Question : TCP/IP MTU size

Would someone please let me know reguarding MTU size.

We have a site, which just got a dedicated T1, with a Netopia router, with 5 global IP addresses, fully opened, nothing filtered, no NAT, etc ...  we put a Cisco 1721 router in to handle the LAN, and site to site tunnels, etc ... using NAT, etc ... (we have done this many, many times) ... in this particular case, the machines on the LAN could get to some website but not others (google.com okay, yahoo.com NOT okay) ... we finially tracked it down to the MTU size, we think. If we change the MTU size on the local PC to, say 576, which is what Microsoft recommends for a WAN, then it works.

http://support.microsoft.com/kb/900926

Shouldn't the router be able to adjust the MTU to the proper size? There is a "don't fragment" bit, but not sure everyone uses it, or perhaps they "are" using it, which means that the router isn't allowed to break a packet into smaller pieces?

Anyway, it seems there should be a simple answer to this. We tried setting the MTU size on the Cisco 1721 router, but that did not work. I'm guessing because the client PC is still sending packets larger than what "someone upsteam from us" will handle??

There has got to be a solution, other than configuring every PC on the LAN with registry hack:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\

We just found the following, and it appears to work... by putting the following line of code on the WAN interface: (?)
ip tcp adjust-mss 1452

see:
http://www.cisco.com/en/US/docs/ios/12_2t/12_2t4/feature/guide/ft_admss.html

This is what we found (here) ... and it seems to work:
http://www.experts-exchange.com/Networking/Misc/Q_21758664.html

Answer : TCP/IP MTU size

The above solution worked!