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Transceiver EEPROM stores identity, calibration, DDM thresholds, and firmware state that your NOS reads at every module insertion. This guide covers the full EEPROM landscape: SFF-8472 (SFP+), SFF-8636 (QSFP28), CMIS 5.x (OSFP/QSFP-DD), memory page structure, digital diagnostic monitoring fields, vendor lock-in mechanisms, and CLI-based debugging workflows for SONiC, Arista EOS, and Cisco IOS-XR. Includes page-level memory maps and practical troubleshooting steps.