Last time I had to change MTU settings was when I setup VPN server at the university, I worked for. I still vividly remember this problem: PPP interface is up, I can ping and ssh into the host on remote side, but scp stucks after 1Kb. Luckily for me, there are whole bunch of VPN manuals/faqs and howtos on the net, and almost every one of them advices to lower MTU in case of a problem.
Today I run into another problem, that can be fixed by lowering MTU, though I'm still not sure what is causing it. My ISP provides me with IPv6 connectivity, as well as IPv4, and some of the features ( like IPTV ) are available only thru v6. So, I configured v6 tunnel on my router, and watched dancing turtle logo on kame.net and dancing Google on ipv6.google.com. But after that I noticed, that some of the sites I frequent before became unavailable. One of them is doc.python.org. That was even more weird, because ping6 worked, and it replied to malformed request, but curl -6 http://doc.python.org never returned.
And to my surprise lowering MTU helped.... Does anyone have an idea why? ( I guess that it's due to wrong MTU settings on one of the v6 routers. )
