The SRX Series Firewall drops packets with the error Dropped by FLOW: First path Out ifp error with no additional diagnostic information provided.

When executing monitor security packet-drops or reviewing flow traceoptions output, packets originating from an IPsec VPN tunnel are observed being dropped with the error Dropped by FLOW: First path Out ifp error.

Two potential causes have been identified:

1. PFE/RE Synchronization Issue - The Packet Forwarding Engine (PFE) is out of sync with the Routing Engine (RE) for the IPsec tunnel.
   To verify, log in to the vty and run show usp ipsec sa, then cross-reference the IPsec SA index with the output of show security ipsec sa.
   
2. Traffic Selector and IKE Gateway in the Same Subnet - If PFE and RE synchronization is confirmed, the issue may be due to the traffic selector and the IKE gateway of the peer residing in the same subnet. This causes the VPN response traffic to arrive on the public interface instead of the st(tunnel) interface, triggering a new session. Since the 4-tuple matches the existing session, the packet is dropped.
   Ensure that the traffic selector and the IKE gateway of the peer are not configured within the same subnet.