What is VTP Pruning?
VTP pruning helps improve proper allocation and use of network bandwidth by reducing unnecessary flooded traffic, such as broadcast, multicast, unknown, and flooded unicast packets.
VTP pruning is disabled by default in Cisco switches. VTP pruning helps to send broadcasts only to those trunk links that actually needs the information. For example, if switch A does not have a port configured for VLAN 7, and broadcast is sent throughout VLAN 7, that broadcast or traffic will not pass through the trunk link to switch A.
VTP pruning should only be enabled on VTP servers, all the clients in the VTP domain will automatically enable VTP pruning. By default, VLANs 2 – 1001 are pruning eligible, but VLAN 1 can’t be pruned because it’s an administrative VLAN. Both VTP versions 1 and 2 support pruning.
To enable VTP pruning on a Cisco IOS switch, you use the vtp pruning VLAN configuration command. Once VTP pruning is enabled, you can optionally configure a prune eligible list if you want to limit the VLANs that can be pruned. The Example below demonstrates how to enable VTP pruning on Switch.
VTP Pruning configuration :
SW1(config-if)#switchport trunk pruning vlan 3-4
In a switched network without VTP pruning enabled. A port on Switch 1 and port on Switch 4 are assigned to VLAN 2. When a broadcast is sent from PC 1 connected to Switch 1. The Switch floods the broadcast to every switch in the network, Switches 3, 5, and 6 have no connected ports to VLAN 2, but will still receive the broadcast sent by switch 1.
However, when VTP pruning is enabled, the broadcast traffic from Switch 1 is not forwarded to Switches 3, 5, and 6 because traffic for VLAN 2 has been pruned (port on Switch 2 connecting switch 3 and port on Switch 1 connecting switch 5).