In a recent post, I shared our frustrating experience with unannounced bugs in FortiOS 7.4.7 on the FortiGate 121G, including slow download speeds for proxy traffic after configuration restoration, proxy chain issues, and CLI access failures. We initially attributed these problems to firmware bugs, as Fortinet’s support team couldn’t pinpoint a cause despite multiple troubleshooting sessions. Today, I’m excited to share that we’ve finally identified and resolved the root cause of the slow download speed issue—and it wasn’t a firmware bug after all. Here’s what we discovered and how we fixed it.
The Problem: Slow Download Speeds for Proxy Traffic
As outlined previously, restoring a configuration backup to another FortiGate 121G (or different models) resulted in significantly reduced download speeds, but only for traffic routed to a proxy server. This affected both direct proxy access and traffic through the FortiGate’s explicit proxy, while non-proxy traffic remained unaffected. The issue persisted across troubleshooting attempts, including restoring the config to a loan unit, which introduced additional problems like CLI inaccessibility and packet capture failures.
The Breakthrough: WAN Interface Misconfiguration
After exhausting firmware-related hypotheses, we turned our attention to the physical and interface-level configurations. The FortiGate 121G’s WAN interface connects to our ISP via a media converter (fiber to RJ45). The WAN port was configured for 100 Mbps full duplex, and the FortiGate’s interface status reported this setting as correct with no obvious errors. However, this configuration was the culprit.
On a hunch, we changed the WAN port setting to auto-negotiate. Immediately, the interface negotiated to 1 Gbps full duplex, aligning with the ISP’s media converter capabilities. The result? The slow download speed issue was resolved instantly. Proxy traffic now performs as expected, with download speeds returning to normal across all affected scenarios.
Why Did This Happen?
The root cause appears to be a mismatch between the FortiGate’s manually set 100 Mbps full duplex configuration and the media converter’s capabilities. While the FortiGate reported the 100 Mbps setting as operational, it likely caused performance degradation, possibly due to duplex mismatches, packet loss, or suboptimal negotiation with the ISP’s equipment. Auto-negotiation allowed the FortiGate to correctly detect and utilize the full 1 Gbps capacity, eliminating the bottleneck.
This issue was particularly deceptive because it only impacted proxy traffic, which may be more sensitive to interface-level performance issues due to the additional processing and session handling involved. Non-proxy traffic, being less resource-intensive, masked the problem in other scenarios.
Key Takeaways and Recommendations
This experience highlights the importance of scrutinizing physical and interface configurations, even when firmware bugs seem like the obvious culprit. Here are my recommendations for FortiGate administrators facing similar issues:
- Verify Interface Settings: Always check the speed and duplex settings of WAN interfaces, especially when connecting to media converters or ISP equipment. Use get system interface physical in the CLI to confirm the negotiated speed and duplex.
- Prefer Auto-Negotiation: Unless explicitly required by your ISP, set WAN ports to auto-negotiate to avoid mismatches. Manual settings can lead to silent performance issues, even if the FortiGate reports them as correct.
- Test Beyond Firmware: If you suspect a firmware bug (as we did with FortiOS 7.4.7), don’t overlook physical or layer-1/layer-2 issues. Use tools like packet captures or interface statistics (diagnose hardware deviceinfo nic) to identify anomalies.
- Engage Your ISP: Confirm with your ISP the expected speed and duplex settings for their equipment, as media converters can introduce unexpected variables.
- Document Troubleshooting: Keep detailed records of configuration changes and their impact, as this helped us isolate the issue after ruling out firmware-related causes.
Does This Absolve FortiOS 7.4.7?
While the slow download speed issue was not a firmware bug, our other concerns with FortiOS 7.4.7—such as proxy chain limitations (monitor failure and FQDN issues) and CLI/packet capture failures on the loan unit—remain unresolved and likely firmware-related. We’re still planning to downgrade to a more stable version like FortiOS 7.2.10 to address these outstanding issues, as Fortinet’s support has yet to provide fixes. However, resolving the download speed problem has restored some functionality and given us breathing room to plan the downgrade carefully.
Community Call to Action
This resolution underscores how complex network issues can stem from unexpected sources. If you’ve encountered similar performance problems with FortiGate firewalls, particularly involving WAN interfaces or media converters, I’d love to hear your experiences in the comments. Have you faced other FortiOS 7.4.7 bugs that we should be aware of? Sharing these insights helps our community build more resilient networks.
Conclusion
Identifying the WAN interface misconfiguration as the cause of our FortiGate 121G’s slow download speeds was a significant win, but it came after weeks of frustration and misdirection. While this issue wasn’t a FortiOS 7.4.7 bug, it reinforces the need for thorough, layered troubleshooting in network administration. We’re now optimizing our configurations and proceeding with our downgrade plan to address the remaining firmware issues. Stay tuned for updates, and thank you for following our journey!