What does the Cisco vtp pruning command do?

VTP pruning is a feature in Cisco switches which stops VLAN update information traffic from being sent down trunk links if the updates are not needed. This is done by forwarding broadcasts and unknown unicast frames on a VLAN over trunk links only if the receiving end of the trunk has ports that are in that VLAN. In other words, if a switch is aware of a VLAN but has no access ports configured it prunes that VLAN so doesn't receive broadcasts. If a new VLAN comes up on the switch, the switch sends a VTP request so that the upstream switch starts sending that VLAN over the trunk. When VTP pruning is enabled on a VTP server, pruning is enabled for the entire management domain.