VLAN trunks are capable of supporting 1-4094 VLANs however there are several mechanisms to reduce the actual number of VLANs whose traffic flows over the trunk.
Allowed VLANs
Trunks allow all VLANs by default, however they can be filtered down by using the switchport trunk allowed command.
Allowed and Active VLANs
In order for a VLAN to be active a VLAN must be in the allowed list configured on the trunk the VLAN must exist in the configuration of the switch and it must be in an active state and not shutdown or suspended. With PVST+ an STP instance is actively running on this trunk for the allowed VLANs on the trunk configuration.
Active and not pruned
VLANs are considered pruned when VTP detects that it does not need to forward frames on a particular trunk and ceases to do so. The active and allowed list consists of any VTP pruned VLANs and VLANs PVST+ considers the port blocking removed.
