Using VLAN Trunk Protocol (VTP)

Using VLAN Trunk Protocol (VTP)
VLAN Trunk Protocol (VTP) was created by Cisco to manage all the configured VLANs
across a switched internetwork and to maintain consistency throughout the network. VTP
enables an administrator to add, delete, and rename VLANs. These changes are then propagated
to all switches.
VTP provides the following benefits to a switched network:
 Consistent configuration of global VLANs across all switches in the network
 Enabling VLANs to be trunked over mixed networks—for example, Ethernet to ATM
LANE or FDDI
 Accurate tracking and monitoring of VLANs
 Dynamic reporting when VLANs are added to all switches
 Plug-and-play VLAN adding to the switched network
To enable VTP to manage your VLANs across the network, you must first create a VTP
server. All servers that need to share VLAN information must use the same domain name, and
a switch can be in only one domain at a time. This means that a switch can share VTP domain
information only with switches configured in the same VTP domain.
466 Chapter 14  VLANs, Trunks, and VTP
A VTP domain can be used if you have more than one switch connected in a network. If all
switches in your network are in only one VLAN, then VTP doesn’t need to be used. VTP information
is sent between switches via a trunk port between the switches.
Switches advertise VTP management domain information, such as the name, as well as a configuration
revision number and all known VLANs with any specific parameters.
You can configure switches to receive and forward VTP information through trunk ports but not
process information updates nor update their VTP database. This is called VTP transparent mode.
You can set up a VTP domain with security by adding passwords, but remember that
every switch must be set up with the same password, which might be difficult. However, if
you are having problems with users adding switches to your VTP domain, then a password
can be used.
Switches detect the additional VLANs within a VTP advertisement and then prepare to
receive information on their trunk ports with the newly defined VLAN in tow. The information
would be VLAN ID, 802.10 SAID fields, or LANE information. Updates are sent out as
revision numbers that are notification +1. Any time a switch sees a higher revision number,
it knows the information it receives is more current and will overwrite the current database
with the new one.
Do you remember the clear config all command we talked about in Chapter 13, “Connecting
the Switch Block”? Well, guess what? It really doesn’t “clear all” after all. It seems that
VTP has its own NVRAM, which means that VTP information as well as the revision number
would still be present if you perform a clear config all. You can clear the revision number
by power-cycling the switch.