The configuration revision number is the only criterion used when
determining if a switch should keep its existing VLAN database or overwrite it
with the VTP update sent by another switch in the same domain with the same
password. Therefore, when a switch is added to a network it is important that
it does not inject spurious information into the domain.

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CAUTION:
If a new switch was at one time attached to another network, it is
feasible that it contains a vlan.dat file in Flash and that its configuration
revision number is higher than that of other VTP servers in the VTP domain to
which it is being added. If no VTP domain was explicitly configured on the
switch, when connected to the network, the new switch is able to derive the VTP
domain name from any VTP update it sees. If there is no password on the domain
and if the new switch is in server mode (default), its VLAN information can
overwrite the VLAN database on other switches in the VTP domain.
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The How to Add a Switch table explains the steps to take to add a
new switch to the network. It is very critical to VLAN stability to add a
switch in this manner.
