Summary

  • Richard Steenbergen's NANOG 95 deck shows that a modern transceiver contains a management CPU and sometimes a DSP, retimer and gearbox. Its CMIS interface can expose application choices, diagnostics and optional firmware controls.
  • The same deck describes vendor locks on third-party modules and gives examples of identity fields used to pass host checks. It also warns that bypassing a host's management driver can race its own polling process.

At the front of a router, an optical module is small enough to lose under a maintenance log. Inside, Steenbergen's original NANOG 95 slides find a management CPU beside the optics. Some modules carry a retimer, DSP and gearbox. One 800G OSFP firmware image he disassembled was 512 KiB, about 380 KiB of it nonzero, and ran on an ARM Cortex-M4. That is one specimen, but it ruins the comforting fiction that the port is a passive socket.

The module talks to the host through a memory-map interface descended from EEPROM. The slides say that this is usually virtual memory now: 128 bytes stay visible while another 128 are selected by page. CMIS lets a module advertise which host and media protocols it can pair, and how its lanes map. Diagnostics can report performance; some modern optics can run PRBS tests from the module itself. The OIF's CMIS 5.3 specification also defines CDB firmware-management facilities, advertised as an optional feature. A failed link can therefore require an answer from the module as well as the router.

Then comes the purchasing rule. Steenbergen says equipment vendors may reject third-party optics, with resale a lucrative incentive. His deck shows cases where changing a vendor name or OUI can satisfy a host check. He acknowledges that compatibility was genuinely messy before CMIS. A copied name is thus evidence of a gate, not proof that every module behind it is interoperable.

The operator still owns the outage when a port goes dark. Steenbergen contrasts SONiC's managed NVMEM path with direct i2c-tools access on Juniper JUNOS EVO, where paging and locking are absent and a polling race is possible. The deck's sharper lesson is operational: before testing a substitute optic, obtain the exact host, firmware, module and diagnostic behavior, then use the host's safe access path. A vendor identity string should not substitute for that record.

Sources: Steenbergen's NANOG 95 original slides · OIF CMIS 5.3.