Does a VTP server switch, with a VTP domain configured on it, advertise the VTP domain name to a client switch, so that the VTP domain is automatically configured on client switch?

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    – Ron Maupin
    Aug 7 '17 at 16:34

If you have not previously configured a switch to be in a VTP domain, it will become part of the VTP domain from the first VTP advertisement it sees. See the sections which I emphasized below:

Configuring VTP:

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.

  • VTP version 3 will not erase the database if it has a higher version number.
    – Ron Trunk
    Mar 30 '16 at 16:02
  • @RonTrunk, that's good to know. VTP is still a major security risk, and we just set all the switches to transparent mode, and use the switchport trunk allowed vlan <vlan list> to control which VLANs are allowed on which switches.
    – Ron Maupin
    Mar 30 '16 at 16:06

I believe:

VTP server does advertise domain name in protocol packets

Cisco client does choose to pick up the domain name when a)it's set to client mode and b)it's domain name is empty

  • The switch doesn't need to be set to client mode. If there is no VTP domain configured, it will pick up the VTP domain from the first VTP advertisement it sees. By default, switches are VTP servers, and they will still get the VTP domain from a VTP advertisement.
    – Ron Maupin
    Mar 30 '16 at 15:39

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