Any NICs in a Tru64 TruCluster environment, whether they be the cluster interconnect or the regular public lan NICs, should have spanning tree protocol (STP) turned off on the port they are connected to. See the TruCluster Server Cluster Administration guide (part number: AA-RHGYE-TE
On a Cisco switch, in enable mode:
no spanning-tree portfast
Don't forget to save the running config to nvram.