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The "%SYS-3-CPUHOG" error message is displayed in the logs of Cisco Catalyst switches

Core issue

The %SYS-3-CPUHOG: Task ran for [dec] msec ([dec]/[dec]), process = [chars], PC = [hex] message is received when a process runs for too long, and does not relinquish the processor. The process is a "CPU hog".

Each process should run only for a small amount of time before relinquishing the processor, so that other processes can run. This problem can be caused by many conditions such as traffic, system load, hardware, operational configuration, a configuration change, initialization of many interfaces, a high momentary error rate, or a sustained abnormal condition.

For more information, refer to What Causes %SYS-3-CPUHOG Messages.


If this message appears during startup or during OIR of a card, no action is required.

If this message appears during an attempt to access a Flash memory device, replace the affected Flash memory device.

If this message occurs during normal router operation, the cause is most probably a software failure. To take advantage of recent fixes, upgrade your system to the latest Cisco IOS Software Release in the release train.

If the error message indicates a Spanning Tree Protocol (STP) process, try to:

  • Reduce the number of VLANs (through VLAN Trunking Protocol (VTP) transparent).
  • Add an allowed VLAN list to all trunks. The switch will not send out a bridge protocol data unit (BPDU) for VLANs that are disallowed on the trunks. This is called pruning (especially for IP phones).

These actions:

  • Remove packets per second of outgoing traffic from every port.
  • Reduce the load on the CPU from the STP Queue Handler process.
  • Greatly reduce the amount of memory permanently allocated by STP.
  • Greatly reduce the amount of memory transiently allocated by STP in order to transmit BPDUs.