VTP Process and Revision Numbers

The VTP update process begins when a switch administrator, from a VTP server switch, adds, deletes, or updates the configuration for a VLAN. When the new configuration occurs, the VTP server increments the old VTP revision number by 1, and advertises the entire VLAN configuration database along with the new revision number.

The VTP revision number concept allows switches to know when VLAN database changes have occurred. Upon receiving a VTP update, if the revision number in a received VTP update is larger than a switch's current revision number, it believes that there is a new version of the VLAN database. Figure 2-2 shows an example in which the old VTP revision number was 3, the server adds a new VLAN, incrementing the revision number to 4, and then propagates the VTP database to the other switches.

VTP Revision Number Basic Operation

© Sync New VLAN Info @ Sync New VLAN Info

© Sync New VLAN Info @ Sync New VLAN Info

Cisco switches default to use VTP server mode, but they do not start sending VTP updates until the switch has been configured with a VTP domain name. At that point, the server begins to send its VTP updates, with a different database and revision number each time its VLAN configuration changes. However, the VTP clients in Figure 2-2 actually do not have to have the VTP domain name configured. If not configured, the client will assume it should use the VTP domain name in the first received VTP update. However, the client does need one small bit of configuration, namely, the VTP mode, as configured with the vtp mode global configuration command.

VTP clients and servers alike will accept VTP updates from other VTP server switches. When using VTP, for better availability, a switched network using VTP needs at least two VTP server switches. Under normal operations, a VLAN change could be made on one server switch, and the other VTP server (plus all the clients) would learn about the changes to the VLAN database. Once learned, VTP servers would then store the VLAN configuration permanently (for example, in NVRAM), whereas clients do not permanently store the configuration.

Figure 2-2

KEY POINT

The support of multiple VTP servers introduces the possibility of accidentally changing the VLAN configuration for the network. When a VTP client or VTP transparent switch first connects via a trunk to the network, it cannot affect the existing configuration, because in these modes the switch does not originate VTP updates. However, if a newly added switch in VTP server mode attaches via a trunk, it has the capability to change the other switch VLAN configurations with its own. It will change the other switch VLAN configurations, assuming the following are true about the newly added switch:

KEY ■ The new link connecting the new switch is trunking. POINT

■ The new switch has the same VTP domain name as the other switches.

■ The new switch's revision number is larger than that of the existing switches.

■ Same password, if configured on the existing switches.

The revision number and VTP domain name can be easily seen with a Sniffer trace; to prevent DoS attacks with VTP, set VTP passwords, which are encoded as message digests (MD5) in the VTP updates. Also, some installations simply use VTP transparent mode on all switches, which prevents switches from ever listening to other switch VTP updates and erroneously deleting their VLAN configuration databases.

Continue reading here: VTP Configuration

Was this article helpful?

+4 -1