http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094c52.shtml

Conclusion

There are some disadvantages to the use of VTP. You must balance the ease of VTP administration against the inherent risk of a large STP domain and the potential instability and risks of STP. The greatest risk is an STP loop through the entire campus. When you use VTP, there are two things to which you must pay close attention:

·         Remember the configuration revision and how to reset it each time that you insert a new switch in your network so that you do not bring down the entire network.

·         Avoid as much as possible to have a VLAN that spans the entire network.

 

 

http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_52_se/configuration/guide/swvtp.html

Adding a VTP Client Switch to a VTP Domain

Before adding a VTP client to a VTP domain, always verify that its VTP configuration revision number is lower than the configuration revision number of the other switches in the VTP domain. Switches in a VTP domain always use the VLAN configuration of the switch with the highest VTP configuration revision number. With VTP versions 1 and 2, adding a switch that has a revision number higher than the revision number in the VTP domain can erase all VLAN information from the VTP server and VTP domain. With VTP version 3, the VLAN information is not erased.

Beginning in privileged EXEC mode, follow these steps to verify and reset the VTP configuration revision number on a switch before adding it to a VTP domain:

 

Command

Purpose

Step 1

show vtp status

Check the VTP configuration revision number.

If the number is 0, add the switch to the VTP domain.

If the number is greater than 0, follow these steps:

a. Write down the domain name.

b. Write down the configuration revision number.

c. Continue with the next steps to reset the switch configuration revision number.

Step 2

configure terminal

Enter global configuration mode.

Step 3

vtp domain domain-name

Change the domain name from the original one displayed in Step 1 to a new name.

Step 4

end

The VLAN information on the switch is updated and the configuration revision number is reset to 0. You return to privileged EXEC mode.

Step 5

show vtp status

Verify that the configuration revision number has been reset to 0.

Step 6

configure terminal

Enter global configuration mode.

Step 7

vtp domain domain-name

Enter the original domain name on the switch.

Step 8

end

The VLAN information on the switch is updated, and you return to privileged EXEC mode.

Step 9

show vtp status

(Optional) Verify that the domain name is the same as in Step 1 and that the configuration revision number is 0.

After resetting the configuration revision number, add the switch to the VTP domain.

Note You can use the vtp mode transparent global configuration command to disable VTP on the switch and then to change its VLAN information without affecting the other switches in the VTP domain.

 

The VTP Domain

A VTP domain (also called a VLAN management domain) consists of one switch or several interconnected switches under the same administrative responsibility sharing the same VTP domain name. A switch can be in only one VTP domain. You make global VLAN configuration changes for the domain.

By default, the switch is in the VTP no-management-domain state until it receives an advertisement for a domain over a trunk link (a link that carries the traffic of multiple VLANs) or until you configure a domain name. Until the management domain name is specified or learned, you cannot create or modify VLANs on a VTP server, and VLAN information is not propagated over the network.

If the switch receives a VTP advertisement over a trunk link, it inherits the management domain name and the VTP configuration revision number. The switch then ignores advertisements with a different domain name or an earlier configuration revision number.

Caution Before adding a VTP client switch to a VTP domain, always verify that its VTP configuration revision number is lower than the configuration revision number of the other switches in the VTP domain. Switches in a VTP domain always use the VLAN configuration of the switch with the highest VTP configuration revision number. If you add a switch that has a revision number higher than the revision number in the VTP domain, it can erase all VLAN information from the VTP server and VTP domain. See the "Adding a VTP Client Switch to a VTP Domain" section for the procedure for verifying and resetting the VTP configuration revision number.

When you make a change to the VLAN configuration on a VTP server, the change is propagated to all switches in the VTP domain. VTP advertisements are sent over all IEEE trunk connections, including Inter-Switch Link (ISL) and IEEE 802.1Q. VTP dynamically maps VLANs with unique names and internal index associates across multiple LAN types. Mapping eliminates excessive device administration required from network administrators.

If you configure a switch for VTP transparent mode, you can create and modify VLANs, but the changes are not sent to other switches in the domain, and they affect only the individual switch. However, configuration changes made when the switch is in this mode are saved in the switch running configuration and can be saved to the switch startup configuration file.

For domain name and password configuration guidelines, see the "VTP Configuration Guidelines" section.

 

 

 

http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094c52.shtml

 

Configuration Revision Number

The configuration revision number is a 32-bit number that indicates the level of revision for a VTP packet. Each VTP device tracks the VTP configuration revision number that is assigned to it. Most of the VTP packets contain the VTP configuration revision number of the sender.

This information is used in order to determine whether the received information is more recent than the current version. Each time that you make a VLAN change in a VTP device, the configuration revision is incremented by one. In order to reset the configuration revision of a switch, change the VTP domain name, and then change the name back to the original name.

Summary Advertisements

By default, Catalyst switches issue summary advertisements in five-minute increments. Summary advertisements inform adjacent Catalysts of the current VTP domain name and the configuration revision number.

When the switch receives a summary advertisement packet, the switch compares the VTP domain name to its own VTP domain name. If the name is different, the switch simply ignores the packet. If the name is the same, the switch then compares the configuration revision to its own revision. If its own configuration revision is higher or equal, the packet is ignored. If it is lower, an advertisement request is sent.