Intelligent flow state synchronization to improve resiliency, availability, and/or performance of redundant network security devices

Example security systems for use between at least one upstream router and at least one downstream router, are described. A group or pool of security devices can be used to provide stateful security to bidirectional packet flows between upstream and downstream routers. The packets of the bidirectional flows are forwarded to particular security devices based on a consistent hash ring process. For a given flow, bidirectional state information is synchronized among some, but not all, of the security devices. The security devices among which such bidirectional flow state information is shared are determined using the same consistent hash ring process.