MSN blocking PMTUD

Hi, it seems that Microsoft Network is blocking the path MTU
discovery (ICMP fragmentation required but df bit set).

That is causing problems because some customers who happen to
use GRE tunnels for their branch offices can't access MSMessenger
of the Hotmail page.

Is there somebody from MSN here? Is there a workaround for this
problem without using other type of tunneling? If not, is there any
other workaround?

Use "ip mtu 1500" on each tunnel interface. Uses more CPU but works.

"ip adjust-mss 1300" is also nice, in conjunction with the above you end
up with the TCP traffic not being fragmented and it still being
transparent for fullsize UDP traffic.