Dynamic Load Balancing: Adaptive Routing for AI Fabrics
On an AI fabric, use adaptive Dynamic Load Balancing rather than static-hash ECMP. DLB re-evaluates the path on every flowlet, a sub-flow chunk delimited by an inter-packet gap, using live ASIC egress queue-depth and port-utilisation telemetry, so GPU collectives stop colliding on a single uplink. It runs in the switch ASIC on Broadcom Tomahawk 4 and 5, with no NIC or collective-library changes.
Adaptive routing on a leaf-spine fabric
A four-spine, two-leaf slice carrying GPU AllReduce traffic. DLB measures local egress queue depth in real time. When Spine-3 saturates, the leaf re-bins the next flowlet onto a less-loaded spine, keeping all four uplinks balanced.
Static ECMP and DLB, axis by axis
Standard ECMP hashes the 5-tuple at flow start and pins the flow for its lifetime. On north-south web traffic, millions of short-lived flows smooth utilisation across paths. On an AI fabric a handful of GPU elephant flows, each holding a whole 400G or 800G uplink for seconds, cause hash collisions: some uplinks saturate while parallel paths sit idle. DLB re-evaluates the path on every flowlet using live ASIC telemetry.
| Axis | Static ECMPhash and pin | DLBadaptive, per-flowlet |
|---|---|---|
| Path decision | Hashes the 5-tuple once at flow start and pins the flow for its entire lifetime. | Re-evaluates the path on every flowlet boundary and moves traffic to a less-congested uplink. |
| Granularity | Whole flow. A single hash choice governs seconds of elephant-flow traffic. | Flowlet: a sub-flow chunk delimited by a small inter-packet gap, safe to re-bind without reordering. |
| Congestion signal | None. The hash is blind to real-time link state. | Live ASIC egress queue-depth and port-utilisation telemetry. |
| Under GPU elephant flows | Hash collisions: two elephants can land on one uplink while another sits idle. | Saturated members drop out of the candidate set, so uplinks stay balanced. |
| Reordering risk | None; the flow is pinned, but at the cost of hot-spots and tail-latency outliers. | None in practice; the inter-packet gap means RoCEv2 and TCP see correct in-order delivery. |
| NIC / collective library | No changes; standard behaviour. | No changes. Runs in the switch ASIC, so RoCEv2 NICs and xCCL stacks are untouched. |
| Hardware | Any ECMP-capable switch. | Broadcom Tomahawk 4 (25.6T) and Tomahawk 5 (51.2T) in 64x400G and 64x800G configurations. |
The OcNOS DLB implementation
DLB is not a control-plane bolt-on. It runs in the Broadcom forwarding pipeline, scoring every ECMP next-hop from live congestion signals and re-binding flowlets in hardware.
Flowlet gap timer
An ASIC-native flowlet inactivity timer splits long elephant flows into chunks safe to spray across paths without TCP or RoCEv2 reordering.
Live queue-depth feedback
DLB consumes per-egress port queue-occupancy and link-utilisation signals from the Tomahawk pipeline to score every ECMP next-hop in real time.
Adaptive next-hop selection
On each flowlet boundary the highest-quality member is selected, so a saturated spine drops out of the candidate set.
TH4 / TH5 native
Validated on Broadcom Tomahawk 4 (25.6T) and Tomahawk 5 (51.2T) spine platforms, in 64x400G and 64x800G port configurations.
Where DLB sits in the fabric
DLB is the single-hop, congestion-aware layer of a larger open Ethernet transport. It rides on lossless RoCEv2, and fabric-wide GLB, coming in OcNOS 7.1, extends it to end-to-end scoring.
Lossless transport underneath
DLB rebinds flowlets on top of a RoCEv2 lossless fabric (PFC, ECN, DCQCN).
Fabric-wide scoring next
Where DLB scores the local next-hop, GLB (OcNOS 7.1) extends the decision to end-to-end path quality across the whole leaf-spine fabric.
The same idea in the NIC
Ultra Ethernet takes path spreading into the NIC: packet spray and multi-path RDMA spread one flow across many paths from UEC-capable NICs.
What DLB delivers in production AI fabrics
By keeping flowlets off the congested uplinks that static ECMP overloads, DLB turns a symmetric leaf-spine fabric into one that stays balanced under GPU collective traffic.
- Higher utilisation. Flowlet rebalancing keeps traffic off the congested uplinks that static ECMP overloads, so a well-run fabric can target utilisation above 90 percent on the same hardware, without buying more uplinks.
- Lower tail latency. Collective completion time tightens because no single link saturates while others sit idle.
- No NIC changes. DLB lives in the switch ASIC. Existing RoCEv2 NICs and xCCL (NCCL, RCCL, oneCCL) collective stacks see correct in-order delivery without code changes.
- PLUS tier. DLB is included in the OcNOS-DC PLUS SKUs, with the same image and support contract as the rest of OcNOS-DC.
Adaptive routing is table stakes for an Ethernet AI fabric
Static ECMP was built for north-south web traffic. Once a fabric carries GPU collectives, adaptive routing is the difference between balanced uplinks and stalled training jobs, and OcNOS delivers it on open hardware today.
ECMP alone is not enough
A handful of elephant flows defeat static hashing. Hash collisions leave some uplinks saturated while parallel paths sit idle.
DLB closes the routing gap
Per-flowlet, congestion-aware path selection keeps an Ethernet fabric balanced under collective traffic, one of the axes where InfiniBand had an edge.
OcNOS is the enabler
DLB today, GLB in OcNOS 7.1. One NOS on validated open hardware from vendors such as Edgecore and UfiSpace, with no NIC or collective-library changes.
Dynamic Load Balancing, answered
What is a flowlet and why does DLB use it?
How is DLB different from static ECMP?
Does DLB require new NICs or changes to my collective library?
Which hardware supports OcNOS DLB?
Go deeper. Take it with you.
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OcNOS-DC Datasheet
Full OcNOS-DC specification: the EVPN-VXLAN and Ethernet for AI feature set, software SKUs, supported hardware platforms, and the solution ordering guide.
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Non-blocking RoCEv2 fabric on Broadcom Tomahawk 4/5 spines: SKU tiers, validated platforms, and deployment architecture.
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Carrier-grade leaf-spine data center fabric: symmetric IRB, Type-2/Type-5 routes, and distributed anycast gateway.
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