# 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 ](https://app.gitbook.com/@mudassirs46/s/network-fundamentals/~/drafts/-MRZ8l67L5MHnaQIEh9W/trunking-protocol-802.1q)/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>