Adaptive Routing on Spectrum-X addresses the fundamental limitation of hash-based ECMP in synchronized AI collective workloads. The key takeaways:

• At scale (512+ GPUs), hash ECMP produces 18-25% load variance across spines; AR reduces this below 3%
• Spectrum-4 AR engine samples queue depth at sub-microsecond intervals and makes per-packet forwarding decisions at line rate with zero added latency
• Per-packet AR requires SuperNIC reorder buffers on every host - BlueField-3 on B200, ConnectX-8 on HGX B300; DGX H100/H200 (CX7) clusters must use per-flowlet mode with a 100 us flowlet timer
• Resilient hashing protects 75% of active flows from disruption during single-link failures in a 4-spine fabric
• AR validation requires telemetry: NetQ per-port utilization spread <5%, CloudAI benchmark >=46.5 GB/s, WJH showing near-zero RoCE OOO NAKs
• IB AR uses a centralized SM-based approach with credit-based OOO prevention; Spectrum-X AR is distributed with host-side reorder - different tradeoffs suited to different deployment models