Solaris 2.x sets the don't fragment bit on all packets it send as part of MTU path discovery. The Solaris 2.x implementation is RFC compliant, but the MTU path discovery protocol will fail when there are broken routers in the path. Typical symptom is not being able to connect from a Solaris 2.x hosts but having no trouble from other hosts or being able to start a TCP/IP connection but not move any significant amount of data.
/usr/sbin/ndd -set /dev/ip ip_path_mtu_discovery 0
(See also 5.19)