A passive interface is specifically designed to monitor copied network traffic received from a switch SPAN or mirror port. Because the Secure Firewall Threat Defense device is not physically placed in the traffic path, it receives a copy of the packets for inspection but does not retransmit them. This provides network and intrusion visibility without affecting production traffic. The trade-off is that a passive deployment cannot directly block, shape, or modify the monitored traffic because the original packets continue through the switch independently. An inline deployment, by comparison, places paired interfaces directly in the forwarding path and can drop malicious traffic. Routed and transparent describe firewall forwarding modes and are not the appropriate SPAN-based IPS interface type. Therefore, a SPAN-port monitoring deployment requires a passive interface. Cisco passive-interface documentation