VTP Pruning
VTP pruning offers a dynamic mechanism that automatically configures the allowed VLAN lists on trunks within a VTP domain.
Based upon whether or not a switch has ports currently active within a VLAN, the switch dynamically indicates to the remote switch on the other side of a trunk that the traffic within the inactive VLAN not be forwarded across a trunk.
VTP pruning blocks unneeded flooded traffic to VLANs on trunk ports that are included in the pruning-eligible list.
Only VLANs included in the pruning-eligible list can be pruned (VLANs 2-1001 by default).
Extended-range VLANs are pruning-ineligible.
VTP pruning is supported in all VTP versions.
Enabling VTP pruning on a VTP server enables pruning for the entire management domain.
Making VLANs pruning-eligible or pruning-ineligible affects pruning eligibility for those VLANs on that trunk only (not on all switches in the VTP domain).
VTP pruning is not designed to function in VTP transparent mode.
VTP pruning is disabled by default.
Enable VTP pruning on the VTP server with the vtp pruning command.
To configure VTP pruning on an interface, use the switchport trunk pruning vlan x,y-z.
VTP pruned VLANs on a trunk are still part of spanning tree. Effectively, VTP pruning does not decrease the number of spanning tree instances.
To enable pruning in the VTP administrative domain:
Switch#configure terminal
Switch(Config)#vtp pruning
Note: By default, pruning is disabled. You need to enable pruning on only one switch in VTP server mode.
Manual Pruning
By default, trunk ports permit traffic for all VLANs.
VTP pruning can only instruct a switch to not request traffic for specific VLANs.
Manual pruning allows an administrator to explicitly decide which VLANs can traverse across the link.
The command to do this is “switchport trunk allowed vlan x,y-z”.
This is the best method to prune VLANs because it limits the number of spanning tree instances to the number of VLANs allowed on the trunk.
Configuration:
Switch#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Swith(config)#int fastEthernet 0/1
!
! -- limit vlans allowed over trunk link
Switch(config-if)#switchport trunk allowed vlan 5,10,15
!
! -- add additional vlan to exisitng trunk
Switch(config-if)#switchport trunk allowed vlan add 11
Switch(config-if)#end
Switch#wr mem
Note: Make sure you replicate the same configuration to both end of trunk link.