Disclaimer

Known affected versions were inferred from fixed-version data and were not explicitly confirmed by the vendor: 9.1.8

The earliest recollection of this bug is traced back to PAN-OS 9.1 - July 22, 2025. This bug is fixed in PAN-OS versions 9.1.9. ( VM-Series firewalls on Microsoft Hyper-V only ) Fixed an issue where, when upgrading to PAN-OS 9.0.8 or later, ethernet packets dropped after adding VLAN tags during egress from a subinterface. To leverage this fix, set the interface level maximum transmission unit (MTU) to 1496 or less. On the VM-Series firewall on Microsoft Hyper-V, when upgrading to PAN-OS 9.1.0 or later, ethernet packets might be dropped after adding VLAN tags during egress from a subinterface. Workaround : Create the Hyper-V Virtual Switch with MTU size 1504, store as persistent and reboot for the changes to take effect. Before upgrading PAN-OS, access the VM-Series firewall CLI and set the MTU size on firewall interfaces to 1504. For more information: https://docs.paloaltonetworks.com/pan-os/9-1/pan-os-release-notes/pan-os-9-1-addressed-issues/pan-os-9-1-9-addressed-issues https://docs.paloaltonetworks.com/pan-os/9-1/pan-os-release-notes/pan-os-9-1-release-information/known-issues/known-issues-related-to-pan-os-9-1-releases