Cisco WS-C2950SX-48-SI Configuration Manual page 196

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 is
possible in off or transparent modes), the database is invalid. The concept of valid databases is new with
VTP version 3 and is directly derived from the fact that there is only one primary server in the network.
An invalid database is only applied 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. The following examples help to define valid
databases:
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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:
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Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide
9-20
If you move from VTP version 1 to VTP version 3, the VLAN database is not deleted. The VLAN
database is marked invalid because it has been generated by a VTP version 1 server, not by a
VTP version 3 primary server.
If a VTP version 3 server with a valid database is moved to transparent mode, you can configure the
VLAN database, but as soon as the database is modified, it becomes invalid. This prevents you from
going back to server mode and advertising this database. If you attempt to do so, the valid database
that is received from the network will overwrite 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 gets advertised to the rest of the domain. There is a difference between configuring
database-related parameters and domain-related parameters on a primary server. In any mode,
configuring a domain-related parameter immediately invalidates all the databases. Domain
parameters are the domain name, the VTP version, and the authentication method (password). In
addition to invalidating the databases, configuring a domain-related parameter also reverts a primary
server to a secondary server. When a domain parameter is changed, the switch is inserted into a new
domain. To prevent the "wrong" database from accidentally being inserted into a VTP domain, a
switch cannot be inserted as a primary server into a new domain (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.
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
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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.
Chapter 9 Configuring VTP
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Release 8.1
78-15486-01