Draytek 2130 manual PPTP/L2TP, It means Max Transmit Unit for packet. The default, setting is

PPTP/L2TP

if you click PPTP/L2TP as the protocol, please manually enter the Username/Password provided by your ISP and all the required information.

User Name	Assign a specific valid user name provided by the ISP.
Password	Assign a valid password provided by the ISP.
Server Address	Specify the IP address of the PPTP server.

WAN IP Network Settings You can choose Static IP or DHCP as WAN IP network setting.

IP Address	Type the IP address if you choose Static IP as the WAN IP
	network setting.
Subnet Mask	Type the subnet mask if you chose Static IP as the WAN IP.
Redial Policy	If you want to connect to Internet all the time, you can choose
	Always On. Otherwise, choose Connect on Demand.

Idle Time Out	Set the timeout for breaking down the Internet after passing
	through the time without any action. The unit is seconds. The
	range is XX ~ XX.

MTU Size	It means Max Transmit Unit for packet. The default
	setting is 1442.
Enable	The router will detect the MAC address automatically. Or,
	check the box to enable MAC address cloning.

Vigor2130 Series User’s Guide 23