What command is used to configure allowed VLANs on a trunk port?

The command used to configure allowed VLANs on a trunk port is "switchport trunk allowed vlan allowed-vlans". This command enables the network administrator to specify which VLANs are permitted to traverse the trunk link. Trunk ports are used to carry traffic from multiple VLANs between switches, and by defining allowed VLANs, the administrator can control the traffic flow and maintain security by restricting access to certain VLANs.

When configuring trunk ports, it's essential to restrict which VLANs can send and receive traffic over the trunk to avoid unnecessary exposure of VLANs that should not communicate with each other. The syntax of the command is straightforward, where "allowed-vlans" can be a list of VLAN IDs or a range that indicates which VLANs are permitted on that trunk port.

This command helps maintain optimal network performance and security by ensuring that only specified VLANs can communicate over the trunk link. Since other options either use incorrect syntax or mix terms in a way that doesn't align with the Cisco IOS command structure, they do not serve as viable alternatives for achieving the desired configuration.