Which VLAN trunking protocol adds four bytes to the Ethernet frames?
802.1Q adds 4 bytes to the Ethernet frame. The process is known as 802.1Q tagging, and inserts a four-byte field into the Ethernet frame header between the source address and the Len/Etype fields. This tag identifies the frame as an 802.1Q frame and includes bits used to identify both the priority and the VLAN ID. The VLAN ID field indicates which VLAN the frame belongs to. An 802.1q trunk can support 4096 different VLANs. After the new tag field is inserted into the frame, the frame’s previous FCS field is recalculated and replaced. The following graphic shows both the ISL and 802.1Q frame formats as well as the original Ethernet frame:
Inter switch link (ISL) is a Cisco proprietary trunking protocol that handles the frame in a different manner. It adds a 26- byte frame header and 4-byte trailer to the frame.
LANE (LAN Emulation) is an IEEE standard for identifying VLANs on ATM networks.
802.10 is a Cisco proprietary method of identifying VLANs on FDDI media by writing VLAN information to the Security Association Identifier (SAID) of the 802.10 frame.
Layer 2 Technologies
Configure and verify trunking