Cisco CCNA mock exam questions sample test – Question 344

.Which statement correctly identifies a difference between Inter-Switch Link (ISL) and 802.1q?

A. 802.1q uses a native VLAN, ISL does not.
B. Cisco devices support only ISL.
C. ISL uses a 12-bit VLAN number field, and 802.1q does not.
D. ISL modifies the original Ethernet frame, while 802.1q encapsulates the original Ethernet frame.

Correct Answer: A

Explanation:
802.1q defines a native virtual LAN (VLAN) on each trunk link, which defaults to VLAN 1. The 802.1q frame tagging method specifies that frames in the native VLAN will not be tagged
while transmitting over a trunk link. The switch on the other end of the link identifies a native VLAN frame by the absence of the 802.1q header. ISL does not have the concept of
native VLANs, and traffic from all VLANs is encapsulated.
While older Cisco devices support both the ISL and 802.1q frame tagging methods, ISL is a deprecated, Cisco-proprietary frame tagging method, and newer Cisco switches only
support the 802.1q standard. When switches from multiple vendors are installed in the network, the 802.1q frame tagging method should be used.
It is incorrect to state that ISL uses a 12-bit VLAN number field and 802.1q does not. ISL uses a 15-bit VLAN ID field, while 802.1q uses a 12-bit VLAN ID field.
ISL encapsulates the original Ethernet frame, adding a 26-byte header and a 4-byte trailer. 802.1q operates by inserting a 4-byte header inside the original Ethernet frame, then
recalculating the checksum (CRC) in the Ethernet trailer.
Objective:
LAN Switching Fundamentals
Sub-Objective:
Configure and verify Layer 2 protocols
References:
Cisco > Technology Support > LAN Switching > Virtual LANs/VLAN Trunking Protocol (VLANs/VTP) > Design > Design Technotes > Inter-Switch Link and IEEE 802.1Q Frame
Format > Document ID: 17056