First checks for IPMP problems (when ifconfig shows FAILED)

IPMP allows solaris to group multiple interfaces together to provide redundancy. This is not trunking or link aggregation, which combines multiple interfaces into one virtual interface for higher throughput.

Sometimes IPMP notices a problem with an interface and marks it as FAILED (this means that the test has failed, not that there is a hardware problem). This is because either be "the link is down" or "the ping test failed" (assuming you have test addresses).

First, I'd recommend understanding IPMP (see "man in.mpathd" and the IPMP documentation)
For troubleshooting steps, I recommend the following infodoc on sunsolve: