Network engineering

400G Cloud Interconnect Architecture: Define the Service Before the Optic

Design a 400G cloud or data-center interconnect around service handoffs, route diversity, capacity and the correct optical architecture.

A 400G interconnect can describe a connection inside a data center, a leased service between sites or a client interface into an optical transport system. These arrangements have different responsibilities and engineering limits. Define the service and ownership boundary before selecting transceivers or drawing a redundant pair of lines.

Identify what crosses the boundary

Document the applications, endpoints and traffic direction. Determine whether the required handoff is routed IP, a Layer 2 service or a specific overlay arrangement. Decide where routing, segmentation and fault isolation belong. Extending a broadcast domain between sites should be an explicit service requirement, not an accidental consequence of a high-bandwidth link.

VXLAN and EVPN can support particular overlay designs, but their availability does not remove the need to plan underlay reachability, MTU, failure behavior and operating ownership. R30 R31

Separate client optics from the line system

The optic that connects a switch to nearby transport equipment is not necessarily the optic or system that carries the inter-site route. Identify whether the channel is a direct supported optical link or includes wavelength multiplexers, amplification, dispersion treatment or other line-system functions.

Some DWDM-PAM4 implementations specify associated transport requirements; those conditions cannot be generalized to every PAM4 or coherent interface. Select a complete supported optical architecture instead of combining components because their rate labels match. R41

Define capacity and availability together

Design dimension	Required decision
Service boundary	Who owns each port, optical segment and routing relationship?
Capacity	Busy-hour demand, growth and allowed congestion
Resilience	Link, route, equipment and site failures to tolerate
Optical path	Direct channel or transport system with documented limits
Operations	Monitoring, escalation and maintenance coordination

An illustrative requirement for 250 Gb/s of usable traffic after one route fails cannot be justified by two 200G paths: only 200 Gb/s of nominal link capacity remains. A single surviving 400G path may satisfy the arithmetic, but the design still needs workload and overhead allowances plus end-to-end validation.

Route diversity must be demonstrated physically. Two services entering the same building conduit can share a failure even when they use different providers or routers.

Keep procurement aligned with ownership

For every handoff, record interface specification, supported modules, connector, route length and responsible operator. Define who provides test access and who diagnoses a low-power or error condition. The module catalogue supports product identification; it does not replace a channel design or service agreement. R25

Do not represent an evaluation list as a certified BOM. Hold unresolved host, optical or transport combinations for engineering confirmation.

Release an architecture that can be operated

Before implementation, approve the topology, capacity model, optical design, responsibility matrix and failure objectives. Establish measurable acceptance criteria covering throughput, errors, recovery and application reachability.

Use the companion implementation workflow for staging and cutover. This architecture document should answer a different question: why this service boundary and physical design meet the project’s requirements, and what assumptions must remain true as the interconnect grows.

Engineering illustrations

These figures illustrate the design concepts discussed here. They are not measured results, certified configurations or a supplied-equipment list.