---
title: "400G ZR Interoperability Reality: Which Vendor Pairs Actually Work and What to Test Before You Commit"
slug: "400g-zr-interoperability-matrix-testing"
type: analysis
category: "Coherent Optics"
tags: ["400G ZR", "interoperability", "OIF", "coherent", "QSFP-DD", "DSP", "DWDM"]
seo_focus_keyword: "400G ZR interoperability vendor matrix"
---

The OIF (Optical Internetworking Forum) 400ZR Implementation Agreement was ratified in 2020 with the explicit goal of enabling multi-vendor interoperability in 400G coherent DWDM. Two and a half years after the first compliant modules shipped, the honest assessment is: interoperability works, within well-defined conditions, and there are enough asterisks on the "works" to fill a whitepaper. Knowing which vendor pairs have been validated and what the OIF implementation agreement actually covers—versus what it leaves to vendor interpretation—is essential before committing a production network to a specific configuration.

## What OIF-400ZR Specifies

The OIF-400ZR IA defines a specific, narrow operating point: 400G using DP-16QAM modulation, FEC using OpenFEC (a concatenated CFEC/UFEC scheme), at a single carrier frequency within the C-band, with a maximum reach of approximately 120 km on a compensated link (EDFA-amplified, no inline DCM) or 80 km on an uncompensated link.

The IA specifies the modulation format, baud rate (approximately 60 GBaud), spectral occupancy (approximately 75 GHz), FEC algorithm, DSP framing, OTU4 client mapping, and key electrical interface parameters for the QSFP-DD host connector. Within these constraints, a module from Vendor A should be interoperable with a module from Vendor B.

What the IA does not specify: the specific DSP implementation, laser linewidth characteristics beyond the minimum requirement, pre-emphasis and equalization algorithms, and—critically—firmware update sequences and initialization timing. Each DSP vendor (Acacia, InPhi/Marvell, Coherent Corp./II-VI, Lumentum, Broadcom) implements the coherent signal processing differently, and these differences are the source of most practical interoperability issues.

## The DSP Version Problem

The most consequential compatibility issue in 400G ZR deployment is DSP firmware versioning. The OIF-400ZR IA defines the protocol, but each DSP generation implements that protocol with different FEC coefficients, different carrier recovery loop parameters, and different chromatic dispersion compensation ranges.

A specific example: early DSP implementations used a CD (Chromatic Dispersion) acquisition range of ±8,000 ps/nm. The specification required a minimum of ±2,400 ps/nm (for uncompensated links up to 120 km on G.652 fiber, which accumulates roughly 2,000–2,400 ps/nm of CD). Early Acacia AC400 and early InPhi Porrima (CP200) DSP implementations had acquisition ranges well within spec but differed in how they signaled acquisition state to the host. If one end acquired lock and began transmitting live traffic before the far end had completed its initial carrier recovery, the mismatched initialization state caused a transient failure that resolved itself within seconds but occasionally triggered the host router's interface error threshold and bounced the link.

This specific issue was addressed in firmware updates released in 2021–2022 for most first-generation 400ZR DSP implementations. But it illustrates the class of problem: OIF-400ZR compliance means protocol compliance, not implementation-level behavioral compatibility, and the implementation differences show up in edge cases like acquisition timing, fault recovery, and behavior under marginal OSNR.

## Which Vendor Pairs Have Been Validated

The most current publicly available interoperability validation data comes from two sources: the OIF's own interoperability demonstrations (conducted at OFC and other industry events since 2021) and operator validation reports from major telcos and cloud providers who have published their findings.

Validated pairs that have been publicly demonstrated to operate in bidirectional coherent ZR mode include:

Acacia (Cisco) AC400/AC1200 with InPhi (Marvell) Porrima CP200 and CP400: demonstrated at OFC 2021 and 2022, with confirmed firmware version requirements published. This pair works reliably at firmware revisions specified in the OIF demo documentation.

Lumentum 400ZR QSFP-DD with Coherent (II-VI) CFP2-DCO ZR: validated in lab testing by multiple European operators. The CFP2 to QSFP-DD pairing works because the ZR specification is form-factor independent—the optical interface is the standard; the electrical host interface is separate.

Broadcom Bakerfield-based implementations (used by Innolight, HG Tech, and others in merchant silicon modules) with Acacia and InPhi: generally validated at the protocol level, with some firmware version sensitivity around CD acquisition timing.

Combinations that have known issues or limited validation: any first-generation 400ZR module at firmware predating mid-2021 against any other first-generation module. The 2020–early 2021 firmware was when the "implementation agreement is ratified but implementations are still maturing" period was most visible. If you have first-generation 400ZR modules in inventory that haven't received firmware updates since deployment, treat their interoperability with new second-generation modules as unvalidated.

## How to Test Before You Commit

The validation process for a 400G ZR vendor pair should cover more than "does the link come up." A complete interoperability test covers:

**Link acquisition testing**: power both ends up simultaneously from a cold start and measure time to link establishment. Repeat 20 times. Any failure to establish link within 60 seconds (the OIF-400ZR acquisition time requirement) is a bug. Any consistent delay beyond 30 seconds warrants investigation.

**Marginal OSNR testing**: use a variable optical attenuator to reduce the received OSNR incrementally and measure FEC error rate and pre-FEC BER at each attenuation step. The FEC threshold (the point where corrected errors appear) and the hard decision threshold (the point where uncorrected errors appear) should be consistent with the OIF-400ZR specification. A DSP pair that shows a larger gap between FEC threshold and hard decision threshold is more robust under impaired conditions.

**Link restoration after failure**: simulate fiber cut and restoration (loopback disconnection and reconnection) and measure time to link re-establishment. Coherent DSP reacquisition times vary from 5 to 60 seconds depending on implementation and condition history.

**FEC performance verification**: at nominal OSNR, FEC corrected error count should be low (on the order of 10^-4 to 10^-5 pre-FEC BER). A link running at consistently high pre-FEC BER with active FEC correction is operating with less margin than specification implies, and multi-vendor pairs may have slightly different pre-FEC BER characteristics at the same optical power level.

**Firmware version documentation**: record the specific firmware version on both ends before and after any firmware update, and re-run the test matrix after updates. Firmware updates that change DSP coefficients can shift interoperability behavior.

## The Operational Reality

Production 400G ZR networks running interoperable multi-vendor configurations exist and are operationally stable—this isn't a theoretical exercise. The conditions for success are: confirmed firmware version compatibility (both ends on validated firmware revisions), tested and documented link acquisition behavior, and a change control process that requires re-validation after DSP firmware updates.

The operational risk of 400G ZR interoperability is not that it doesn't work—it's that the conditions under which it works are specific, and changes to those conditions (new firmware, new module generation on one end, changed optical path characteristics) can shift interoperability behavior without obvious warning. Treating the validation matrix as a living document, updated with each significant change, is the practice that distinguishes networks that manage 400G ZR coherent well from those that manage it reactively.
