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[ScreenOS] How can I reduce High CPU (flow), due to high rate of Multicast traffic?

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Article ID: KB11126 KB Last Updated: 29 Aug 2019Version: 4.0
Summary:
In some cases, Multicast traffic can cause the flow CPU to go high on a Juniper security device.
Symptoms:
When Multicast traffic hits the firewall at a very high rate, the flow CPU has been known to go high, i.e. 80-99 %.  In a specific case, this was due to a customer sending compressed HD video feed through an ISG2000 running ScreenOS 5.4.0r8a. 

Is Multicast traffic processed by hardware/ASIC for ASIC based firewalls?
Solution:
In ScreenOS, there is a flow command that will allow Multicast sessions to be processed by hardware as opposed to software in the CPU.  The command is :

set flow multicast install-hw-session

It's been noticed that Flow CPU can be greatly reduced with this command when Multicast traffic is causing High CPU. 


Note:  This is also documented in the ScreenOS 6.1 Release Notes: ScreenOS 6.1 Technical Documentation.
Modification History:
2019-08-21: Content reviewed for accuracy.
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