VLAN Trunking Protocol

VLAN Trunking Protocol, or VTP, is a Cisco proprietary protocol allowing you to pass VLAN information between switches.  There are three modes that VTP can be configured in

• Server (Default)
• Client
• Transparent

VTP Server mode is the default, and it will send VTP updates to other VTP servers or client devices over a trunk or port channel.  VTP Client mode will accept updates from VTP servers only.  If your switch is operating in VTP client mode, you cannot add new VLANs to it, and you would need to configure the VLANs on the server and have them propagate to the client.  From what it sounds like, the VTP client will accept updates from VTP servers and update its VLAN.dat file.  While this is true, the client can update the servers if your switch is in VTP client mode and has a higher revision number than what is on the servers. If there are VLAN mismatches, you will lose whatever VLANs are not consistent (perhaps even all of them, depending on how the switch was decommissioned before reinstalling).   VTP transparent mode is just how the name makes it seem to operate transparently to the VTP service.  A switch operating in VTP transparent mode will not send VTP updates or update its VLAN database if it receives a VTP update message, but it will forward it to the next switch.   Let’s take this network, for example,

The transparent switch receives the VTP update message and forwards it along.  Also, a transparent switch will allow you to add, delete and modify VLANs.

When putting an older switch into production, following these steps is the easiest way to ensure your current VTP configuration does not overwrite anything on your network.

• Off the production network, console into the device, and set the VTP mode to transparent.
• Issue the command del flash:VLAN.dat and accept any prompts to delete the file.
• At this point, I always reload the switch to be safe.
• When the switch comes back up, set the VTP mode to your desired mode (client or server)

Setting up VTP is pretty simple out of the box (that is, a new switch). The switch is set up in VTP server mode but will not begin forwarding VTP updates until a domain name is configured.  There are other configuration options, like pruning unused VLANs, setting a password, and defining the version, to name a few, but they are not required to get VTP up and running.  Let’s get VTP configured, and we want to set the domain name to itlearninz.com the password to Cisco, and let’s set VTP to prune unused VLANs

SW1(config)#vtp domain itlearninz.com
Changing VTP domain name from NULL to itlearninz.com
SW1(config)#
*Mar 22 14:32:39.167: %SW_VLAN-6-VTP_DOMAIN_NAME_CHG: VTP domain name changed to itlearninz.com.
SW1(config)#vtp password P@55w0rd
Setting device VTP password to P@55w0rd
SW1(config)#vtp pruning
Pruning switched on

The VTP domain and VTP password are case-sensitive and must match on both ends.

The VTP parameters you can configure are;

• Domain – Set a domain name for your VTP implementation
• File – Configures a file on Flash where VLAN information is stored
• Interface – what the source interface is for VTP updates
• Mode – Client, Server, or Transparent
• Password – Applies an MD5 hash to all VTP updates (is easily seen in IOS by typing.
• Pruning – The VLAN must be configured on a switchport; if it is not and VTP pruning is enabled, it will not propagate to that switch VLAN database, and you can see what VLANs VTP is propagating to your switch by typing show interface trunk on the upstream switch and look at what is circled

SW1#show int trunk | begin pruned
Port        Vlans in spanning tree forwarding state and not pruned
Et0/0       1,10,20,30
Et0/1       1,10,20,30,40
Et0/2       1,10,20,30,40