VLAN Trunking Protocol (VTP)

In this blog, we will learn what VLAN Trunking Protocol easily in detail, Practical Commands for configuring VTP, and Practical Lab with configuration and testing.

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 is the operation of VTP:

i) Modification of VLAN information (eg: VLAN name) 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 is 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 than 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 of 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,

or 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 operating 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.

 

VTP Pruning provides a way to preserve bandwidth by configuring switches to only send

broadcasts and unknown unicasts to the trunks to other switches that need the information

(contain ports in a particular VLAN). VTP pruning is disabled by default, which means

broadcasts and unknown unicasts in every VLAN are forwarded to all switches in the network.

Ex: SW1 does not have any ports configured for VLAN 2, thus broadcasts sent to VLAN 2

should not be forwarded to the trunk to SW1 (pruning flooded traffic to conserve bandwidth).

 

Note: VLAN 1 can never be pruned because it is an administrative VLAN.

The [no] vtp pruning privileged or global configuration command is used to enable or disable

VTP pruning respectively. VTP pruning only needs to be enabled on a single VTP server mode

switch throughout a VTP domain, as the setting will be propagated to other switches and cause

them to enable VTP pruning as well. VTP pruning utilizes VTP advertisements for its operation.

 

When PVST is in use, STP optimization for each VLAN and VTP pruning are important to

ensure STP changes that occur in a particular VLAN will not affect other STP instances for other

VLANs, which results in a more stable network.

 

VTP

SW1(config)# vtp mode server

SW1(config)# vtp domain mdabusayed

SW1(config)# vtp password mdabusayed

Create VLANs, set names

SW2(config)# vlan 10

SW2(config-vlan)# name IT

Client switches:

SW3(config)# vtp mode client

SW3(config)# vtp password mdabusayed

Transparent switches:

SW3(config)# vtp mode transparent

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Note: I used most of the pictures from the internet. Those guys created this picture all credit goes to them.