Calculate the aggregate and per-VLAN bandwidth carried on an Ethernet trunk link.
A trunk link carries traffic for multiple VLANs simultaneously by inserting an 802.1Q tag (4 bytes) into each Ethernet frame to identify which VLAN it belongs to. Total trunk demand is the sum of each VLAN's average traffic, plus a small overhead contribution from the extra 4 bytes added to every frame — the overhead's proportional impact depends on average frame size, since small frames are affected more (percentage-wise) than large ones.
Trunk bandwidth
bandwidth = vlans × avg_traffic_per_vlan + (4 bytes/frame overhead)
802.1Q inserts a 4-byte VLAN tag into the Ethernet header of every frame crossing a trunk link, which slightly increases each frame's size. For small frames this overhead is a larger percentage of the total; for large frames it becomes negligible.
The 802.1Q standard supports up to 4094 usable VLAN IDs (0 and 4095 are reserved) on a single trunk, but the practical limit is set by the total bandwidth those VLANs demand and the switch's hardware capabilities, not by the tagging protocol itself.
Size for a blend: sum the average traffic across all VLANs as a baseline, then apply a peak factor (see the Peak Bandwidth Calculator) since VLANs rarely all peak simultaneously, but provisioning purely for the sum of averages risks congestion during correlated bursts.