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When the ip inspect interface configuration command is issued, the router may drop a Synchronize/Acknowledge (SYN/ACK) reply instead of send it

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Core issue

This occurs when you originate a TCP connection from an interface on another router, and the ip inspect interface configuration command has not been issued.

The output of the debug ip packet detail privileged EXEC command indicates that the SYN/ACK reply  is dropped by inspect, as shown:

IP: s=192.168.128.16 (FastEthernet0.2), d=192.168.192.69 (FastEthernet0.3), len 48, dropped by inspect

TCP src=23, dst=3403, seq=143608234, ack=3669485014, win=5840 ACK SYN

Resolution

Remove the ip inspect interface configuration command from the interface of the router that  is supposed to send the SYN/ACK reply.  For more information, refer to Cisco bug ID CSCec78231. The Bug Toolkit provides information on which Cisco IOS  Software version includes the fix for this defect.