Regarding VLAN configurations, which of the following is true?

The statement that a single interface can send mixed VLAN traffic is correct because, in many networking environments, it is possible to configure an interface to handle both tagged and untagged traffic simultaneously. This is a common practice in VLAN configurations, allowing for greater flexibility in network design. By enabling mixed VLAN traffic, a device can communicate with devices on different VLANs while still accommodating devices that do not support VLAN tagging, thus ensuring compatibility across varied network setups.

In many scenarios, especially when dealing with trunk ports, interfaces can pass traffic from multiple VLANs tagged with appropriate identifiers while still being able to accept untagged traffic, typically associated with a default or native VLAN. This versatility allows network administrators to create more complex and efficient networks, as they can support various client devices and VLAN configurations without needing to dedicate separate physical interfaces for each VLAN.

The other options do not hold true in the context of VLAN configurations: it is not mandatory for all interfaces to have an untagged VLAN assigned, tagged and untagged traffic can coexist on the same interface, and specific port settings are often necessary to ensure proper VLAN operation and management.