Direct answer
FEC corrects some received-data errors; passive tests assess the cabling. Link-up or a low post-FEC reading does not replace agreed loss, length and polarity acceptance. Keep both-end FEC state, observation windows and cabling-test records.
What FEC assesses
Cisco explains FEC as redundant information enabling a receiver to correct a limited set of errors. Implementation can reside in the host or particular optics. Check the selected products rather than prescribing one configuration for every 400G/800G link. Cisco FEC explanation
This guide addresses new-link handover, optic replacement and fault investigation. Keep operating statistics and passive-cabling records together while preserving their different test objects.
Pre-FEC and post-FEC
The NVIDIA MLNX-OS 3.10.2202 monitoring guide distinguishes errors before and after correction; its Effective BER concerns errors FEC did not correct. Its definitions are software-specific, not universal platform thresholds. NVIDIA monitoring guide
A corrected-error count and a BER are different quantities. Check counter units, sampling, normalization and reset events in the equipment documentation before comparing results.
Evidence comparison
This is an editorial acceptance framework without universal pass limits.
| Record | Question it helps answer | What it does not establish alone |
|---|---|---|
| Link-up state | Is a link currently established? | Complete passive-cabling acceptance |
| Pre-FEC statistic | What errors appear before decoding correction? | That fiber alone caused those errors |
| Post-FEC statistic | What errors remain after correction? | Stability at every temperature and duration |
| Corrected-error count | What correction activity occurred in the window? | A comparable rate with an unknown window |
| OLTS record | Agreed cabling loss, length and polarity | Host-and-optic operating acceptance |
Fluke Networks describes Tier 1 OLTS measurements of total insertion loss, length and polarity. Link statistics assess a different set of items. Fluke Networks testing guide
Build a comparable operating record
Agree device, port, optic IDs, rate, FEC mode and software version before defining the observation window. Retain beginning and ending counters and record restarts, resets and optic changes.
NVIDIA UFM 6.24.4 lists Active_FEC, Total_Raw_BER and Effective_BER separately, alongside a time-since-counter-clear field. Retain original field names and software versions rather than merging layers into a generic error column. NVIDIA telemetry fields
Organize a controlled investigation
Save both-end configuration and raw statistics before changes. Review optic pairing, end faces, route and passive-test records. Change one documented condition at a time and retain before/after measurements.
A changing pre-FEC reading is an investigation lead; disposition depends on the selected equipment and acceptance specification. Do not disable required FEC or convert a single zero-residual-error observation into a lifetime guarantee.
Procurement FAQ
- Does zero post-FEC validate a trunk? It does not replace passive tests. Verify the observation duration, conditions and counter meaning.
- Does an increasing correction count prove damaged fiber? No. Check units, rate, window, both optics and cabling evidence before assigning a cause.
- Can BER readings from two platforms be compared directly? Confirm measurement location, field definitions and sampling first.
- Is there a universal 400G/800G acceptance threshold? This guide provides none. Use the selected equipment, optics and project specification.
- What belongs in the handover pack? Cabling reports, both-end configuration, raw statistics, observation windows, change records and explicit acceptance criteria.
Prepare an acceptance review
Bring passive-test reports and both-end operating records before deciding on cable changes or retesting. Related pages support cable and optical-route review and do not confirm FEC or BER performance.
Related products and technical guides
Sources and review date
- Cisco FEC explanation
- NVIDIA MLNX-OS 3.10.2202 monitoring
- NVIDIA UFM 6.24.4 telemetry fields
- Fluke Networks fiber testing guide
Sources checked on 2026-10-10. Manufacturer examples apply only to the cited products. Selection tables and procurement records are editorial recommendations, not ARRANEX performance, certification or supply commitments.
Sources checked: 2026-10-10