This article describes an issue observed on SRX platforms running Junos 24.1 releases where traffic fails when using an IRB interface along with PMI (Packet Management Infrastructure) and Layer 2 (L2) switch mode enabled simultaneously. While basic connectivity appears functional, certain application traffic is not forwarded correctly through the device.

• Ping from LAN to internet succeeds
• Limited traffic such as FTP works
• Application traffic (e.g., SSH) fails and times out
• SRX, acting as a transit device, does not forward certain packets
• Flow trace shows missing ACK packets on the service side
• Traffic appears to be processed via PMI fast path, but not transmitted

Root Cause

PMI fails to correctly handle packets for IRB interfaces when L2 switch mode is enabled. Certain traffic is processed through the PMI fast path but not properly forwarded, leading to failures in protocols like SSH.

Workaround

Disable PMI:

set security flow power-mode-disable
commit