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Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
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Chapter 9 Configuring VTP
Understanding How VTP Version 3 Works
Valid Databases
A switch advertises a database only if it is valid. The only way to validate a database is to become the
primary server. If a switch modifies a database that has been generated by a primary server (this situation
is possible in off or transparent modes), the database is invalid. An invalid database is applied only
locally on a switch and is overwritten by any database that is received on the network if the switch is a
VTP client or server. Some examples of valid and invalid databases are as follows:
•
When you move from VTP version 1 to VTP version 3, the VLAN database is not deleted but is
marked invalid because it has been generated by a VTP version 1 server, not by a VTP version 3
primary server.
•
If you move a VTP version 3 server with a valid database to transparent mode, you can configure
the VLAN database, but as soon as the database is modified, it becomes invalid. This situation
prevents the switch from going back to server mode and advertising this database because the valid
database that is received from the network overwrites the changes made while in transparent mode.
If a server moves to transparent mode and then back to server mode with no changes to the database
configuration, its database is still valid.
•
If you modify a database on a primary server (such as a VLAN configuration), the database stays
valid and is advertised to the rest of the domain. In any mode, when you configure a domain-related
parameter (such as the domain name, VTP version, and the authentication method [password]), all
the databases become invalidated. In addition to invalidating the databases, configuring a
domain-related parameter also reverts a primary server to a secondary server.
•
When you change a domain parameter, the switch is inserted into a new domain. To prevent the
wrong database from accidentally being inserted into a VTP domain, you cannot insert a switch as
a primary server into a new domain because it could potentially erase a valid configuration. Because
it has an invalid database, a newly inserted switch in a domain immediately accepts the network
configuration instead of erasing it.
Database Revision Number
Each VTP instance is associated with a database revision number. The database revision number is
incremented when the value of the database that is covered by the advertised checksum is modified.
When a device receives a VTP advertisement from the same primary server for an instance in the same
domain, the following occurs:
•
If the database revision number in the advertisement is less than that of the receiving device, the
advertisement is ignored and a summary advertisement with the current revision number is
transmitted on the trunk on which the original advertisement was received.
•
If the database revision number in the advertisement is the same as that of the receiving device, then
the following occurs:
–
If the checksum of the advertisement is exactly the same as the checksum of the current
configuration known to the device, then no action is taken.
–
If the checksum of the advertisement is not exactly the same as the checksum of the current
configuration known to the device, the device’s configuration is unaffected, but the device
indicates to the database manager that a configuration error condition has occurred.