The earliest recollection of this bug is traced back to PAN-OS 8.1 - July 22, 2025. This bug is fixed in PAN-OS versions 9.1.1, 8.1.12. ( PA-5250, PA-5260, and PA-5280 firewalls with 100GB AOC cables only ) When you upgrade the first peer in a high availability (HA) configuration to PAN-OS 9.0.3 or a later PAN-OS 9.0 release, the High Speed Chassis Interconnect (HSCI) port does not come up due to an FEC mismatch until after you finish upgrading the second peer. ( PA-5250, PA-5260, and PA-5280 firewalls with 100GB AOC cables only ) Fixed an issue where after you upgraded the first peer in a high availability (HA) configuration to a PAN-OS 9.0 release, the High Speed Chassis Interconnect (HSCI) port did not come up due to an FEC mismatch until after you finished upgrading the second peer. ( PA-5250, PA-5260, and PA-5280 firewalls with 100GB AOC cables only ) When you upgrade the first peer in a high availability (HA) configuration to PAN-OS 8.1.9-h4 or a later PAN-OS 8.1 release, the High Speed Chassis Interconnect (HSCI) port does not come up (due to an FEC mismatch) until after you finish upgrading the second peer. ( PA-5250, PA-5260, and PA-5280 firewalls with 100GB AOC cables only ) Fixed an issue where after you upgraded the first peer in a high availability (HA) configuration to PAN-OS 8.1.9-h4 or a later] release, the High Speed Chassis Interconnect (HSCI) port did not come up due to an FEC mismatch until after you finished upgrading the second peer. For more information: https://docs.paloaltonetworks.com/pan-os/8-1/pan-os-release-notes/pan-os-8-1-addressed-issues/pan-os-8-1-12-addressed-issues https://docs.paloaltonetworks.com/pan-os/8-1/pan-os-release-notes/pan-os-8-1-release-information/known-issues/known-issues-related-to-pan-os-8-1-releases https://docs.paloaltonetworks.com/pan-os/9-1/pan-os-release-notes/pan-os-9-1-addressed-issues/pan-os-9-1-1-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