Together, the primary and secondary interfaces form a pair of decryption forwarding interfaces. Only interfaces that you have enabled to be Decrypt Forward interfaces are displayed here. Your security chain type (Layer 3 or Transparent Bridge) and the traffic flow direction (unidirectional or bidirectional) determine which of the two interfaces forwards allowed, clear text traffic to the security chain, and which interface receives the traffic back from the security chain after it has undergone additional enforcement.