VTP Pruning
A switch must forward broadcast frames out all available ports in the broadcast domain because broadcasts are destined everywhere there is a listener. Multicast frames, unless forwarded by more intelligent means, follow the same pattern. In addition, frames destined for an address that the switch has not yet learned or has forgotten must be forwarded out all ports in an attempt to find the destination. When forwarding frames out all ports in a broadcast domain or VLAN, trunk ports are included. By default, a trunk link transports traffic from all VLANs, unless specific VLANs are removed from the trunk with the clear trunk command. In a network with several switches, trunk links are enabled between switches and VTP is used to manage the propagation of VLAN information. This causes the trunk links between switches to carry traffic from all VLANs.
VTP pruning makes more efficient use of trunk bandwidth by reducing unnecessary flooded traffic. Broadcast and unknown unicast frames on a VLAN are forwarded over a trunk link only if the switch on the receiving end of the trunk has ports in that VLAN. In other words, VTP pruning allows switches to prevent broadcasts and unknown unicasts from flowing to switches that do not have any ports in that VLAN. VTP pruning occurs as an extension to VTP version 1. When a Catalyst switch has a port associated with a VLAN, the switch sends an advertisement to its neighbor switches that it has active ports on that VLAN. The neighbors keep this information, enabling them to decide if flooded traffic from a VLAN should use a trunk port or not.
By default, VTP pruning is disabled on IOS-based and CLI-based switches. On IOS-based switches, the vtp pruning command in the VLAN database configuration mode, the can be used to enable pruning while the set vtp pruning enable command can be used to enabled VTP pruning on CLI-based switches.