Trunk Bandwidth Calculator
Estimate the total bandwidth demand on an 802.1Q trunk link carrying multiple VLANs.
Inputs
Used to estimate 802.1Q tag overhead (4 bytes/frame).
Total Trunk Bandwidth Needed
806.25Mbps
Base Bandwidth (before tagging)
800.00Mbps
802.1Q Tag Overhead
6.250Mbps
Recommended Link Speed
1 Gbps
Step by step
Values used
Number of VLANs = 8; Avg. Traffic per VLAN = 100 Mbps; Average Frame Size = 512 bytes
Trunk bandwidth
bandwidth = vlans × avg_traffic_per_vlan + (4 bytes/frame overhead)
Total Trunk Bandwidth Needed
= 806.25 Mbps
Base Bandwidth (before tagging)
= 800.00 Mbps
802.1Q Tag Overhead
= 6.250 Mbps
Recommended Link Speed
= 1 Gbps
How it works
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.
Formula
Trunk bandwidth
bandwidth = vlans × avg_traffic_per_vlan + (4 bytes/frame overhead)
- n_vlan
- Number of VLANs
- \bar{T}_{vlan}
- Average traffic per VLAN
- L_frame
- Average frame size in bytes
Frequently Asked Questions
What is 802.1Q tagging 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.
How many VLANs can a single trunk carry?
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.
Should I size the trunk for average or peak VLAN traffic?
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.