Ferritaas Incident Intelligence
Defect Intelligence Report Bug 0773918 Severity: High (Hardware Link Flap)

SFP Transceiver DOM I2C Bus Hang Causes False Link-Down Alarms

Digital Optical Monitoring (DOM) query collisions lock the internal hardware chassis I2C serial bus on FortiGate appliances, preventing transceiver status updates and spuriously declaring fiber links down.

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⏱️ Est. Triage: 6–8 mins | Automated CLI Available

Technical Root Cause Analysis

The FortiOS kernel uses an internal I2C bus controller to read digital optical diagnostics (RX/TX power, laser bias, voltage, and temperature) from plugged SFP/SFP+ transceivers.

When multiple interfaces are polled simultaneously, or when non-Fortinet coded optics stretch the I2C clock line (clock stretching bug), the I2C bus state machine enters a permanent deadlock. The kernel receives timeouts on read operations and assumes the physical optic has been unplugged, taking the link down and triggering spurious failovers across HA clusters and SD-WAN steering members.

Affected Firmware & Blast Radius Matrix

FortiOS Branch Vulnerable Builds Confirmed Clean Build Status & Workaround
FortiOS 7.0 7.0.0 – 7.0.11 7.0.12+ Increase DOM polling timer to 60s
FortiOS 7.2 7.2.0 – 7.2.3 7.2.4+ Automated I2C reset patch added
FortiOS 7.4 Not Affected 7.4.x Branch Clean Native asynchronous I2C queue

Platform Impact: FG-100F, FG-200E, FG-200F, FG-600E, FG-1100E using SFP/SFP+ optical modules in enterprise fiber aggregation links.

Step 01: Free Verification CLI (Safe Read-Only)

Execute these commands to inspect optical transceiver power levels, diagnostic registers, and I2C kernel logs:

Diagnostic Commands

# 1. Query optical power levels and diagnostic parameters
get system interface transceiver <interface_name>

# 2. Run low-level hardware transceiver register check
diagnose hardware test transceiver

# 3. Check low-level interface carrier status and error stats
diagnose netlink interface get <interface_name>

# 4. Search kernel dmesg for I2C timeout and bus hang signatures
diagnose dmesg | grep -i -E "sfp|i2c|transceiver"

Remediation & Workaround Steps (Teaser Preview)

Follow the complete non-disruptive optical recovery runbook in the Ferrite interactive platform:

Step 02: Clear Hung Chassis I2C Controller via CLI

Execute low-level hardware controller reset to release the locked clock line without rebooting.

🔒 Interactive CLI Available in Ferrite Runbook #08

Step 03: Adjust Digital Optical Monitoring (DOM) Polling Frequency

Increase transceiver telemetry query intervals to prevent concurrent bus read collisions.

🔒 Interactive CLI Available in Ferrite Runbook #08

Step 04: Audit Optical Power Budget (RX Sensitivity Margin)

Verify optical attenuation to distinguish genuine fiber degradation from software bus hangs.

🔒 Interactive CLI Available in Ferrite Runbook #08
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Frequently Asked Questions

What causes Bug 0773918?

I2C bus collisions during concurrent DOM queries cause the hardware bus controller to lock, preventing link presence detection.

Can this be fixed without rebooting?

Yes. Resetting the I2C bus driver via targeted CLI commands restores communication without dropping traffic.

Which firmware releases contain the fix?

FortiOS 7.0.12 and 7.2.4 added automatic bus lockup detection and hardware watchdog recovery.