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 — same QSFP-DD form factor as a grey 400G optic, just with a coherent transceiver inside. 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.
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
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.
Long-haul / amplified DCI
Tunable modulation (16-QAM down to QPSK), higher tx power, oFEC for longer reach. Reach depends on the line system — typically 800–2500 km on amplified DWDM. Right for cross-region AI fabric extension.
The OcNOS-DC implementation
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.
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.
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.
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.
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.
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.