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Switchport trunk encapsulation

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

switchport trunk encapsulation is a Cisco IOS interface command that selects the encapsulation protocol for a trunk port.

switchport trunk encapsulation selects the protocol a Cisco IOS trunk port uses: 802.1Q (dot1q) or ISL. On switches supporting both, manually choosing encapsulation is required before manually setting trunk mode. section 12.3.2

Configure a trunk with 802.1Q

Attempting switchport mode trunk while encapsulation is Auto is rejected. Configure the protocol first, then set trunk mode: switchport trunk encapsulation dot1q followed by switchport mode trunk. Verify the result with show interfaces trunk, which should display 802.1q and trunking. 

SW1(config-if)# switchport trunk encapsulation dot1q     #1
SW1(config-if)# switchport mode trunk                    #2
SW1(config-if)#
#1 Manually configures 802.1Q encapsulation
#2 The command works this time.

Review the command sequence showing why encapsulation must be selected before trunk mode on switches supporting both protocols. section 12.3.2

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes
Explains protocol choices, DTP interaction, command ordering, rejection behavior, and verification, but not frame-format internals.
Examples
Uses rejected and successful G0/0 configurations plus show interfaces trunk output displaying 802.1q and trunking.
What it assumes
Assumes interface configuration basics while teaching why dual-protocol switches require explicit encapsulation before trunk mode.
What you can do after
Operate/configure a trunk by selecting dot1q or ISL, enabling trunk mode, and verifying encapsulation and status.
How it connects
Connects trunk ports with DTP, 802.1Q, ISL, and switchport mode trunk.
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