Which Cisco IOS command controls the VLAN assigned to frames received on a trunking interface without an 802.1Q tag?

The command that controls the VLAN assigned to frames received on a trunking interface without an 802.1Q tag is indeed related to the concept of a native VLAN. When frames are sent over a trunk link, they are usually tagged with the VLAN information to distinguish which VLAN they belong to. However, if a frame arrives on a trunk link without a tag, it is considered untagged.

The correct answer specifically uses the command "switchport trunk native vlan-id" to configure the native VLAN for the trunk link. The native VLAN is the VLAN that will be used for untagged traffic on the trunk. By default, VLAN 1 is set as the native VLAN on Cisco switches, but network administrators can change this to a different VLAN based on their network design and requirements. This is particularly important for ensuring that frames with no tagging are handled correctly and assigned to the appropriate VLAN for processing.

The other options relate to different aspects of VLAN and interface configurations. For example, "switchport access vlan" is used to specify the VLAN for access ports (not trunking interfaces) and does not apply to how untagged frames are treated on trunk ports. "switchport trunk allow" is used to regulate which VLANs are permitted to