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This is something I've fixed recently and thought I'd document for benefit of others, as it's so obscure.

When anyone on my network downloads a custom StarCraft 2 map, the map download will fail (or appear to fail and later work) and all connections (even on other computers) will be dropped; they'll sign out of battlenet, skype, msn, and pings will time out for a brief moment.

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The solution is to fire up cmd and reduce MTU size;

 netsh interface ipv4 set subinterface "Local Area Connection" mtu=1432 store=persistent

Where "Local Area Connection" is the device name for the adapter you connect to bnet through.

The problem was the router; Belkin F5D7633-4. Even the latest firmware only allows TCP/IP packets to be ~1492 bytes, despite 1500 being ethernet max. My theory is that the router was fragmenting the packets, and then either this process or the response from battlenet to these router-fragmented packets caused the router to crash.

The MTU value I determined following this process;

  • Open an administrative MSDOS command prompt
  • ping -l 1472 -f
  • Reduce 1472 until you no longer get the "packet needs to be fragmented" error message
  • Add 28 more to this (since you specified ping packet data size, not including a packet header of 28 bytes)
  • This value is your max MTU
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Yikes, a router that can't handle the default Ethernet MTU size!? Maybe it's time to get a new router... – BlueRaja - Danny Pflughoeft Feb 15 '12 at 17:18

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