VTP V3 allows each router or LAN device to transmit advertisements in frames on its trunk ports. These frames are sent to a multicast address where they can be received by all neighboring devices. They are not forwarded by normal bridging procedures. An advertisement lists the sending device's VTP management domain, its configuration revision number, the VLANs which it knows about, and certain parameters for each known VLAN. By hearing these advertisements, all devices in the same management domain learn about any new VLANs that are configured in the transmitting device. This process allows you to create and configure a new VLAN only on one device in the management domain, and then that information is automatically learned by all the other devices in the same management domain.

Once a device learns about a VLAN, the device receives all frames on that VLAN from any trunk port by default, and if appropriate, forwards them to each of its other trunk ports, if any. This process prevents unnecessary VLAN traffic from being sent to a device. An extension of VTP called VTP pruning has been defined to limit the scope of broadcast traffic and save bandwidth. Beginning with Release 5.1(1), the Cisco NX-OS software supports VTP pruning.

VTP also publishes information about the domain and the mode in a shared local database that can be read by other processes such as Cisco Discovery Protocol (CDP).

From Cisco NX-OS Release 7.2(0)N1(1), VTP V3 supports the following modes:

• Transparent—Allows you to relay all VTP protocol packets that it receives on a trunk port to all other trunk ports. When you create or modify a VLAN that is in VTP transparent mode, those VLAN changes affect only the local device. A VTP transparent network device does not advertise its VLAN configuration and does not synchronize its VLAN configuration based on received advertisements. You cannot configure VLANs 1002 to 1005 in VTP client/server mode because these VLANs are reserved for Token Ring.

• Server— Allows you to create, remove, and modify VLANs over the entire network. You can set other configuration options like the VTP version and also turn on or off VTP pruning for the entire VTP domain. VTP servers advertise their VLAN configuration to other switches in the same VTP domain and synchronize their VLAN configuration with other switches based on messages received over trunk links. Beginning with Release 5.1(1), the server mode is the default mode. The VLAN information is stored on the bootflash and is not erased after a reboot.

• Client— Allows you to create, change, and delete VLANs on the local device. In VTP client mode, a switch stores the last known VTP information including the configuration revision number, on the bootflash. A VTP client might or might not start with a new configuration when it powers up.

• Off— Behaves similarly to the transparent mode but does not forward any VTP packets. The off mode allows you to monitor VLANs by using the CISCO-VTP-MIB without having to run VTP. On Cisco Nexus 7000 Series devices, because VTP is a conditional service, its MIB is loaded only when the corresponding feature is enabled. The CISCO-VTP-MIB does not follow this convention. It is loaded by the VLAN manager and will always return the correct values whether the VTP process is enabled or disabled.

  Note

  VTP client will move to transparent mode if there is any failure during updating VLAN database received from server. Following syslog message is displayed on console. “VTP-2-VTP_MODE_TRANSPARENT_CREATE_SEQ_FAILED: VTP Mode changed to transparent since VTP vlan create/update failed”. User need to change back the VTP mode to client to get latest database from server.

The VLAN architecture requires all flooded traffic for a VLAN to be sent across a trunk port even if it leads to switches that have no devices that are active in the VLAN. This method leads to wasted network bandwidth.

VTP V3 Pruning optimizes the usage of network bandwidth by restricting the flooded traffic to only those trunk ports that can reach all the active network devices. When this protocol is in use, a trunk port does not receive the flooded traffic that is meant for a certain VLAN unless an appropriate join message is received.

A join message is defined as a new message type in addition to the ones already supported by version 1 of the VTP V3 protocol. A VTP V3 implementation indicates that it supports this extension by appending a special TLV at the end of the summary advertisement messages that it generates. In VTP V3 transparent mode, VTP relays all VTP packets, and pruning requires that the switch processes TLVs in the VTP V3 summary packets.

VTP allows you to enable or disable the VTP protocol on a per-port basis to control the VTP traffic. When a trunk is connected to a switch or end device, it drops incoming VTP packets and prevents VTP advertisements on this particular trunk. By default, VTP is enabled on all the switch ports.

VTP maintains a list of trunk ports in the Spanning Tree Protocol (STP) forwarding state by querying STP at bootup and listening to the notifications that are generated by STP.

VTP sets a trunk port into the pruned or joined state by interacting with STP. STP notifies VTP V3 when a trunk port goes to the blocking or forwarding state. VTP V3 notifies STP when a trunk port becomes pruned or joined.