# Unexpected Denial-of-Service Protection Behavior During Unified ISSU

The denial-of-service (DoS) protection application freezes its state when the in-service software upgrade is initiated. Any suspicious control flow, protocol, or priority remains suspicious until unified ISSU completes.

Freezing the DoS protection state prevents any active control flows from interfering with the system while the unified ISSU is in progress. However, no new control flows, protocols, or priorities are monitored for suspicious activity, and no suspicious activity can be detected until the upgrade is completed.

| Note: Because of this limitation on DoS functionality, we recommend that you do not initiate unified ISSU until all suspicious control flows, protocols, and priorities have been resolved.

When the unified in-service software upgrade is completed, the DoS protection application resumes attending to all dynamic state that was frozen at the beginning of the unified ISSU process.

Some suspicious control flows might remain in a suspicious list based on your configuration, if the upgrade software version has DoS protection classification algorithms that are better or different than in the previous version. Because flows are discovered and monitored at 1-second intervals, the new conditions might cause these flows to be removed. You do not need to explicitly clear the flows when unified ISSU is completed.

#### Related Documentation

- Application Support for Unified ISSU