VTP Pruning on Cisco Switches

  • Work-from-home


Co Admin
Mar 15, 2007
VTP Pruning

Recall that, by definition, a switch must forward broadcast frames out all available ports in the broadcast domain because broadcasts are destined everywhere there is a listener. Unless forwarded by more intelligent means, multicast frames follow the same pattern.
In addition, frames destined for an address that the switch has not yet learned or has forgotten (the MAC address has aged out of the address table) must be forwarded out all ports in an attempt to find the destination. These frames are referred to as unknown unicast.
When forwarding frames out all ports in a broadcast domain or VLAN, trunk ports are included if they transport that VLAN. By default, a trunk link transports traffic from all VLANs, unless specific VLANs are removed from the trunk. Generally, in a network with several switches, trunk links are enabled between switches, and VTP is used to manage the propagation of VLAN information. This scenario causes the trunk links between switches to carry traffic from all VLANs, not just from the specific VLANs created.
Consider the network shown in Figure 6-4. When end user Host PC in VLAN 3 sends a broadcast, Catalyst switch C forwards the frame out all VLAN 3 ports, including the trunk link to Catalyst A. Catalyst A, in turn, forwards the broadcast on to Catalysts B and D over those trunk links.
Catalysts B and D forward the broadcast out only their access links that have been configured for VLAN 3. If Catalysts B and D do not have any active users in VLAN 3, forwarding that broadcast frame to them would consume bandwidth on the trunk links and processor resources in both switches, only to have switches B and D discard the frames.
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.
VTP pruning occurs as an extension to VTP version 1, using an additional VTP message type.
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 whether flooded traffic from a VLAN should use a trunk port.

NOTE: Even when VTP pruning has determined that a VLAN is not needed on a trunk, an instance of the Spanning Tree Protocol (STP) will run for every VLAN that is allowed on the trunk link. To reduce the number of STP instances, you manually should “prune” unneeded VLANs from the trunk and allow only the needed ones. Use the “switchport trunk allowed vlan” command to identify the VLANs that should be added or removed from a trunk.

Enabling VTP Pruning

By default, VTP pruning is disabled on IOS-based switches. To enable pruning, use the following global configuration command:
“Switch(config) # vtp pruning”
If you use this command on a VTP server, it also advertises that pruning needs to be enabled for the entire management domain. All other switches listening to that advertisement also will enable pruning.
When pruning is enabled, all general-purpose VLANs become eligible for pruning on all trunk links, if needed. However, you can modify the default list of pruning eligibility with the following interface-configuration command:
“Switch(config) # interface type mod/num”
“Switch(config-if)# switchport trunk pruning vlan {add | except | none | remove} vlan-list”
By default, VLANs 2 through 1001 are eligible, or “enabled,” for potential pruning on every trunk.
Use the following keywords with the command to tailor the list:
* vlan-list—An explicit list of eligible VLAN numbers (anything from 2 to 1001), separated by commas or by dashes.
* add vlan-list—A list of VLAN numbers (anything from 2 to 1001) is added to the already configured list; this is a shortcut to keep from typing a long list of numbers.
* except vlan-list—All VLANs are eligible except for the VLAN numbers listed (anything from 2 to 1001); this is a shortcut to keep from typing a long list of numbers.
* remove vlan-list—A list of VLAN numbers (anything from 2 to 1001) is removed from the already configured list; this is a shortcut to keep from typing a long list of numbers.

TIP Be aware that VTP pruning has no effect on switches in the VTP transparent mode. Instead, those switches must be configured manually to “prune” VLANs from trunk links. In this case, pruning is always configured on the upstream side of a trunk. (The downstream side switch doesn’t have any ports that belong to the pruned VLAN, so there is no need to prune from that end.) By default, VLANs 2 to 1001 are eligible for pruning. VLAN 1 has a special meaning because it sometimes is used for control traffic and is the default access VLAN on switch ports. Because of these historical reasons, VLAN 1 is never eligible for pruning. In addition, VLANs 1002 through 1005 are reserved for Token Ring and FDDI VLANs and are never eligible for pruning.

Command Recall:

“interface type mod/num switchport trunk pruning vlan {add | except |
none | remove} vlan-list”

  • Like
Reactions: nrbhayo and !Nena!
Nov 1, 2010
Hi red rose
thanks for the post ... how is the weather in karachi? better than here in the UK i bet :)
send us some sunshine :)
PM me ur email add :)
  • Like
Reactions: nrbhayo