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.
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