Wireless network intermittent losing broadband connecting – MTU problem 

 September 25, 2017

By  NickLitten

Feb 2014 Update — If you are here looking for details on how to fix VPN failure after doing a Windows8.1 or Windows10 upgrade then use Windows Troubleshooter to Change the mode to Windows compatibility

1. Right click VPNUI in the Cisco install folder. (for example “C:\Program Files (x86)\Cisco\Cisco AnyConnect Secure Mobility Client\”)

Wireless network intermittent losing broadband connecting - MTU problem 1

2. Choose Troubleshoot compatibility.

3. Wait for the Wizard to complete and take the setting it recommends.

4. NOTE: Alternatively click PROPERTIESSimply set to Compatible with Windows 7. (This has worked for me every time)

Wireless network intermittent losing broadband connecting - MTU problem 2

5. Restart the VPN service. (No need for me, worked after the compatibility mode fix)

6. Start AnyConnect from the start menu.

7. Is it Working?

Detailed instructions here /how-fix-vpn-problem-after-win81-update-its-same-windows10-just-use-compatibility-wizard 🙂

On a customer site yesterday who had problems with a home wireless network which seemed to randomly drop the internet connect on three of his wireless PC’s around the house. His Wireless network intermittent losing broadband connecting - MTU problem 3setup was COX Cable Broadband (used to be COMCAST) plugged into a Netgear Wireless Router (WGR614) and each PC using USB Wireless to connect. Various different makes; a netgear WPN111 and Zoom and a DLINK.

After some tinkering I found that two of the PC’s had different MTU settings: Possibly as a result of running various internet tweaking programs which seemed to upset the connection slightly?

MTU Size was causing problem

Problem solved by resetting the router and all PC’s to the same MTU. We opted for a MTU=1492. A simple fix but one that has cured a lot of head scratching and seemingly random wireless network flakiness in the past. Heres a great little overview of MTU:


Wireless network intermittent losing broadband connecting - MTU problem 4A 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.

Please note that you can cause damage to your system if you make a wrong move when editing the Windows Registry, so you do so at your own risk.

Wireless network intermittent losing broadband connecting - MTU problem 5Microsoft states: “MTU (Maximum Transfer Unit) specifies the maximum transmission unit size of an interface. Each interface used by TCP/IP may have a different MTU value specified. The MTU is usually determined through negotiation with the lower driver, using that lower driver value. However, that value may be overridden.”

What this means in English “Data that flows around networks travels in the form of small chunks of data called packets. If sufficient bandwidth/speed is available, the larger the size of these packets, the more data can be transferred at one time. The value of the Maximum Transmission Unit size specifies the size of these data packets. The MTU size is specified with the Windows Registry, and can be edited to optimise the speed of your connection!”

So, we now know that the MTU (Maximum Transmission Unit) is the largest defined packet size that can be transferred within one frame of a network (thats a networking fact). But, data fragmentation occurs when a transmitted packet is smaller than the packets frame length. As all fragments need to be reassembled, the MaxMTU size should be correct or connection performance levels will reduce.

You can edit this manually by doing it the hard way:​

To change the MTU settings for PPP connections, add the ProtocolType DWORD value and the ProtocolMTU DWORD value to the following registry key:


To do so, follow these steps.

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:

322756 How to back up and restore the registry in Windows
  1. Click Start, click Run, type regedit in the Open box, and then click OK.
  2. Locate and then click the following subkey in the registry:
  3. Add a

    subkey (if it does not already exist). To do so:

    1. On the Edit menu, point to New, and then click Key.
    2. Type Protocols, and then press ENTER.
  4. Add a

    (zero) subkey to the


    subkey. To do so:

    1. Click the

      subkey that you created step 3.

    2. On the Edit menu, point to New, and then click Key.
    3. Type 0 (zero), and then press ENTER.
  5. Click the

    subkey that you created in step 4.

  6. On the Edit menu, point to New, and then click DWORD Value.
  7. In the Value data box, type ProtocolType, and then click OK.
  8. On the Edit menu, click Modify.
  9. In the Value data box, type 800, make sure Hexadecimal is selected under Base, and then click OK.
  10. On the Edit menu, point to New, and then click DWORD Value.
  11. Type PPPProtocolType, and then press ENTER.
  12. On the Edit menu, click Modify.
  13. In the Value data box, type 21, make sure Hexadecimal is selected under Base, and then click OK.
  14. On the Edit menu, point to New, and then click DWORD Value.
  15. Type ProtocolMTU, and then press ENTER.
  16. On the Edit menu, click Modify.
  17. Under Base, click Decimal, type the MTU size that you want in the Value data box, and then click OK.
  18. Quit Registry Editor.
  19. Restart your computer.

All of these techniques are deailed on Microsoft Support


IBM i Software Developer, Digital Dad, AS400 Anarchist, RPG Modernizer, Shameless Trekkie, Belligerent Nerd, Englishman Abroad and Passionate Eater of Cheese and Biscuits. Nick Litten Dot Com is a mixture of blog posts that can be sometimes serious, frequently playful and probably down-right pointless all in the space of a day. Enjoy your stay, feel free to comment and remember: If at first you don't succeed then skydiving probably isn't a hobby you should look into.

Nick Litten

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