Measure what a 4-byte 802.1Q tag or an 8-byte QinQ stack costs in frame size, MTU and bandwidth.
The tag is inserted after the source MAC address, before the EtherType, which pushes the frame past the classic 1518-byte maximum and is why switches must accept 1522-byte baby-giant frames. Because the tag sits inside the MTU budget, every tag you add takes a byte of payload away. Unaccounted tags are a classic cause of intermittent large-packet loss: the frame passes on an access port and gets dropped on the trunk, so pings work while file transfers stall.
802.1Q Tag Overhead
Each 802.1Q tag adds 4 bytes — a 2-byte TPID (0x8100) and a 2-byte TCI holding priority, DEI and the 12-bit VLAN ID — so a tagged frame is 1522 bytes and a QinQ frame 1526.
Each 802.1Q tag adds 4 bytes — a 2-byte TPID (0x8100) and a 2-byte TCI holding priority, DEI and the 12-bit VLAN ID — so a tagged frame is 1522 bytes and a QinQ frame 1526. The tag is inserted after the source MAC address, before the EtherType, which pushes the frame past the classic 1518-byte maximum and is why switches must accept 1522-byte baby-giant frames. Because the tag sits inside the MTU budget, every tag you add takes a byte of payload away.
Unaccounted tags are a classic cause of intermittent large-packet loss: the frame passes on an access port and gets dropped on the trunk, so pings work while file transfers stall.
This calculator takes 4 inputs: IP packet size, Tag stack, Frame rate, Port MTU. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Not on a compliant switch — 802.1Q says the tag rides on top, so the port must carry 1522 bytes to keep a 1500-byte IP MTU. It only costs you payload when an intermediate device enforces a hard 1518-byte limit.