Switchport trunk encapsulation
Cisco switches have traditionally supported two encapsulation methods for trunks: ISL and dot1q . Newer switches no longer support ISL, so there is only one encapsulation method available. For this reason, the switchport trunk encapsulation command was removed. This is the reason that in newer IOS versions, this option is unavailable.
For example, in switches and platforms that support ISL, if you try to put an interface into trunk mode, you get this error message:
SW1(config)#interface fa0/14 SW1(config-if)#switchport mode trunk Command rejected: An interface whose trunk encapsulation is "Auto" can not be configured to "trunk" mode.
In order to set the mode to trunk, the encapsulation must be changed:
SW1(config-if)#switchport trunk encapsulation ? dot1q Interface uses only 802.1q trunking encapsulation when trunking isl Interface uses only ISL trunking encapsulation when trunking negotiate Device will negotiate trunking encapsulation with peer on interface
By setting it to dot1q, you can then successfully set the switchport mode to trunk:
SW1(config-if)#switchport trunk encapsulation dot1q SW1(config-if)#switchport mode trunk SW1(config-if)#
On newer platforms where ISL is not supported, there is no option to set the encapsulation:
CW2960#config terminal Enter configuration commands, one per line. End with CNTL/Z. CW2960(config)#interface gi1/0/14 CW2960(config-if)#switchport trunk encapsulation ? % Unrecognized command CW2960(config-if)#switchport trunk encapsulation ▒
This command is unavailable since dot1q is the only encapsulation method available and is active by default.