What is the default behavior of a trunk port when it receives transit traffic without a VLAN tag?

The default behavior of a trunk port when it receives transit traffic without a VLAN tag is to drop the traffic. In a typical network configuration, trunk ports are designed to carry traffic for multiple VLANs, and they expect to receive traffic that is tagged with a VLAN identifier. When untagged traffic arrives at a trunk port, the port has no context to associate that traffic with any VLAN. As a result, the trunk port will drop the untagged frames, as it cannot process them according to any specified VLAN.

This behavior is important for maintaining network security and efficient traffic management. Ensuring that only properly tagged packets are forwarded helps in managing traffic across different broadcast domains defined by VLANs. This also prevents any confusion or misrouting of packets that could occur if untagged traffic was treated as belonging to any arbitrary VLAN.

In contrast, options suggesting that the traffic is forwarded, or tagged with the lowest or highest VLAN ID permitted on the trunk would not occur because it is crucial for trunk ports to maintain VLAN integrity and avoid potential issues that arise from untagged traffic.