• WAN Port • Advanced Setup • Dynamic DNS • Virtual Servers • Options   Options Backup DNS Backup DNS (1) IP Address: . . . Backup DNS (2) IP Address: . . . These DNS (Domain Name Servers) are used only if the primary DNS is unavailable. MTU MTU (Maximum Transmission Unit): (1..1500) bytes This setting only affects PPPoE ,L2TP and PPTP connections.