Which statement correctly identifies a difference between Inter-Switch Link (ISL) and 802.1q?
- 802.1q uses a native VLAN, ISL does not.
- Cisco devices support only ISL.
- ISL uses a 12-bit VLAN number field, and 802.1q does not.
- ISL modifies the original Ethernet frame, while 802.1q encapsulates the original Ethernet frame.
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.
LAN Switching Fundamentals
Configure and verify Layer 2 protocols