000 sh int trunk on the left-hand side 69 00:06:05,000 --> 00:06:08,000 notice the trunking is changed to dot1q 70 00:06:08,000 --> 00:06:11,000 previously it did negotiate to use ISL. 71 00:06:11,000 --> 00:06:23,000 in the Wireshark capture, scrolling up, we should see some messages. 72 00:06:23,000 --> 00:06:29,000 Here’s switch 2 trunk desirable. 73 00:06:29,000 --> 00:06:32,000 Wanting to use 802.1q rather than ISL. 74 00:06:32,000 --> 00:06:39,000 so notice the trunk operating type is 802.1q 75 00:06:39,000 --> 00:06:43,000 switch 2 is negotiating switch 1 76 00:06:43,000 --> 00:06:47,000 which is still using dynamic auto to set up a trunk 77 00:06:47,000 --> 00:06:49,000 and that’s whats happened. 78 00:06:49,000 --> 00:06:52,000 Switch 1 is using auto mode 79 00:06:52,000 --> 00:07:01,000 but is currently using trunking using 802.1q with switch 2 80 00:07:01,000 --> 00:07:07,000 because switch 2 is configured with desirable mode 81 00:07:07,000 --> 00:07:09,000 and is initiating the trunk. 82 00:07:09,000 --> 00:07:14,000 So there some examples of using dynamic trunking protocol 83 00:07:14,000 --> 00:07:18,000 to dynamically configure a trunk between 2 switches. 84 00:07:18,000 --> 00:07:25,000 The recommendation once again is don’t use DTP 85 00:07:25,000 --> 00:07:30,000 so turn off DTP and manually configure your trunk 86 00:07:30,000 --> 00:07:35,000 and your encapsulation between both switches.