Site to site tunnel file sharing problems

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Hello!

I have 2 Cisco 2801 series routers (one at our corporate office, and
one at our branch location). I have setup a site to site vpn over an
IPSec tunnel. The tunnel works perfectly fine in most ways. I can
ping any address from either location at the other location. I can
browse to any share on any computer. I can copy files that are
approx. 3KB or smaller to PCs at the other location. I CANNOT,
however, copy files larger than approx. 3KB. I get an error message
after about 30 seconds that says "Cannot copy (name of file): The
specified network name is no longer available." I can then take that
exact file, remove enough information from it to take it below approx.
3KB, copy the file again, and it finishes in no time flat. I am sure
this is a result of the MTU fragmentation.

Oh, the best part: This only happens on some computers. From certain
computers, it doesn't matter what the size of the file being copied,
it works perfect, other computers, it is extremely consistant. I have
found a way to, on a PC by PC basis, resolve this issue (but would
like a blanket resolution)...here's the resolution to fix one PC at a
time: open regedit, navigate to HKEY_LOCAL_MACHINE\System
\CurrentControlSet\Services\Tcpip\Parameters and create a new DWORD
called EnablePMTUDiscovery and set the value to 0. Restart.

Is there a way to get this same result by making a change to the
router. I have tried these two things with no avail:

Router# config t
Router(config)# int tunnel0
Router(config-if)# ip mtu 600

&

Router(config-if)# ip tcp adjust-mss 600

Any Ideas?!?!?!? Thanks guys!