Trunk Bandwidth
Calculate the aggregate and per-VLAN bandwidth carried on an Ethernet trunk link.
About Trunk Bandwidth
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
Worked example
With the default values below, Trunk Bandwidth returns Total Trunk Bandwidth Needed of 806.25. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Number of VLANsaccepts 1–4,094
- 8
- Avg. Traffic per VLANminimum 0 Mbps
- 100 Mbps
- Average Frame Sizeaccepts 64–1,518 bytes
- 512 bytes
Results
- 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
Step by step
- 1Values used
- 2Trunk bandwidth
- 3Total Trunk Bandwidth Needed= 806.25 Mbps
- 4Base Bandwidth (before tagging)= 800.00 Mbps
- 5802.1Q Tag Overhead= 6.250 Mbps
- 6Recommended Link Speed= 1 Gbps
Assumptions and limits
- The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
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.