First, log in to the server as root and see if the command rscadm status reports a good status. If it does, then RSC is working and there is an Ethernet configuration problem. Use the rscadm show command to check that Ethernet configuration variables are set correctly.
You can also:
Log in to RSC through the serial port and use the GUI or the shownetwork command to see the current settings.
Log in to another machine on the network and use the ping command to see if RSC is alive. Use RSC's name (for instance, servername-rsc), not the server name, as the argument to the ping command.
Run SunVTS diagnostics to check the Ethernet connection; the external Ethernet test requires that the device be connected to a functional 10-Mbit hub.
Run SunVTS diagnostics to check the SSP and paddle card.