Original subtitles

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The default DTP mode depends on the Cisco IOS software version you're using

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and the platform of switch.

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In this example, I’ve got 2 switches that I've just booted up

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in other words, they have no configuration

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here switch 1 and let's change the name to S1, here’s switch 2.

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So let’s see what DTP mode the switches are using.

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You can view DTP setting by using the command sh dtp interface

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and specifying an interface, so I'll specify gigabit 0/0

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which is this link over here.

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What we can see in the output is that DTP is enabled

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the mode at the moment is access port.

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So the switch has negotiated with switch 2

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to use an access port rather than a trunk port

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and the reason why it’s because it's configured for dynamic auto

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not dynamic desirable

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as an example sh int g0/0 switchport

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shows us I'll just make this bigger

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that this port gigabit 0/0 is configured for dynamic auto as the administrative mode

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and the operational mode or mode that it's using at the moment is static access.

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DTP negotiation is on but because it’s configured for dynamic auto

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it ends up being an access port.

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Under this side show dtp interface g0/0

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you can see that it’s negotiated as an access port

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because dynamic auto is configured on the switch.

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sh int g0/0 switchport

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once again gigabits 0/0 is enabled as a switchport, it's configured for dynamic auto

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it’s operating as an access port even though DTP is enabled.

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So, let’s change that by going on to gigabit 0/0

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and typing switch port and we have a few options

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so let’s say mode dynamic

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which is setting the trunking mode to dynamically negotiate access

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or trunk mode and we can either use auto or desirable

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so I’m gonna configure this is desirable

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so previously on switch 1 when we type sh int g0/0 switchport

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it was operating as a static access port.

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What is it doing now?

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so sh int gigabit 0/0 switchport

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this side is using dynamic auto but the mode negotiated is a trunk port

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using the ISL encapsulation, it’s not using 802.1Q its using ISL.

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On this side sh int g0/0 switchport

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notice this side is dynamic desirable switch 1 is dynamic auto

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but we've configured switch 2 to use desirable

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so switch 2 initiated trunking with switch 1

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and it negotiated to use ISL and hence it becomes a trunk

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so when both sides are configured for auto it doesn’t become a trunk

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when 1 side is configured for auto

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and the other side is using desirable a trunk will be formed.

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So once again to prove that

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let's go back into the interface

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and I'll change the mode to auto

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I'll shut it down and then no shut it

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so that they can negotiate once again

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on this side notice static access it is no longer a trunk

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and on this side once again notice it's using a static access port

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because neither side are initiating trunking

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the port remains an access port and doesn’t become a trunk.

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If either side or both sides are configured

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to use desirable the port will become a trunk port.

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So let’s configure on both sides as dynamic desirable

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I'll shut it down and then no shut it

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sh int g0/0 switchport

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both side are configured for dynamic desirable

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so the link has become a trunk port

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you can also use the command sh int trunk

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and as we can see here gigabit 0/0 is a trunk port

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mode is desirable and the encapsulation is ISL native VLAN is 1

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all VLANs are allowed across the port

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we'll see something similar on switch 1

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so sh interface trunk

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notice desirable ISL is being used

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I'll set this side back to auto

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shut it down, no shut it, do sh int trunk

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sh int trunk we haven't got a trunk configured at the moment.

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Let’s see this side show interface trunk

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took a while but it's now negotiated to use a trunk with switch 2

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the mode on the left is auto

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the mode on the right is desirable.

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So these 2 switches on gigabit 0/0

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have negotiated to use trunking

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because 1 side in this case switch 2 is initiating the trunking.

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However, we change that again to auto.

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sh int trunk

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we don’t see any trunking.

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On this side, we don’t see any trunking.

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If we look at the interface you can see it’s an access port

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so no trunking is taking place.