The /etc/rc2.d/S69inet startup script, which runs when the machine boots, determines whether a machine is a router or a host. This decision also determines whether the routing protocols (RIP and RDISC) should run in router mode or host mode.
The /etc/rc2.d/S69inet script concludes that a machine is a router if the following two conditions exist:
More than one /etc/hostname.interface file exists.
More than one interface was configured "up" by the ifconfig command. (See the ifconfig(1M) man page.)
If only one interface is found, the script concludes that the machine is a host. See "Configuring Both Router Network Interfaces". An interface that is configured by any means other than an /etc/hostname.interface file is not taken into account.