Which command is used to control which VLANs are permitted to cross an interface configured as a trunk?

The command that controls which VLANs are permitted to cross an interface configured as a trunk is indeed the one that specifies particular VLAN IDs for trunking. In a trunking configuration, multiple VLANs can be carried over a single network link, but it's often necessary to restrict the specific VLANs that are allowed to traverse that link.

Using the command that specifies "switchport trunk allowed vlan vlan-ids" enables network administrators to define explicitly which VLANs will be allowed on the trunk interface. This is an essential part of maintaining network security and efficiency, as it prevents unauthorized VLANs from accessing the trunk and ensures that only valid traffic is transmitted across the link.

In contrast, the command that sets a specific VLAN for access mode is designed for ports that belong to a single VLAN, which is not applicable in a trunking context. The command that sets the trunk mode is important for enabling trunking, but it does not dictate which VLANs are allowed. Lastly, a command that mentions "vlan allow permitted" doesn't exist in standard Cisco IOS command syntax. Therefore, the correct command helps ensure optimal control over VLAN traffic in a complex networking environment.