CyberCertPrepNetwork FundamentalsSwitching & Routing BasicsEASYFree question
What is the purpose of 802.1Q VLAN tagging in Ethernet frames, and what is the maximum size of a tagged Ethernet frame?
- A.802.1Q tags are 8 bytes and reduce the maximum payload size from 1500 to 1492 bytes per frame
- B.802.1Q inserts a 4-byte tag in the Ethernet frame after the source MAC address; the tag contains the 12-bit VLAN ID (VID, 0-4094), 3-bit CoS/PCP priority bits, and a 1-bit DEI (Drop Eligible Indicator). A standard Ethernet frame is 1518 bytes maximum; with an 802.1Q tag, it becomes 1522 bytes (a "baby giant" frame)
- C.802.1Q tags are added by the end device (PC) to identify its VLAN; switches read but do not modify tags
- D.802.1Q VLAN tags are only used on access ports; trunk ports forward frames without modification
Why B is correct
IEEE 802.1Q tag: inserted between source MAC and EtherType. 4 bytes total: 2-byte TPID (0x8100) + 2-byte TCI (3-bit PCP/CoS + 1-bit DEI + 12-bit VLAN ID). Standard Ethernet max = 1518 bytes (14 header + 1500 payload + 4 FCS). Tagged = 1522 bytes (add 4-byte tag). Switches must be configured to accept 'baby giant' frames or jumbo frames for tagged traffic. End devices typically do not add 802.1Q tags; the switch adds/strips them on trunk/access ports. Access ports strip the tag before delivering to end devices.
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