DTP (Dynamic Trunking Protocol)

There are 2 ways to configure a Trunk Port. 1. Statically using (#switchport mode trunk) & another is dynamically using DTP (Dynamic Trunking Protocol).

• DTP auto negotiates between the two trunk ports to form a trunk port. There are 2 modes in DTP. DTP mode auto & DTP mode desirable.

• In DTP Desirable mode Switch asks first to the other end to form a trunk or else it becomes an Access port.

• In DTP Auto mode it waits until the other end Switch asks first or else it becomes an Access port.

  Trunk configuration:

• Select the type of Trunking (802.1Q/ISL/Negotiate (using DTP)):

  • Command is (# switchport trunk encapsulation (dot1q /isl/negotiate)

• Select the Administrative Mode:

  • Always use Trunk (Statically)

    • Command is (# switchport mode trunk)

  • Always not use Trunk

    • Command is (# switchport mode access)

  • Negotiate (Dynamically using DTP)

    • Command is (# switchport mode desirable)

      (asks (negotiates with) the other end to trunk & if negotiation success then it forms a Trunk or else it becomes an access port)

    • Command is (# switchport mode auto)

      (wait until the other end asks (negotiates) to be a trunk or else it becomes an access port)

• To check Trunk port settings (#show interfaces gigabitEthernet 0/1 switchport)

The individual combinations of port settings lead to following results:

• dynamic auto + dynamic auto = access

• dynamic auto + dynamic desirable = trunk

• dynamic desirable + dynamic desirable = trunk

• dynamic auto or dynamic desirable + trunk = trunk

• dynamic auto or dynamic desirable + access = access

Reference:

• CCNA 200-301 OCG, Volume 1, Pg. 180-183 - Wendell Odom.

• https://community.cisco.com/t5/switching/why-dtp-is-used/td-p/1377495