The DNS-323 is set to auto. The 1000mbps setting is greyed out.
When I initially noticed that the unit was not connected at 1000mbps the relevant setting was not greyed out. I tried choosing the 1000mbps setting and the unit dropped out of the network and became unresponsive, prompting me to have to reset to factory defaults.
Since then, across three different routers and two different CAT6 cables the 1000mbps option has been greyed out and the unit is connecting at 100mbps.
Keep in mind this unit has been functioning correctly at 1000mbps for several years. I can really tell the difference too, backups from my pc's take longer now.