Switched network creates one broadcast domain, similar to that of a VLAN powered network where all nodes belonging to the same VLAN are part of the same broadcast domain, receiving all broadcasts sent on their network.
Broadcasts can actually create problems by flooding the VLAN network.
Because the trunk links interconnecting network switches will carry these broadcasts to every switch in the network.
A switch must forward broadcast frames out all available ports in the broadcast domain because broadcasts are destined everywhere there is a listener.
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.
FIG:VLAN without Pruning
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.
FIG:VLAN With Pruning
To configure VTP pruning, perform this task in privileged mode:
Step 1 Enable VTP pruning in the management domain.
set vtp pruning enable
Step 2 (Optional) Make specific VLANs pruning-ineligible on the device. (By default, VLANs 2-1000 are pruning-eligible.
clear vtp pruneeligible vlan_range
Step 3 (Optional) Make specific VLANs pruning-eligible on the device.
set vtp pruneeligible vlan_range
Step 4 Verify the VTP pruning configuration.
show vtp domain
Step 5 Verify that the appropriate VLANs are being pruned on trunk ports.
show trunk