DTP modes on Cisco Catalyst Switches

In this short port we explore the possible DTP modes and configure them on a Catalyst switch. We also learn what the outcome would be if we combine them.

When configuring the Dynamic Trunking Protocol, Cisco recommends to set trunking as static. But here is the behaviour if we don’t set it to “switchport mode trunk”.

For Catalyst 2960 and Catalyst 3560, I found that the default Dynamic Trunking Protocol mode is “dynamic auto”.
Key behaviours:
  • Dynamic auto+ dynamic auto = access
  • dynamic auto+ access/trunk = access/trunk
  • Dynamic desirable + access = access
  • Dynamic desirable + else = trunk

Default modes

Here are a couple of outputs that reveal the default Dynamic Trunking Protocol modes, on Catalyst 2960 and Catalyst 3560:

on Catalyst 2960
on a 3560

A combination of the above

I’m going to set different DTP modes on both ends of the link, and see the behaviour of DTP on Catalyst switches. I have SW1 and SW0.

Dynamic desirable + Access –> Access

When I set DTP mode to “dynamic desirable” on SW1 and “access” on SW0, each¬†port on both sides of the link will operate in static access mode:
dtp dynamic desirablemode access

Dynamic Desirable + else –> Trunk

When I set DTP mode to “dynamic desirable” on SW1 and leave the default value on SW0, the port on SW1 will operate in static access mode. However, the port on SW0 will operate as a trunk:

dtp dynamic desirable mode

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