Where does a VTP server or client store the interface voice VLAN configuration?

The interface voice VLAN configuration is stored in the running-config. This is because the running-config is the current operational configuration of a device, which includes configurations that have been applied or modified but not yet saved to the startup-config.

When you configure a voice VLAN on an interface, that configuration is immediately applied to the device and is reflected in the running-config. If the device were to restart or lose power, this configuration would not persist unless it is saved to the startup-config. Therefore, to see the current state of the device settings, you would refer to the running-config, where all active configurations, including voice VLAN settings, are held.

The startup-config, while critical for saving settings across reboots, would only contain the information that has been saved. Flash memory is used primarily for storing the operating system and other files necessary for booting, while ROM typically holds the bootstrap and basic IOS images that facilitate device initialization. Thus, the location that directly holds the interface voice VLAN configuration at any given time is the running-config.