Why is my Dataplane CPU Higher with Tap Mode traffic?

Why is my Dataplane CPU Higher with Tap Mode traffic?

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Created On 09/26/18 13:53 PM - Last Modified 06/11/20 21:26 PM


Symptom


  • Reported high DP CPU after enabling Security rules for Tap zone traffic with a deny action


Environment


  • Any platform (Virtual and Physical)
  • Any PAN-OS
  • Security rules for Tap zone traffic with a deny action


Cause


 
  • In a common packet flow scenario, with non-tap transit traffic the Firewall would see TCP SYN and pass the traffic through first-packet processing to create a session. First-packet processing tends to be CPU intensive. All subsequent packets for that session will match the session and hence do not need to go through first-packet processing again. Therefore this is less load on the CPU.
  • Also, for transit traffic, if the policy is set to deny then the session would not be created and the end client/server would drop that session and not continue sending anymore data.
  • Tap mode is different because it will not drop traffic and terminate the TCP sessions. TCP session traffic will continue to be seen despite the deny rule. The result is the Palo Alto device will perform first-packet processing on the SYN packet of a session and deny the session meaning no session would be created. Every subsequent TCP packet for that session will be processed as first-packet again but will be dropped due to the tcp-reject-non-syn option.


Resolution


 

  1.  For tap mode, the tap interface will always drop the packet so it is recommended to configure rules to allow for any TAP interface.

 

 



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