Size a switch CAM table from host count and VLAN membership, and see how long before growth fills it.
A switch learns a MAC per VLAN, not per box, so a trunked hypervisor with interfaces in several VLANs consumes several entries. Compound growth against the hardware limit gives the year the table runs out, which is the useful figure for a refresh plan. A full CAM table does not error — it floods unknown unicast out every port in the VLAN, so throughput quietly collapses and traffic that should be switched becomes visible to every host in the segment.
MAC Table Size
Entries = hosts × VLANs per host + static entries, because a MAC is learned once per VLAN it appears in.
Time to exhaustion
years to full = ln(capacity ÷ entries) ÷ ln(1 + growth).
Entries = hosts × VLANs per host + static entries, because a MAC is learned once per VLAN it appears in. A switch learns a MAC per VLAN, not per box, so a trunked hypervisor with interfaces in several VLANs consumes several entries. Compound growth against the hardware limit gives the year the table runs out, which is the useful figure for a refresh plan.
A full CAM table does not error — it floods unknown unicast out every port in the VLAN, so throughput quietly collapses and traffic that should be switched becomes visible to every host in the segment.
This calculator takes 5 inputs: Unique MAC addresses, VLANs each MAC appears in, Static and reserved entries, Hardware MAC table size, Annual host growth. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Aging (300 s by default) releases entries for hosts that go quiet, so the steady-state count is usually below the raw host count. Size for the peak, though: a boot storm or a scan can populate every entry at once.