Though Solaris is slowly disappearing(at least at my current company), there are still legacy machines which will remain and sometimes you have to debug them especially over network speed. Solaris has a legacy of having problems with auto-negoting. Sometimes you may think you are running full-duplex and you are not.
If you have access to the network switch, it’s easy to check. If you don’t you have to rely on solaris commands.
The NDD command will give you configuration information on the interface.
# ndd /dev/bge0 link_duplex
# ndd /dev/bge0 link_autoneg
# ndd /dev/bge0 link_status
# ndd /dev/bge0 link_speed
Here duplex is full with auto-negotiating off and the link is down.
NDD can do much more but I only needed to check the interface settings.