Edimax Technology BR-6204WG manual This is optional. You can specify the maximum size

Use the following	The ISP gives you a static IP to be used
IP address	to connect to the L2TP server.
IP Address	This is the IP address that your ISP has given you to
	establish a L2TP connection.
Subnet Mask	Enter the Subnet Mask provided by your ISP (e.g.
	255.255.255.0)
Gateway	Enter the IP address of the ISP Gateway
User ID	Enter the User Name provided by your ISP for the
	PPTP connection. Sometimes called a Connection ID
Password	Enter the Password provided by your ISP for the
	PPTP connection
L2TP Gateway	If your LAN has a L2TP gateway, then enter that
	L2TP gateway IP address here. If you do not have a
	L2TP gateway then enter the ISP’s Gateway IP
	address above
MTU	This is optional. You can specify the maximum size of
	your transmission packet to the Internet. Leave it as it
	is if you to not wish to set a maximum packet size.
Connection Type	If you select “Continuous”, the router will always
	connect to the ISP. If the WAN line breaks down and
	links again, the router will auto-reconnect to the ISP.
	If you select “Connect On Demand”, the router will
	auto-connect to the ISP when someone want to use
	the Internet and keep connected until the WAN idle
	timeout. The router will close the WAN connection if
	the time period that no one is using the Internet
	exceeds the “Idle Time”.
	If you select “Manual”, the router will connect to ISP
	only when you click “Connect” manually from the
	Web user interface. The WAN connection will not be
	disconnected due to the idle timeout. If the WAN line
	breaks down and latter links again, the router will not
	auto-connect to the ISP.
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