GE MULTILINK 1601-9026-A3 - Manual (Page 54)

50 MULTILINK ETHERNET COMMUNICATIONS SWITCH – QUICKSTART GUIDE
QUICKSTART GUIDE
From this screen we can see that the port named C5 is actually port 21.
Z Note this port number.
Z from the Administration menu open the Set menu.
Z Open the STP type setting.
Z Set STP to Rapid Spanning Tree (RSTP).