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400G ZR / ZR+ Coherent DCI

An AI training run that spans two data halls used to need a transponder shelf at each end. With 400G ZR and ZR+ coherent pluggables, the long-haul interface drops straight into the spine in the same QSFP-DD form factor as a grey 400G optic, and OcNOS-DC supports both with EVPN inter-DC carrying tenant traffic across the link.

Two Fabrics, One Optical Pluggable

Each spine ships a 400G ZR or ZR+ coherent module in one of its QSFP-DD cages. The fiber goes straight into the metro / long-haul DWDM line system. EVPN inter-DC tunnels carry VXLAN tenant traffic across the link, tracking the OcNOS-DC fabric on each side.

400G ZR/ZR+ coherent DCI between AI data centers Two AI data center boxes, one on each side. Each contains spine and leaf switches. The two spines are connected by a 400G ZR/ZR+ coherent pluggable optical link, drawn as a wave pattern crossing the gap. EVPN inter-DC tunnel symbol overlays the link. Bottom band lists key DCI properties. DATA CENTER A · AI POD DATA CENTER B · AI POD Spine-AQSFP-DD ZR+ port Spine-BQSFP-DD ZR+ port 400G ZR / ZR+ coherent · DWDM line EVPN inter-DC tunnel Leaf-A1 Leaf-A2 Leaf-A3 Leaf-B1 Leaf-B2 Leaf-B3 GPU pods · Site A GPU pods · Site B 400G COHERENT PLUGGABLE · TRANSPONDER-FREE · OPENZR+ · EVPN INTER-DC

Why coherent pluggables changed DCI economics

Until coherent pluggables existed, any inter-DC link beyond ~10 km needed a transponder: a separate piece of optical equipment between the switch and the line system. That meant a second box per site, a second vendor relationship, second-set spares, and the operational complexity of managing a separate optical layer. Coherent pluggables collapsed all of that into a QSFP-DD module that fits into the spine itself.

Two flavours matter today. 400G ZR covers up to ~120 km of unamplified or single-amp metro fiber, perfect for two AI halls in the same metro. 400G ZR+ (per the OpenZR+ MSA) supports higher tx power, FEC tuning, and additional modulation modes, covering longer hauls or links that traverse external amplifiers and ROADM systems. Both are pluggable into the same QSFP-DD cage; the choice depends on your link budget.

ZR vs ZR+ at a glance

400G ZR: IEEE 802.3ct

Metro DCI (≤ ~120 km)

Industry-standard 400G coherent: short metro reach, fixed configuration, lower cost per port. Right for two AI halls in the same metro region.

400G ZR+: OpenZR+ MSA

Long-haul / amplified DCI

Tunable modulation (16-QAM down to QPSK), higher tx power, oFEC for longer reach. Reach depends on the line system and FEC tuning: typically 400-800 km single-amp, up to ~1200-1800 km on multi-amp DWDM systems. Right for cross-region AI fabric extension.

The OcNOS-DC implementation

Native Pluggable

QSFP-DD on the spine

400G ZR and ZR+ pluggables drop directly into supported spine platforms: Edgecore AS9736-64D, AS7726-32X, UfiSpace S9700-class, and other listed HCL gear with QSFP-DD coherent-capable ports.

CMIS Management

Standardized control

OcNOS speaks CMIS (Common Management Interface Specification) to the module: set frequency, modulation, tx power, line FEC; read back DDM, BER, OSNR for diagnostics.

EVPN Inter-DC

Tenant tunnel extension

VXLAN VNIs and EVPN routes propagate across the coherent link. The two fabrics behave like one extended fabric for tenant L2 and L3 services.

Telemetry

Coherent-aware gNMI

Per-module pre-FEC BER, post-FEC BER, tx/rx power, frequency offset, OSNR, and CMIS state stream over gNMI / OpenConfig, folded into your existing observability dashboard.

Operations

Same NOS, same skills

The DCI link is just another L3 interface to OcNOS-DC. BGP, BFD, route-policy, segment routing: everything works the same way as on a grey 400G port.

Hardware

Validated optics

Pluggables qualified against the platforms in the Hardware Compatibility List. We test the combination, not just each side in isolation.

What this gives the AI architect

  • No transponder shelf. The optical interface lives in the spine port. CapEx and rack space drop materially compared to transponder-based DCI.
  • Single management plane. Coherent diagnostics arrive in the same gNMI feed as the rest of fabric telemetry. No separate optical OSS to learn.
  • Fabric extension at GPU scale. 400G into the spine is enough headroom for a single long-haul link to carry a meaningful slice of an inter-DC training run; bond multiple ZR+ modules for higher capacity.
  • Open hardware path. ZR/ZR+ is an MSA, not a vendor proprietary. Optics from Coherent (Lumentum), Innolight, Cisco, Ciena, etc. all interoperate through CMIS.
  • Standard EVPN inter-DC. No new control plane to learn. The same EVPN tools you already use for intra-fabric carry tenants across the WAN.
EVPN-VXLAN EVPN multi-homing BGP underlay AI fabric topologies RoCEv2 lossless fabric DCI in feature matrix Coherent-capable platforms OcNOS-DC DC Fabric solution IPoDWDM

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FAQ

Frequently asked questions

What is coherent DCI with 400G ZR/ZR+?
Coherent DCI is data center interconnect built on 400G ZR and ZR+ coherent pluggable optics that plug straight into a switch port. Because the coherent interface lives in the QSFP-DD cage, the long-haul link no longer needs a standalone transponder shelf between the switch and the optical line system.
What is the difference between 400G ZR and 400G ZR+?
400G ZR (IEEE 802.3ct) is a fixed-configuration metro optic covering up to roughly 120 km, suited to two halls in the same metro. 400G ZR+ follows the OpenZR+ MSA with tunable modulation (16-QAM down to QPSK), higher tx power, and oFEC for longer, amplified reaches. Both share the same QSFP-DD cage, so the choice depends on your link budget.
Do I still need a transponder for coherent DCI?
No. The coherent optic sits in the spine switch port itself, so the transponder shelf, its separate vendor relationship, spares, and optical OSS go away. The fiber from the pluggable feeds the metro or long-haul DWDM line system directly.
How does OcNOS manage coherent pluggables?
OcNOS uses CMIS (Common Management Interface Specification) to set frequency, modulation, tx power, and line FEC, and to read back DDM, BER, and OSNR. Per-module pre-FEC and post-FEC BER, tx/rx power, frequency offset, OSNR, and CMIS state stream over gNMI and OpenConfig into existing telemetry dashboards.
How does tenant traffic cross a coherent DCI link?
EVPN inter-DC carries the traffic. VXLAN VNIs and EVPN routes propagate across the coherent link so the two OcNOS-DC fabrics behave like one extended fabric for tenant Layer 2 and Layer 3 services, using the same EVPN tooling as the intra-fabric network.
What optical transport options connect AI data centers?
Two data centers running AI fabrics are connected either by a traditional optical transport system, with a separate transponder shelf between the switch and the line system, or by coherent DCI, where a 400G ZR or ZR+ pluggable sits directly in the spine switch port and feeds the DWDM line system. Coherent DCI removes the transponder shelf and its separate vendor relationship. OcNOS-DC drives the pluggable and carries tenant traffic across the link with EVPN inter-DC, so the two fabrics behave like one.
Which DCI platforms support open and interoperable transport?
Open coherent DCI runs on switches whose ports accept standards-based 400G ZR and OpenZR+ pluggables, so the optic interoperates across multiple vendors and rides a third-party ROADM as an alien wavelength. IP Infusion delivers this on validated open hardware running OcNOS-DC, with the wavelength, modulation, and FEC set from the switch and the optical link streamed over gNMI and OpenConfig. Because the optics and the line system stay open, the transport layer is a competitive, multi-vendor purchase.
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