Which switch command enables a trunking protocol that appends a four byte CRC to the packet?

You need to configure a new Company switch for trunking. Which switch command enables
a trunking protocol that appends a four byte CRC to the packet?

A.
CompanySwitch(config-if)#switchport trunk encapsulation dot1q

B.
CompanySwitch(config-if)#switchport trunk encapsulation itef

C.
CompanySwitch(config-if)#switchport trunk encapsulation fddi

D.
CompanySwitch(config-if)#switchport trunk encapsulation isl

E.
None of the other alternatives apply

Explanation:
The Inter-Switch Link (ISL) protocol is a Cisco proprietary method for preserving the source
VLAN identification of frames passing over a trunk link. ISL performs frame identification in
Layer 2 by encapsulating each frame between a header and trailer. Any Cisco switch or
router device configured for ISL can process and understand the ISL VLAN information. ISL

is primarily used for Ethernet media, although Cisco has included provisions to carry Token
Ring, FDDI, and ATM frames over Ethernet ISL. (A Frame-Type field in the ISL header
indicates the source frame type.)
When a frame is destined out a trunk link to another switch or router, ISL adds a 26-byte
header and a 4-byte trailer to the frame. The source VLAN is identified with a 10-bit VLAN ID
field in the header. The trailer contains a cyclic redundancy check (CRC) value to ensure the
data integrity of the new encapsulated frame. Figure 6-3 shows how Ethernet frames are
encapsulated and forwarded out a trunk link. Because tagging information is added at the
beginning and end of each frame, ISL is sometimes referred to as double tagging.