I am trying to configure a D-link DES 3526 switch. When I add a VLAN with some untagged ports (via the web interface), the system says that it cannot add untagged vlan ports. What's wrong?
You can only have 1 untagged vlan per port, on a port setup for access. On a port setup for truncking you can have 1 untagged and as many tagged vlans as you like. If you want to switch which vlan is untagged simply go to the other vlan and mark that port as untagged.