Saturday, 25 July 2009

VTP (VLAN Trunking Protocol) configuration

Things to remember when setting up VTP:

VTP Modes:

  • Server - This is the default setting for a switch. You can create, edit and remove VLANS for the VTP domain.
  • Client - A client is not able to create, edit or delete VLANS, but it will synchronize with the VTP server.
  • Transparent - A switch in this mode will not advertise it’s VLANS, nor will it synchronize with the VTP server. However, it will forward VTP advertisements received.

VTP v2 - provides support for Token Ring. If you don’t have a Token Ring environment, there is no need to enable VTP v2.

VTP sumary advertisements - these are sent out every 5 minutes by default. If the VTP domain in the advertisement is different that the switch’s VTP domain, then the advertisement is ignored. Once the switch determines the VTP domain is the same, it then looks at the revision number. If the switch has a revision number that is equal to or higher then the advertisement revision number, the advertisement is ignored.

Configuration on a Cisco 3550 switch that you want to be a “server”:

MDF# config t

MDF (config)# vtp mode server (default is server)

MDF (config)# vtp domain cisco (default is NULL and is case sensitive)

MDF (config)# vtp password cisco

Configuration on a Cisco 3550 switch that you want to be a “client”:

MDF# config t

MDF (config)# vtp mode client (default is server)

MDF (config)# vtp domain cisco (default is NULL and is case sensitive)

MDF (config)# vtp password cisco

Commands used with VTP troubleshooting:

“show vtp status”

“show vlan brief”

“delete flash:vlan.dat” to remove all VTP and VLAN information from the switch. A “write erase” will not delete this file, you must delete it manually.

WARNINGS:

Be sure to check the VTP information on a switch before adding it to a production network. A switch configured as a server with the same domain and a higher revision number can down an entire network!

How is this possible? For example, you recently replaced all of your old Cisco 2950 switches with new 3560s. The 2950s had a revision number of 25. When you added the 3560s, you did it all at once one evening. You also made a few changes to your VLANs and the new revison number is only 17. Lets say a 3560 failed and you replaced it with one of the old 2950s that is configured as a VTP server. The 2950s old VLAN information will replace all of the new VLANs on the other 3560s and all of your updates are lost.

No comments: