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VLAN Trunking Protocol
- VLAN Trunking Protocol (VTP) is the Cisco-proprietary L2 protocol used to exchange and
maintain a consistent VLAN database across Catalyst switches in an administrative domain.
VTP minimizes misconfiguration and eases the configuration task by reducing manual
configuration needs. VTP facilitates centralized VLAN management – VLAN configuration
(eg: addition, deletion, and renaming of VLANs) only needs to be made on a single switch and
the configuration will be propagated to all other switches in the same VTP domain.
Ex: The changing of the name for a particular VLAN on a switch will be distributed to other
switches. Hence no configuration is needed on those switches.
- Below lists the operation of VTP:
i) Modification of VLAN information in a VTP server.
ii) Increments the VLAN configuration revision number (+1).
iii) The VTP server sends out a VTP advertisement with a higher revision number.
iv) VTP clients notice the configuration when the revision number larger than its current one.
v) VTP clients synchronize their VLAN configurations with the new VLAN information.
- By default, VTP servers flood VTP advertisements throughout a VTP domain every 10 minutes,
or whenever there is a VLAN configuration change. VTP advertisements are sent as multicasts.
A VTP domain is also known as a VLAN management domain.
- A higher configuration revision number indicates that the received VLAN information is more
current than the current information. A switch would ignore a VTP advertisement with a revision
number that is the same or lower with its current revision number.
- Caution: Inserting a VTP client or server with a higher configuration revision number into a
VTP domain can overwrite the VLAN information on existing VTP servers and clients.
- The methods that can be used to reset the VTP configuration revision number of a switch are:
i) Change the VTP domain name, and change back to the original VTP domain name.
ii) Change to VTP Transparent mode, and change back to VTP Server mode.
- Below list the VTP operation modes:
Server
(default)
There must be at least one VTP server in a VTP domain. Only VTP server
switches are allowed to create, add, modify, and delete VLANs. Changes
made on a VTP server switch will be advertised throughout the entire VTP
domain. Can save VLAN config in NVRAM.
Client Can receive and forward VTP advertisements, but cannot create, add, modify,
nor delete VLANs. They process received advertisements and synchronize
their VLAN configuration. Cannot save VLAN configuration in NVRAM.
They synchronize the VLAN configuration with other switches upon reboot.
Transparent Do not participate in the VTP domain (ignore VTP advertisements), but still
forward VTP advertisements. Switches operate in this mode can create, add,
modify, and delete their own VLANs but do not advertise the configuration to
other switches (locally significant only). Can save VLAN config in NVRAM.
- Note: VTP advertisements are only propagated across (or sent over) trunk links. VTP
advertisements can be sent over all types of trunk links, eg: ISL, 802.1Q, and ATM LANE.
Additionally, all switches in the VTP domain must be configured to use the same VTP version.
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Copyright © 2008 Yap Chin Hoong
yapchinhoong@hotmail.com