As of this writing, LAG (or trunking) can only be used on a network interface that is not associated with the LAN cluster interconnect. You must install at least the inaugural patch kit (IPK) in order for LAG to work on V5.1A. LAG did not exist on V5.0A or V5.1. For additional information, see chapter 9 and Tru64 UNIX Network Administration: Connections Guide.