Figure 3: Conditions that must be met before an alias IP is added

&PingHost: the subroutine for checking the availability
           of an IP (see Figure 1).
$ALIAS_IP: the IP Address of the alias.
$ROUTER:   the IP Address of the system&'s default router.
$PARTNER:  the IP Address of our partner running Alias Defender.
$PRIMARY:  a true/false value for which true means the system
           is the default system for running the alias IP. Used
           to reduce collisions during the adding of the alias IP.

sub CheckAlias{
    # Subroutine for checking to see if the alias is alive.
    # A return value of "true" means that we should add the alias.
    #
    # For a return value of true, the following must be meet:
    #     if we are the primary
    #         the alias must be down and router must be up.
    #
    #     if we are not the primary
    #         the alias and partner must be down while the router 
    #         must be up.
    #

    if (&PingHost("$ALIAS_IP") eq "true"){
        return false;
    }else{
        if (&PingHost("$ROUTER") eq "true"){
            if ($PRIMARY eq "true"){
                # Alias is down, router is up and we are the primary.
                # Do not care if partner is alive. Bring up alias.
                return true;
        }else{
                if (&PingHost("$PARTNER") eq "true"){
                    &DisplayLogMessage("WARNING!! Could not \
                    failover. $ALIAS_IP is NOT alive but the primary \
                    $PARTNER is alive.");
                    return false;
                }else{
                    # Alias is down, router is up, partner is down
                    # and we are are the secondary. Bring up alias.
                    return true;
                }
            }
        }else{
            # Router is down. In this situation, a failover is not 
            # done because of the possibility the switch or network 
            # is down.
            &DisplayLogMessage("ERROR!! Could not bring up $ALIAS_IP \
            due to router unavailability. Switch/Network down?");
           return false;
        }
    }
}