Which option is the most effective placement of an IPS device within the infrastructure?

A.
Inline, behind the internet router and firewall

B.
Inline, before the internet router and firewall

C.
Promiscuously, after the Internet router and before the firewall

D.
Promiscuously, before the Internet router and the firewall

Explanation:
BD
Firewalls are generally designed to be on the network perimeter and can handle dropping a lot of the nonlegitimate traffic (attacks, scans etc.) very quickly at the ingress interface, often in hardware.
An IDS/IPS is, generally speaking, doing more deep packet inspections and that is a much more
computationally expensive undertaking. For that reason, we prefer to filter what gets to it with the firewall line of
defense before engaging the IDS/IPS to analyze the traffic flow.
Source: https://supportforums.cisco.com/discussion/12428821/correct-placement-idsips-network-architecture