NetGear DGN1000 - MTU, Partial Loss of Internet Connection, and Performance

MTU (Maximum Transmission Unit) is the largest packet a network device transmits. The best MTU setting for NETGEAR equipment is often just the default value. MTU is sometimes presented as something that can be easily changed to improve performance, but in practice this may cause problems. Leave MTU unchanged unless:

1. You have problems connecting to your ISP, or other Internet service, and either the technical support of the ISP or of NETGAER recommends changing MTU. These may require an MTU change:
   
   • A secure Web site that won't open, or only displays part of a Web page
   • Yahoo email
   • MSN
   • AOL's DSL service
   • 
     
2. You use VPN, and have severe performance problems.
3. You used a program to "optimize" MTU for performance reasons, and now you have connectivity or performance problems.

• An easy solution to most problems is to change MTU to 1400.
• If you are willing to experiment, gradually reduce MTU as described in "Details of MTU", below.

How to Change a Computer's MTU

Note: If you change MTU on one computer, change it on your other computers, switches, and routers, as well. Instructions for changing MTU on other NETGEAR devices is found in the Reference Manuals, in the sections explaining Advanced Features.

The third party Dr. TCP software can be used to change the MTU setting.

1. Download it from this link, choosing the zip file or the exe file at the top of the page.
2. Run the utility.
3. In the Adapter Settings pull down, select the Ethernet driver and adapter used to connect with the network.
4. In the MTU box, type the MTU size you are trying.
5. Click in any other box, without changing the data there.
6. Click Save.
7. Click Exit.
8. Restart the computer.

Details About MTU

A packet sent to a device larger than its MTU is broken into pieces. Ideally, MTU would be set to the same — large — value on all your computers, routers and switches, as well as on all the parts of the Internet that you access. But you cannot control the MTU on the Internet, and in practice the optimum MTU size on your LAN is related to your hardware, software, wireless interference, etc.

• Tweaking MTU size may work well in one situation, but cause performance and connection problems in others.
• When network devices with different MTU settings communicate, packets are fragmented to accommodate the one with the smallest MTU.
• Windows XP sets MTU automatically, that is, it optimizes computer MTU for you. This Microsoft article explains resolving lack of connection to a broadband ISP using Windows XP: How-To Configure Broadband Connections Using PPPoE.
• Once a network device fragments a packet, the data stays fragmented until arriving at the destination computer.

Setting MTU size is a process of trial-and-error: start with the maximum value of 1500, then reduce the size until the problem goes away. Using one of these values is likely to solve problems caused by MTU size:

• 1500. The largest Ethernet packet size; it is also the default value. This is the typical setting for non-PPPoE, non-VPN connections. The default value for NETGEAR routers, adapters and switches.
• 1492. The size PPPoE prefers.
• 1472. Maximum size to use for pinging. (Bigger packets are fragmented.)
• 1468. The size DHCP prefers.
• 1460. Usable by AOL if you don't have large email attachments, etc.
• 1430. The size VPN and PPTP prefer.
• 1400. Maximum size for AOL DSL.
• 576. Typical value to connect to dial-up ISPs.