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S-Series: High CPU (switch packet processing) when a GRE tunnel traverses the switch (source and dest elsewhere) and netflow is enabled

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TitleS-Series: High CPU (switch packet processing) when a GRE tunnel traverses the switch (source and dest elsewhere) and netflow is enabled
Symptoms
  • High CPU ( Switch Packet Processing task runs up to the max of 60% )
  • Large number of flows on port where GRE tunnel enters the switch ( up to 500,000 )
  • potential dropped frames
Environment
  • S-Series
  • Firmware 8.62.04
  • GRE Tunnel traverses through the switch or Vxlan
  • Netflow enabled 
Cause
The issue can happen if there is a GRE tunnel traversing the switch toward Purview and a lot of customer traffic mirrored within that tunnel. It may or may not show symptoms dependent on the amount of traffic in the tunnel.

Enabling netflow leads to building of flows past tunnel headers (Vxlan, GRE, MPLS, Mac in Mac, etc) to the inner customer headers and netflow records are exported for the inner headers. In some environments that may lead negative consequences like exceeding flow limits, high CPU usage, and dropped packets.
Resolution
Upgrade to Firmware 8.63.01

In this version a new CLI command has been added to give user control over whether flows are built past tunnel headers (and netflow records exported for inner headers). You can then configure:
 
set netflow export-inner-headers disable


Workaround :

Try removing Netflow configuration  from the switch. If the CPU drops then you can work around by keeping netflow disabled until you can upgrade

 
Additional notes
See also the following article which has exactly the same symptoms for a similar reason but with a different root cause and a different fix in firmware
S-Series: High CPU (switch packet processing) when a GRE tunnel traverses the switch (source and dest elsewhere) and Vxlan is enabled

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