What is VTP pruning?

VTP pruning is a feature that automatically stops unnecessary Ethernet frames flooding over trunk link. If unnecessary flooding can be suppressed by VTP pruning, the bandwidth of the trunk link can be used more effectively.

Example of Unnecessary Ethernet Frame Flooding

In VTP, it is assumed that switches are connected to each other by trunk ports. Trunk ports belong to all VLANs by default. As a result, Ethernet frames of unwanted VLANs may be flooded on the trunk port, consuming extra bandwidth.

For example, consider the network diagram shown in the following figure: SW3 does not have an access port for VLAN 10; even if an Ethernet frame of VLAN 10 is forwarded to SW3, it will be discarded. When flooding SW3 with Ethernet frames of VLAN 10, extra bandwidth is consumed.

Figure Unnecessary flooding example
Figure Unnecessary flooding example

To prevent Ethernet frame flooding of unwanted VLANs on trunk ports, there are two options.

  • Use switchport trunk allowed vlan command to manually limit the VLAN to which the trunk port belongs.
  • Enable VTP pruning.

The switchport trunk allowed vlan command is time-consuming because it requires the administrator to identify the areas where unwanted flooding occurs and enter the commands manually. VTP pruning, on the other hand, can automatically stop unwanted flooding as long as it is enabled.

For more information about the switchport trunk allowed vlan command, see the following article.

VTP pruning behavior

VTP pruning notifies that Ethernet frames for a particular VLAN are not needed by using VTP pruning messages.

Let’s consider again the previous network diagram: since SW3 does not have an access port for VLAN 10, SW3 will discard VLAN 10 Ethernet frames even if it receives them. If VTP pruning is enabled, SW3 will send a VTP pruning message to notify that Ethernet frames of VLAN 10 are not needed.

Figure VTP pruning message
Figure VTP pruning message

As a result, SW2 will not flood Ethernet frames of VLAN 10 to Fa0/8 in the direction of SW3.

Figure Stopping Flooding by VTP Pruning
Figure Stopping Flooding by VTP Pruning

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