An administrator is configuring a transparent Cisco FTD device to receive ERSPAN traffic from multiple switches on a passive port but the FTD is not processing the traffic What is the problem?

While Layer 3 connectivity for GRE traffic is necessary, the core issue is the FTD's operational mode (transparent) preventing it from processing that Layer 3 encapsulated traffic, not just the switch's connectivity.

There isn't a specific "ERSPAN port" mode on FTD; ERSPAN traffic is received on a standard interface which then needs to be configured to handle GRE, typically in routed mode. A "passive port" is for non-encapsulated SPAN.

ERSPAN encapsulates mirrored traffic within GRE tunnels, which are Layer 3 protocols. To properly receive and decapsulate ERSPAN traffic, the Cisco FTD device needs to operate in routed mode to handle Layer 3 routing and GRE tunnel termination, rather than transparent (Layer 2) mode.

While matching monitor session IDs and flow IDs is crucial for ERSPAN configuration, the fundamental problem here is the FTD's transparent mode being incompatible with processing Layer 3 GRE-encapsulated ERSPAN traffic.