switchport trunk allowed vlan add 104. The keyword add appends a VLAN to the existing allowed VLAN list without replacing the current list. That is the critical detail in the prompt: the engineer must insert the new VLAN without modifying anything previously configured. A command that specifies only VLAN 104 replaces the existing allowed list with VLAN 104, which can interrupt traffic for existing VLANs. The all option allows every VLAN and changes the policy more broadly than requested. Cisco CCNA 200-301 v1.1 Network Access expects engineers to understand the operational difference between setting a trunk allowed list and adding to it. On production trunks, using the wrong command can remove active VLANs and create an outage. The safe command is switchport trunk allowed vlan add 104.