VTP for VLAN management

Mod Comms

VTP Version 3

To overcome the false update issue, VTP version 3 [2] has introduced the primary and secondary server concept. Enter the vtp version 3 command to tell the switches to use VTP version 3. The vtp mode server command only sets a switch to indicate a secondary server. However, all VTP version 3 switches do not accept a VLAN update unless the update is generated by a primary server. The only way to change the VLAN database is first to promote a secondary server to a primary server with the vtp primary vlan command (Listing 6), modify the VLAN database, and finally demote it back to a secondary server for safety. By changing to transparent mode and then back to server mode, the primary server will act as a secondary server again.

Listing 6

Promoting Secondary Server

SW1#vtp primary vlan
This system is becoming primary server for feature vlan
No conflicting VTP3 devices found.
Do you want to continue? [confirm]
SW1#show vtp status
VTP Version                   : 3 (capable)
VTP version running           : 3
VTP Domain Name               : admin
VTP Pruning Mode              : Disabled
VTP Traps Generation          : Disabled
Device ID                     : aabb.cc00.0400
Feature VLAN:
VTP Operating Mode                : Primary Server
Number of existing VLANs          : 23
Number of existing extended VLANs : 2
Configuration Revision            : 2
<--Output Omitted-->

Some other features are also added in version 3; for example, increasing the VLAN propagating range from 1-1005 in previous versions to 1-4095. VTP version 3 also supports propagating Multiple Spanning Tree Protocol (MSTP) instance tables.

VTP Pruning

The VTP pruning feature can automatically block unnecessary traffic that passes through trunk links to avoid wasting bandwidth. In the scheme depicted in Figure 4, SW3 does not need any VLAN traffic other than VLAN10, so it notifies SW2 to prune all other VLAN traffic. SW2 only needs VLAN30 traffic, but it combines the SW3 request and tells SW1 to prune all VLANs other than VLAN10 and VLAN30.

Figure 4: Enabling VTP pruning can reduce unnecessary bandwidth usage on the trunk link.

The configuration is surprisingly easy. Just go to the VTP server and enter the command

vtp pruning

and all switches in the VTP domain will enable the pruning feature.


VTP is a convenient tool for VLAN management, but after hearing too many sad stories about the false database update incident, I strongly recommend using version 3 only, rather than version 1 or 2. VTP is a Cisco proprietary protocol, but you might consider an equivalent open standard called Generic VLAN Registration Protocol (GVRP) [3] for non-Cisco products.

The Author

Jan Ho is a network engineer living in Hong Kong. He writes network tutorials at https://www.jannet.hk/en/.

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