GRE tunnel packet size is inclusive in PAN-OS by design

GRE tunnel packet size is inclusive in PAN-OS by design

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Created On 10/24/23 16:34 PM - Last Modified 10/24/23 18:28 PM


Symptom


  • Traffic is able to go through a GRE tunnel terminating on Palo Alto only if the overhead of GRE is included as outer layer in tunnel interface MTU configuration. 

Environment


  • GRE tunnels terminating on Palo Alto firewalls

Cause


  • GRE tunnel overhead is inclusive in packet size in PAN-OS

Resolution


  • For example, traffic is able to go through Palo Alto Firewall (from the source server to the internet), from the Server(MTU = 1500), through an AWS Transit Gateway, to the Firewall (MTU = 1500), if the customer sets MTU to higher values on the tunnel to 1600 and 8500, however, if you set it to 1500 it does not work.   
  • A configured MTU of 1500 on a GRE tunnel on Palo Alto firewalls actually only provides 1476 MTU by design 
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