illumos gate

ifconfig has the following option:

primary

Defines the interface as the primary. The interface is defined
               as the preferred one for the delivery of client-wide
               configuration data. Only one interface can be the primary at
               any given time. If another interface is subsequently selected
               as the primary, it replaces the previous one. Nominating an
               interface as the primary one will not have much significance
               once the client work station has booted, as many applications
               will already have started and been configured with data read
               from the previous primary interface.

Neither ipadm nor nwamd has an optional to allow designating an interface as such, but it would be convenient to have this.