What is VTP Pruning?
VTP (VLAN Trunking Protocol) pruning is a feature in Cisco network switches that optimizes the use of bandwidth by reducing unnecessary traffic related to VLANs (Virtual Local Area Networks).
Simply put, it is a technique for controlling the distribution of VLAN details throughout a network.This technique helps to ensure that VLAN configurations are consistent and accurate across all devices in the network, reducing the chance of errors or misconfigurations. By centralizing VLAN information and automating the distribution process, VLAN pruning simplifies network management and improves overall network performance..
In a network without VTP pruning, all VLAN traffic is flooded over trunk links, even if the VLAN does not exist on the other side of the trunk. This can lead to inefficient use of bandwidth, as traffic for VLANs not needed on certain switches is still sent over the trunk link.
Here’s how VTP Pruning Enhances Network Efficiency:
Reduces Unnecessary Traffic: VTP pruning stops the traffic for certain VLANs from being sent across trunk links if no ports on the remote switch belong to these VLANs. This reduces unnecessary broadcast, multicast, and unicast traffic.
Bandwidth Optimization: By limiting VLAN traffic only to trunk links that lead to switches with ports in that VLAN, VTP pruning helps in optimizing the use of available bandwidth on trunk links.
Dynamic Management: VTP pruning is dynamic. If a switchport is added to a VLAN that was previously pruned on a trunk link, VTP pruning automatically adjusts and allows traffic for that VLAN on the trunk link.
Integration with VTP: VTP pruning works in conjunction with the VLAN Trunking Protocol, which is used to manage VLAN information in a network. VTP maintains VLAN configuration consistency by managing the additions, deletions, and name changes of VLANs across a network.
Compatibility and Configuration: It’s important to note that VTP pruning is compatible only with switches running VTP. Also, both ends of the trunk link must support and be configured for VTP pruning for it to work effectively.
VTP pruning is disabled by default in Cisco switches. VTP pruning helps to send broadcasts only to those trunk links that need the information. For example, if switch A does not have a port configured for VLAN 7, and the 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 t
SW1(config)#interface Fa0/1
SW1(config-if)#switchport trunk pruning vlan 3-4
Summary,
In a switched network without VTP pruning enabled. A port on Switch 1 and a 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).
In summary, VTP pruning is a useful feature for managing and optimizing VLAN traffic across trunk links in a network, particularly in environments with multiple VLANs and extensive trunking configurations. It helps in making efficient use of network resources by eliminating unnecessary traffic.
