Original subtitles

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Now on links to devices that don’t support DTP

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the port will remain an access port

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so on this link between switch 2 and router 2

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the default mode is being used which is dynamic auto.

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so DTP is enabled but it's using dynamic auto.

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I’ll configure it to use desirable.

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sh int g0/2 switchport

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notice the administrative mode is dynamic desirable

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but the operational mode is static access.

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So it’s not trunking it's remained as an access port in the default VLAN of 1.

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and that’s because router 2 isn’t configured to use DTP

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this interfaces are routed interface

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so the switch port interface

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at the moment that interfaces is down, so let’s enable it

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and see if it makes any difference.

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But notice if we type the command sh switchport it’s not gonna work

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sh int f0/0 this is a routed port not a switchport interface.

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So it’s acting as a PC or traditional router on switch 2

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sh int g0/2 switchport

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notice the mode has remained a static access

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even though it’s configured as desirable

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that will be true for both dynamic auto and dynamic desirable.

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Unless both sides can negotiate to use DTP

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it will just remain an access port in the default VLAN which is VLAN 1.

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Now let’s look at some negotiation

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so let’s do some Wireshark captures

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so I start capturing traffic on the hub, in this case on port 2

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the switches are using gigabit 0/3, so sh int g0/3 switchport

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at the moment its configured to use dynamic auto

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in other words DTP but auto mode.

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The same is true on switch 2, notice dynamic auto.

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In the Wireshark captures we see Spanning Tree BPDUs

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we see some CDP messages and notice here we see dynamic trunk protocol.

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We can see a message from switch with MAC address ending in cc:03

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configured for auto access mode

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sh int g0/3

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notice this is the Mac address of switch 2 ending in cc:03

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so in the output, we can see the trunk status

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values configured for auto access

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it’s using ISL and the sender ID is switch 2.

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We can also see a message from switch 1

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notice MAC address ff0/3 sh int g0/3

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so there’s the command, notice the MAC address ends in ff0/3.

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This switch is also using DTP auto access mode scrolling down

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we can see it wants to negotiate to use ISL

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and there’s the MAC address of the switch.

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So at the moment, neither side is initiating trunking.

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So if we type sh int trunk

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you’ll notice gigabit 0/3 is not in the list of trunk ports.

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on either side sh int g0/3 switchport

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at the moment it’s operating as an access port.

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We can see that on both sides but if we change 1 side so int g0/3

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switchport mode dynamic desirable sh int trunk

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Notice that gigabit 0/3 is configured to use trunking

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with ISL encapsulation because this side is configured as desirable

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sh int trunk this side is configured as auto

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so they’ve negotiated to set up a trunk.

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In the Wireshark capture, I scroll down.

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Here we have a message from switch 2

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notice the mode is trunk desirable.

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So the switch is initiating the trunking using ISL with switch 1.

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Switch 1 is still using trunk auto

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but switch 2 is using dynamic desirable trunking

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so it’s initiating the trunking.

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What happens if we change the encapsulation?

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so int g0/3

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switchport trunk encapsulation dot1q

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sh int trunk on the left-hand side

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notice the trunking is changed to dot1q

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previously it did negotiate to use ISL.

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in the Wireshark capture, scrolling up, we should see some messages.

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Here’s switch 2 trunk desirable.

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Wanting to use 802.1q rather than ISL.

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so notice the trunk operating type is 802.1q

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switch 2 is negotiating switch 1

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which is still using dynamic auto to set up a trunk

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and that’s whats happened.

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Switch 1 is using auto mode

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but is currently using trunking using 802.1q with switch 2

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because switch 2 is configured with desirable mode

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and is initiating the trunk.

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So there some examples of using dynamic trunking protocol

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to dynamically configure a trunk between 2 switches.

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The recommendation once again is don’t use DTP

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so turn off DTP and manually configure your trunk

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and your encapsulation between both switches.