How to read the SFP optical module

SFP modules store both static identification and real-time diagnostic data, which can be read via EEPROM or network commands to verify compatibility and monitor performance.Understanding SFP EEPROM Data

Every SFP module contains a small EEPROM that holds two main types of information :

• Serial-ID / Base EEPROM (A0h / 0xA0): This includes the module identifier, connector type, vendor name (bytes 20–35), vendor OUI, part number, and revision. It is typically 96–128 bytes depending on the form factor.
• Digital Diagnostic Monitoring (DDM/DOM, A2h / 0xA2): If supported, this page provides real-time measurements such as temperature, supply voltage, laser bias current, transmit (TX) power, and receive (RX) power, along with alarm and warning thresholds. For QSFP or QSFP28 modules, the memory map is a superset of SFP, with multiple pages and expanded telemetry for higher-speed lanes.

Checking SFP Modules on Linux

On Linux systems, you can use commands like ethtool to read SFP information from network cards :

ethtool -m

Replace with your network interface name (e.g., eth0, p4p1). This command can display:

• Module type and identifier
• Connector type (LC, SC, etc.)
• Transceiver type and encoding
• Nominal bitrate and supported distances
• Vendor name, part number, and revision
• Real-time diagnostics if DDM is supported You can also check link status and speed using:

ethtool Checking SFP Modules on Cisco Switches

Cisco devices provide CLI commands to view SFP details and diagnostics :

• show inventory — lists all installed hardware with Product ID (PID), Version ID (VID), and serial number.
• show interface transceiver detail — displays full DDM metrics including temperature, voltage, TX/RX power, and vendor information.
• show interfaces status — shows port status, speed, duplex, and media type. These commands help verify correct installation, detect optical problems, and monitor module health.

Practical Tips

• Always verify that the SFP module is compatible with your switch or network card before installation.
• Use DDM readings to monitor for overheating, low optical power, or voltage issues.
• For troubleshooting, compare the module's EEPROM data with the expected vendor and part number to detect mismatched or unsupported optics.
• Periodically check SFP health to prevent unexpected link drops and maintain network stability. By combining EEPROM reading, Linux commands, or switch CLI diagnostics, you can fully monitor and manage SFP optical modules for reliable network operation.