This topic was migrated from the BotBlox ticketing system after being summarized and anonymized.
Problem:
Customer experienced intermittent loss of communication to all devices downstream of interconnected SwitchBlox Rugged (BB-SWR-G-1) switches during power-up. The failure appeared as the switch-to-switch link not forming, causing all devices on downstream switches to disappear from the network. Swapping the inter-switch connection to a different port would immediately restore communication.
Answer:
Root cause was traced to an interoperability issue between the customer’s KSZ8081R Ethernet PHY and the KSZ8567S switch chip used on SwitchBlox Rugged.
The customer’s custom PCBAs used a KSZ8081RNBIA PHY, which has known silicon errata. BotBlox identified that similar symptoms (failed links, packet loss, link formation issues) can occur unless the PHY errata are addressed in firmware. The recommended fix was to disable auto-negotiation (force 100BASE-T operation) on the KSZ8081R and ensure the relevant Microchip errata workarounds were implemented.
What Worked:
- Swapping the inter-switch connection to a different port restored communication.
- Power cycling often restored communication temporarily.
- Systems using managed switches did not exhibit the same issue.
- Customer implemented firmware changes to disable auto-negotiation for testing.
What Didn’t Work:
- Original configuration with KSZ8081R auto-negotiation enabled.
- Certain power-up sequences where the switch-to-switch link never formed.
- Simply reseating cables/connectors did not reliably restore the link.
Final Status:
Likely resolved through KSZ8081R PHY configuration and errata mitigation.
BotBlox concluded the issue was not a SwitchBlox timeout or port-disable mechanism, but rather a PHY interoperability issue involving:
- KSZ8081R PHY on the customer’s boards
- KSZ8567S switch silicon on SwitchBlox Rugged
- Auto-negotiation behavior and known PHY errata
Additionally, forcing the KSZ8081R to 100 Mbps full duplex without auto-negotiation can create a duplex mismatch with an unmanaged switch, so forcing 100 Mbps half duplex or using a managed switch capable of matching settings is recommended.
Conclusion:
Intermittent downstream dropouts were caused by a KSZ8081R PHY interoperability/auto-negotiation issue rather than a SwitchBlox hardware fault. Implementing the PHY errata fixes and carefully configuring auto-negotiation settings was the recommended solution.
(TS2511-0056)