Feb 02

VTP Pruning and manual pruning

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#wr mem

    Note: Make sure you replicate the same configuration to both end of trunk link.

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