Here is an example drawn from a working trunk on a 6500
description example of a trunk
switchport trunk encapsulation dot1q
switchport trunk native vlan 105
switchport trunk allowed vlan 105,123,127
switchport mode trunk
I'm using Packet Tracer but I don't see this line exist
Switch(config-if)#switchport trunk encapsulation dot1q
What is this line for?
To correctly deliver the traffic on a trunk port with several VLANs, the device uses the IEEE 802.1Q encapsulation (tagging) method that uses a tag that is inserted into the frame header. This tag carries information about the specific VLAN to which the frame and packet belong. This method allows packets that are encapsulated for several different VLANs to traverse the same port and maintain traffic separation between the VLANs. The encapsulated VLAN tag also allows the trunk to move traffic end-to-end through the network on the same VLAN.
A trunk is a point-to-point link between the device and another networking device. Trunks carry the traffic of multiple VLANs over a single link and allow you to extend VLANs across an entire network.
you will get more understanding from the URL
Some switches support multiple encapsulation types.
If the command is not available then don't worry about it. It simply means that the switch only supports that encapsulation so there is no need for the command.