How large is the additional tag included within each frame when using IEEE 802.1Q?

When using IEEE 802.1Q for virtual LAN (VLAN) tagging, the protocol adds a tag to each Ethernet frame to indicate the VLAN to which the frame belongs. This additional tag is known as the VLAN header and is 4 bytes in size.

The 4-byte tag includes several important fields: a 2-byte Tag Protocol Identifier (TPID) and a 2-byte Tag Control Information (TCI) field. The TPID is generally set to the value of 0x8100 to indicate that the frame is VLAN-tagged, while the TCI contains information such as the VLAN ID and priority level for quality of service (QoS) purposes.

Understanding the size of this tag is crucial for network design and troubleshooting, as it impacts the overall frame size and potentially the performance of network equipment. Therefore, recognizing that the additional tag size is 4 bytes is essential for anyone involved in advanced networking tasks involving VLANs.