The default DTP mode depends on the Cisco IOS software version you're using
and the platform of switch.
In this example, I’ve got 2 switches that I've just booted up
in other words, they have no configuration
here switch 1 and let's change the name to S1, here’s switch 2.
So let’s see what DTP mode the switches are using.
You can view DTP setting by using the command sh dtp interface
and specifying an interface, so I'll specify gigabit 0/0
which is this link over here.
What we can see in the output is that DTP is enabled
the mode at the moment is access port.
So the switch has negotiated with switch 2
to use an access port rather than a trunk port
and the reason why it’s because it's configured for dynamic auto
not dynamic desirable
as an example sh int g0/0 switchport
shows us I'll just make this bigger
that this port gigabit 0/0 is configured for dynamic auto as the administrative mode
and the operational mode or mode that it's using at the moment is static access.
DTP negotiation is on but because it’s configured for dynamic auto
it ends up being an access port.
Under this side show dtp interface g0/0
you can see that it’s negotiated as an access port
because dynamic auto is configured on the switch.
sh int g0/0 switchport
once again gigabits 0/0 is enabled as a switchport, it's configured for dynamic auto
it’s operating as an access port even though DTP is enabled.
So, let’s change that by going on to gigabit 0/0
and typing switch port and we have a few options
so let’s say mode dynamic
which is setting the trunking mode to dynamically negotiate access
or trunk mode and we can either use auto or desirable
so I’m gonna configure this is desirable
so previously on switch 1 when we type sh int g0/0 switchport
it was operating as a static access port.
What is it doing now?
so sh int gigabit 0/0 switchport
this side is using dynamic auto but the mode negotiated is a trunk port
using the ISL encapsulation, it’s not using 802.1Q its using ISL.
On this side sh int g0/0 switchport
notice this side is dynamic desirable switch 1 is dynamic auto
but we've configured switch 2 to use desirable
so switch 2 initiated trunking with switch 1
and it negotiated to use ISL and hence it becomes a trunk
so when both sides are configured for auto it doesn’t become a trunk
when 1 side is configured for auto
and the other side is using desirable a trunk will be formed.
So once again to prove that
let's go back into the interface
and I'll change the mode to auto
I'll shut it down and then no shut it
so that they can negotiate once again
on this side notice static access it is no longer a trunk
and on this side once again notice it's using a static access port
because neither side are initiating trunking
the port remains an access port and doesn’t become a trunk.
If either side or both sides are configured
to use desirable the port will become a trunk port.
So let’s configure on both sides as dynamic desirable
I'll shut it down and then no shut it
sh int g0/0 switchport
both side are configured for dynamic desirable
so the link has become a trunk port
you can also use the command sh int trunk
and as we can see here gigabit 0/0 is a trunk port
mode is desirable and the encapsulation is ISL native VLAN is 1
all VLANs are allowed across the port
we'll see something similar on switch 1
so sh interface trunk
notice desirable ISL is being used
I'll set this side back to auto
shut it down, no shut it, do sh int trunk
sh int trunk we haven't got a trunk configured at the moment.
Let’s see this side show interface trunk
took a while but it's now negotiated to use a trunk with switch 2
the mode on the left is auto
the mode on the right is desirable.
So these 2 switches on gigabit 0/0
have negotiated to use trunking
because 1 side in this case switch 2 is initiating the trunking.
However, we change that again to auto.
sh int trunk
we don’t see any trunking.
On this side, we don’t see any trunking.
If we look at the interface you can see it’s an access port
so no trunking is taking place.