To assign VLAN 99 as the Native VLAN on a trunk port, you need to use the command switchport trunk native vlan 99. This command specifically sets VLAN 99 as the Native VLAN, which means that traffic from VLAN 99 on this trunk port will not be tagged. Choosing any other VLAN number or not setting a Native VLAN would either leave the default VLAN 1 as native or incorrectly tag the management VLAN traffic, potentialy causing traffic misrouting or security issues.