Adding a Cisco switch to a VTP domain

Adding a Cisco switch to a VTP domain
By James on October 16, 2007
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Categories: Cisco Tags: IOS

Adding a switch to a VTP domain is fairly easy to do, but done incorrectly, can bring down a whole network. Fortunatly I have not had this happen to me, but I have heard horror stories. There are a few simple steps to take to make sure everything stays running smoothly.

The very first step to complete as soon as you are ready to put a new switch on a network and join it to the domain, is to make sure that the vtp mode is set to transparent. Setting the mode to transparent ensures that the “Configuration Revision” is set to 0. If the switch has been used in a lab and has vlans configured and a configuration revision that is higher than the domain server, even if the switch being added is in client mode, it will overwrite the server and propagate all of it’s vlans across the network.


This is directly taken from Cisco’s website:

A recently added switch can cause problems in the network. It can be a switch that was previously used in the lab, and a good VTP domain name was entered. The switch was configured as a VTP client and was connected to the rest of the network. Then, you brought the trunk link up to the rest of the network. In just a few seconds, the whole network can go down.

If the configuration revision number of the switch that you inserted is higher than the configuration revision number of the VTP domain, it propagates its VLAN database through the VTP domain.

This occurs whether the switch is a VTP client or a VTP server. A VTP client can erase VLAN information on a VTP server. You can tell this has occurred when many of the ports in your network go into the Inactive state but continue to assign to a nonexistent VLAN.

To make sure this does not happen, before you connect the switch to the network, make sure to set the vtp mode to transparent.
C3750-Client(config)#vtp mode transparent
C3750-Client(config)#exit
C3750-Client#show vtp status
C3750-Client#sh vtp status
VTP Version : 2
Configuration Revision : 0
Maximum VLANs supported locally : 1005
Number of existing VLANs : 7
VTP Operating Mode : Transparent
VTP Domain Name :
VTP Pruning Mode : Disabled
VTP V2 Mode : Disabled
VTP Traps Generation : Disabled
MD5 digest : 0×18 0×17 0xE9 0×22 0×49 0×96 0×0C 0×7E
Configuration last modified by 10.10.10.20 at 3-1-93 00:03:25

Now that this switch won’t overwrite the server, configure the vtp domain and password, and then change the mode to client.
C3750-Client(config)#vtp domain Test
Changing VTP domain name from NULL to Test
C3750-Client(config)#vtp password Testpassword
C3750-Client(config)#vtp mode client
Setting device to VTP CLIENT mode

Show the vtp status to confirm.
C3750-Client#show vtp status
VTP Version : 2
Configuration Revision : 62
Maximum VLANs supported locally : 1005
Number of existing VLANs : 38
VTP Operating Mode : Client
VTP Domain Name : Test
VTP Pruning Mode : Enabled
VTP V2 Mode : Enabled
VTP Traps Generation : Disabled
MD5 digest : 0xCD 0×9D 0xFF 0xC3 0×6F 0×63 0×5F 0xF5
Configuration last modified by 10.10.10.2 at 10-16-07 22:16:43

And then a show vlan and check the output to make sure all the vlans have propagated.
C3750-Client#show vlan

The only rule you really need when adding switches is to make sure the switch is in TRANSPARENT mode first.


url:http://jklogic.net/adding-a-cisco-switch-to-a-vtp-domain/