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[ScreenOS] With CPU protection enabled, why do drop counters increase even when utilization is less than the threshold?

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Article ID: KB25962 KB Last Updated: 09 May 2013Version: 4.0
Summary:
This article describes the issue of drop counters increasing with CPU protection being enabled, even though the utilization is less than the set threshold.
Symptoms:
With CPU protection being enabled and the threshold set to 90%, the drop counters are increasing; even though the utilization is less than threshold. The command to enable CPU protection is set cpu-protection threshold 90.
FW-> get cpu-protection
Current usage: 44%, High CPU threshold: 90%
ID Src-IP Dst-IP Protocol Src-Port Dst-Port Drop Timeout
---------------------------------------------------------
Class    Traffic       Drop        Pass
---------------------------------------------------------
0        Critical      0           7849487
1        BC            0           34545
2        Non-first     0           561875
3        First         22321471   22991316
4        Other         0          81219

FW-> get cpu-protection
Current usage: 46%, High CPU threshold: 90%
ID Src-IP Dst-IP Protocol Src-Port Dst-Port Drop Timeout
---------------------------------------------------------
Class    Traffic           Drop            Pass
---------------------------------------------------------
0        Critical         0                7932142
1        BC               0                34894
2        Non-first        0                567178
3        First            22464716         23272478
4        Other            0                81916
Cause:

Solution:
This is expected behavior.

  • The firewall does not immediately stop dropping traffic, when CPU utilization goes below the threshold.

  • Rather, it uses an internal logic to gradually reduce the drop frequency and ultimately stops dropping packets, when the flow has stabilized.

  • The firewall maintains a drop frequency that indicates the number of packets to be dropped per 100 packets and it changes as per the flow.

  • This drop frequency is maintained internally and cannot be viewed on the firewall.

  • The calculation includes a combination of the following parameters:

    • Current utilization with respect to utilization during the last second.

    • CPU threshold.

    • The packets that are dropped during the last second.

  • In a production environment, it is not possible to predict the drop rate, as it will depend on the traffic flow.

Note: At times, it is also possible that the CPU usage is below the threshold due to packets being dropped by cpu-protection. So, actually the traffic is not below the threshold; rather, it is maintained below the threshold by dropping packets.

Note: See KB27138 for further detail on cpu-protection throttling.

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