Networking_
Building the Leaf-Spine Cable Plant for a GPU Fabric
Details the physical leaf-spine fiber plant construction for AI scale-out fabrics, covering trunk routing, polarity control, inspection sequences, and verification steps that crews follow when connecting GPU racks via InfiniBand or Ethernet.
Key facts
- MPO trunk cables for scale-out networks are factory-terminated and polished; field crews perform only routing, patching, cleaning, and testing.
- GPU-to-GPU NVLink traffic stays on copper backplanes inside each rack and never traverses the MPO fiber plant.
- Leaf-spine fabrics separate the intra-rack NVLink domain from the inter-rack compute fabric carried on multimode or single-mode fiber.
- Polarity must be verified end-to-end using a calibrated MPO continuity tester before any trunk is placed into service.
- Bend radius violations during trunk installation remain the leading cause of early link failures in raised-floor and overhead pathways.
- An OTDR trace plus insertion-loss measurement with a power meter constitutes the minimum acceptance test for each MPO link.
- Labeling follows the rack-row and port-grid scheme supplied by the network design team so that any patch can be traced without tracing cables.
Leaf and spine switch placement within the row
Leaf switches mount in the GPU racks or adjacent network racks so each leaf faces the servers it serves. Spine switches sit in dedicated spine racks at the row end or middle to keep fiber runs short while preserving airflow. The layout fixes trunk lengths and the number of vertical managers needed on each aisle side.
Placement decisions set how many MPO trunks exit each leaf switch and enter each spine. Crews mark floor tiles or overhead basket sections before any pull so later additions do not block existing trunks.
MPO trunk routing and slack management
Trunks are pulled from leaf patch panels to spine patch panels along pre-measured pathways that respect the OEM-specified minimum bend radius. Slack is stored in figure-eight loops on designated shelves rather than tight coils. Each trunk receives a unique identifier at both ends before it leaves the reel.
Overhead and under-floor routes stay separate from power cables to limit EMI. Where trunks cross aisles they rest on dedicated bridges so later rack moves do not pinch fibers.
Polarity control and patch-cord selection
Type-B or Type-C MPO trunks are selected per the network design so transmit fibers align with receive fibers at the far end. Patch cords at leaf and spine ends must match the trunk polarity method; mixing types produces crossed links found only during traffic tests. Crews verify polarity with a dedicated MPO tester before any transceiver is inserted.
Color coding on patch cords and trunks matches the labeling scheme so operators can confirm the correct cord by sight during maintenance.
Inspection, cleaning, and mating sequence
Every MPO connector is inspected with a fiberscope before it touches a port or adapter. Dust or film on the end-face raises insertion loss and can damage the opposing connector. Cleaning uses only the OEM-approved one-click tool or cassette; compressed air alone does not remove embedded particles.
Mating occurs only after both sides pass inspection. The connector is aligned, pushed until it latches, then given a gentle tug to confirm retention. Adapters remain capped until the moment of connection.
Link testing and documentation
Each completed link receives an OTDR trace from both directions plus a power-meter insertion-loss reading. Results are recorded against the link identifier and stored in the project database before the next trunk is tested. Any trace showing an event above the agreed loss threshold triggers re-inspection and possible replacement.
Documentation includes measured length, polarity result, and both OTDR files so future troubleshooting can compare against the baseline without disturbing live traffic.
Common failure modes encountered in the field
The most frequent failures are dirty end-faces that produce high loss after a quick visual check. These occur when crews skip the fiberscope step or reuse a contaminated cleaning tool. The second most common issue is polarity reversal from a mismatched patch cord after the trunk has been tested and labeled.
Bend-radius violations during the initial pull or later rack moves create macrobends visible as elevated loss on the OTDR. Mislabeling leads to incorrect patching found only during fabric bring-up. Leviathan Systems crews require a second technician to verify labels and polarity before any trunk is tied down.
Standards referenced: TIA-568.3-D Optical Fiber Cabling Component Standard · IEC 61754-7 Fibre optic interconnecting devices and passive components · TIA-942 Telecommunications Infrastructure Standard for Data Centers
Frequently asked_
How do crews confirm that a new MPO trunk has not been damaged during installation?
An OTDR trace is taken from each end and compared against the expected loss and event table for that length. Any macrobend or high-loss event is flagged before the trunk is placed in service. The power-meter insertion-loss measurement provides a second confirmation that total loss stays within the design budget.
What determines whether Type-B or Type-C trunks are used on a given fabric?
The choice follows the polarity method already selected in the network design documents. Type-B trunks reverse the fiber order inside the connector; Type-C uses a pair-wise flip. The patch cords at each end must match the trunk type so that transmit lands on receive at every transceiver.
Why is the NVLink fabric kept entirely separate from the MPO plant?
NVLink runs over copper backplanes inside the rack and does not leave the rack enclosure. The MPO fiber plant carries only the scale-out InfiniBand or Ethernet traffic between racks. Mixing the two domains creates unnecessary conversion points and violates the topology that the GPU software stack expects.
When should patch cords be installed relative to trunk testing?
Trunks are tested and accepted before any patch cords are attached. This isolates trunk performance from patch-cord variables. Once the trunk passes, the correct polarity patch cords are added and the end-to-end link is re-verified with the power meter.
What single step prevents the majority of field fiber failures?
End-face inspection with a fiberscope immediately before mating catches contamination that would otherwise produce high loss or transceiver damage. Crews that skip this step account for most repeated service calls on new fabrics. Leviathan Systems enforces this check on every connector.