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
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.
isl adds a total 30 bytes to the frame while dot1q only add 4 bytes. I think A is the correct answer.
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Question is which protocol “appends” 4 bytes CRC. While 802.1Q inserts just tag and then original FCS is recomputed the ISL encapsulates original packet (with its own FCS) and add new FCS(CRC) at the end.
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