400G ZR+ DCI (Data Center Interconnect): Optics Selection Guide

What a 400G ZR+ DCI link has to deliver

Data center interconnect is the link between two buildings, campuses or metro sites that have to behave like one facility. The traffic is storage replication, VM migration, database sync and AI training checkpointing, and it tolerates delay far less than transit traffic. Spans are usually 10 to 120 km of the operator's own fibre, sometimes with a line system, sometimes on a bare dark fibre pair.

400G coherent pluggables changed how those links are built. Instead of a transponder shelf and separate line system at each end, a ZR or ZR+ module sits directly in a router or switch port, carries its own coherent DSP and tunable laser, and places the client signal on a DWDM channel. Selection reduces to three questions: how far the span goes, how many channels share the fibre, and which line-rate modes it can carry at margin.

ZR, ZR+ and OpenZR+: what the labels cover

400G ZR is the metro DCI case: one 400G channel on the DWDM grid across the 80 to 120 km class, on a dark fibre pair with an amplifier or an existing line system. ZR+ keeps the same footprint and adds reach and flexibility: amplified metro and regional spans, a stronger FEC scheme, and fallback to 300G, 200G or 100G line rate when the span will not carry 400G at margin. OpenZR+ is the multi-vendor profile of the same idea, so modules from different vendors interoperate on one channel. On a fixed span the fallback modes make the link deployable: a span that will not take 400G at the required margin will usually take 200G comfortably, and the module changes rate instead of the link being rebuilt.

Recommended optics for DCI and metro transport

The reach column is a planning class, not a promise: the real limit is span loss, the amplifier chain and the channel plan. Coherent form factors and grey optics for the same campus are on the 400G QSFP-DD and OSFP page, and the line-system side is covered under coherent optics.

Span loss, amplifiers and unamplified links

Coherent reception buys roughly 20 dB over grey optics, which is what makes a 400G channel viable at metro DCI distance. It does not remove the physics: a long span still needs an amplifier, an amplified span needs the power and gain to be right, and an unamplified span cannot exceed the loss the receiver closes over at margin. Ask for the measured span loss when the link is ordered, not the design figure; fibre laid years ago is usually lossier than the drawing says. Where loss sits between an unamplified and an amplified design, a lower line rate is cheaper than an extra amplifier site.

Line-rate modes and the margin trade

Coherent pluggables trade rate against reach: 400G on a shorter or cleaner span, 300G, 200G or 100G on a span that will not support the top rate at margin. The modes are set in the module, so a link can be commissioned at a lower rate and raised later. State the target rate and acceptable fallback range in the RFQ, and whether the two ends must interoperate with a different vendor's module.

Power, cooling and the port budget

A coherent 400G module dissipates more heat than a grey optic in the same cage, and the DSP is the reason. Check three things before ordering: the line card port power budget, chassis airflow, and the density achievable once the coherent ports are populated. On a high-density AI fabric, spreading coherent ports across several line cards is usually better for thermal margin than concentrating them on one.

Coding, host support and management

The module must be supported by the host port, coded for the platform you name, and readable through the host: optical power, temperature, laser bias and alarms should be available, and it should report the line rate it locked to. Modules supplied for a branded router or switch port are compatible parts and are not manufactured by the brand of the host, with no claim of relationship to that brand. Hosts in this class include Cisco, Juniper and Nokia platforms, alongside other vendors' line cards and white box routers. Name the platform and release so the coding and mode set match at first insert.

How to choose, in order

1. Measure the span loss first, and note whether amplifiers or a line system are in the path.
2. Decide the target rate and the fallback range you will accept on that span.
3. Choose the form factor the host port supports: QSFP-DD, OSFP or CFP2-DCO.
4. Confirm the channel plan and spacing against the rest of the DWDM spectrum in use.
5. Check port power, airflow and density before committing to the count.
6. Name the host platform and release, and state the coding and interop requirement.

Frequently asked questions

What is the difference between 400G ZR and 400G ZR+?
ZR is the metro DCI case on the 80 to 120 km class with a single channel. ZR+ adds amplified metro and regional reach plus fallback rates of 300G, 200G and 100G.

Do I still need a transponder shelf?
Not for a point-to-point link: the pluggable goes into the router or switch port. Transponder and muxponder cards remain the better fit where many client signals have to be aggregated onto few wavelengths.

Can a ZR+ module interoperate with another vendor's module on one channel?
Where both implement the same profile, yes, and that is what OpenZR+ is for. Confirm the profile, the FEC and the grid on both ends first.

How much power does a coherent pluggable draw?
More than a grey optic in the same cage, because of the DSP. Confirm the port power class and chassis airflow against the platform data sheet before filling a shelf.

What should be in the RFQ for a DCI link?
The span distance and measured loss, target rate and fallback range, host platform and release, channel plan, and whether the link must interoperate with another vendor's optics.

Send the span distance, the measured loss, the target rate and the host platform, and we confirm the module, coding and supported mode set for each end. Related reading: 400G transponder and muxponder cards for shelf-based aggregation, 800G DCO transponder card for the next step up in capacity, and all optics selection guides.