Which IP address should use?

Hi,
Which IP Address should use? VIP Address or Primary IP Address?
When I use VIP address, the output as follow.
[root@nagios01 libexec]# ./check_cisco_firewall.sh -V 2c -C public -H 192.168.50.17 -M failover
Unknown – Primary = Down, Secondary = Down | Actives_Nodes=0
[root@nagios01 libexec]#

Thanks.