Global Load Balancing: Fabric-Wide Adaptive Routing
DLB makes the right call at one hop; GLB makes the right call across the whole fabric. On the OcNOS 7.1 roadmap, Global Load Balancing extends adaptive routing from a per-port view to end-to-end path quality, closing the multi-hop hot-spot gap on 3-stage Clos AI fabrics up to the 16,384-GPU ceiling. GLB layers on top of DLB rather than replacing it.
Scoring the whole path, not just the local hop
A 3-stage Clos slice (leaf, spine, super-spine) carrying GPU AllReduce. Every tier streams queue-occupancy and link-utilisation telemetry back toward the ingress leaves. GLB picks the path with the best end-to-end score, not the best local-egress score, so a clean uplink that lands on a congested downlink is no longer chosen blindly.
DLB and GLB, by scope of the path decision
DLB scores each ECMP next-hop using local egress queue-depth, which is optimal on a 2-tier leaf-spine. Scale to a 3-tier Clos and you can pick a spine with a clean uplink, only to land on a super-spine whose downlink back to the egress leaf is congested. The local view is correct; the end-to-end view is wrong. At 1,024-GPU and larger fabrics, where 3-stage Clos with super-spines becomes standard, this is the dominant remaining source of tail-latency outliers.
| Axis | DLBlocal, shipping today | GLBglobal, OcNOS 7.1 roadmap |
|---|---|---|
| Scope of decision | Per-hop: each switch ranks its own ECMP next-hops. | End-to-end: ingress leaves rank complete leaf-to-leaf paths. |
| Signal used | Local egress queue-depth and link-utilisation on this switch. | Congestion telemetry aggregated from every tier, back to ingress. |
| Best fit | 2-stage fabrics and the leaf-to-spine hop in 3-stage. | 3-stage Clos with super-spines, up to the 16,384-GPU ceiling. |
| Hot-spot caught | Local egress congestion only. | Downstream hot-spots the local hop cannot see. |
| Rebinding | Flowlet-aligned, in-order for RoCEv2 and TCP. | Flowlet-aligned, same in-order guarantee, full-fabric input. |
| Hardware | TH4 and TH5 today. | Same TH4 and TH5 hardware, no new silicon. |
| Relationship | Independent decision at each switch. | Layers on top of DLB; does not replace it. |
One decision, informed by the whole fabric
GLB reuses the adaptive-routing machinery operators already run and adds fabric-wide awareness on top. It builds directly on DLB and RoCEv2, and stays aligned with where Ultra Ethernet is heading.
The DLB decision
GLB extends the DLB flowlet decision rather than replacing it. Mixed fabrics work correctly: non-GLB switches simply contribute local-only path quality during a rolling upgrade.
A lossless RoCEv2 fabric
GLB rebinds at flowlet boundaries, preserving the in-order delivery that RoCEv2 RDMA needs. The transport stays production-grade; only the path input gets smarter.
Ultra Ethernet signalling
The path-quality plane is being designed to interoperate with Ultra Ethernet Consortium signalling as UEC NIC ecosystems mature, so the roadmap stays forward-compatible.
How the planned GLB implementation fits together
GLB is enabled by OcNOS as the network operating system on open Broadcom-based hardware. The design keeps the control plane quiet and gives operators the telemetry they need to trust the fabric's decisions.
Path-quality publish
Every spine and super-spine publishes per-port queue-occupancy and utilisation deltas to a fabric-wide adjacency. Updates are sub-millisecond over existing in-band signalling, with no extra control-plane chatter.
End-to-end aggregation
Ingress leaves combine local egress quality with downstream telemetry into an aggregate score per candidate path. The worst hop dominates the score, the same intuition operators use when troubleshooting.
Flowlet-aligned
Like DLB, GLB rebinds at flowlet boundaries, preserving in-order delivery for RoCEv2 and TCP. The difference is what feeds the decision: full-fabric quality, not local-port quality.
Layered on DLB
GLB extends the DLB decision; it does not replace it. Mixed fabrics with GLB-capable and DLB-only switches behave correctly, so brownfield upgrades from 7.0 are safe.
Up to the 16k-GPU ceiling
Reference designs use 256 spine switches and 128 super-spine switches, each a 64x800G Tomahawk 5, sized to the 16,384-GPU architectural ceiling.
gNMI for the ops team
Per-path scores, rebind events, and worst-hop attribution stream over gNMI and OpenConfig, so SREs can correlate fabric decisions with xCCL collective job behaviour without a black box.
What to expect when GLB lands
GLB is on the OcNOS 7.1 roadmap. It targets the same hardware and licensing operators already run, so adopting it is an upgrade rather than a forklift.
- OcNOS 7.1 roadmap. GLB targets the 7.1 OcNOS-DC train, on the same TH4 and TH5 hardware running DLB today. Schedule and feature scope at the OcNOS releases page.
- Same SKU. Planned for OcNOS-DC PLUS: no per-feature paywall, no new license keys at upgrade time.
- In-place upgrade. Brownfield upgrade from 7.0 to 7.1 is supported; mixed-version fabrics keep working with DLB-only behaviour during the upgrade window.
- UEC-aligned. The path-quality plane is being designed to interoperate with Ultra Ethernet Consortium signalling once UEC NIC ecosystems mature. See Ultra Ethernet (UEC).
- Architecture review available. If you are sizing a 1,000-plus GPU fabric, we will run a sizing exercise that includes the GLB telemetry plane. Get a first-pass leaf-spine layout with the AI fabric design suite.
DLB is the floor today, GLB raises the ceiling next
Adaptive routing is already solving real tail-latency at 2-tier scale. GLB carries that same discipline up into the 3-stage fabrics where the largest clusters live, on open hardware you already validate.
DLB solves the common case
On 2-tier leaf-spine and the leaf-to-spine hop, local adaptive routing already removes most ECMP collisions. That is shipping on TH4 and TH5 today.
GLB solves the scale case
At 1,024 GPUs and up, multi-hop hot-spots become the dominant outlier. GLB scores complete paths so the ingress leaf stops chasing a clean uplink into a congested downlink.
OcNOS is the enabler
One NOS, one feature roadmap: DLB today, GLB next, RoCEv2 and UEC-aligned throughout, on validated open hardware rather than a single-vendor fabric.
Global Load Balancing, answered
How is GLB different from DLB?
When is GLB available?
Do I have to replace DLB to use GLB?
How large a fabric does GLB support?
Go deeper. Take it with you.
The product datasheet plus short, technical downloads that go further than this page.
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.
Get the datasheetOcNOS 800G Lossless AI Fabric
Non-blocking RoCEv2 fabric on Broadcom Tomahawk 4/5 spines: SKU tiers, validated platforms, and deployment architecture.
Get the briefEVPN-VXLAN Data Center Fabric
Carrier-grade leaf-spine data center fabric: symmetric IRB, Type-2/Type-5 routes, and distributed anycast gateway.
Get the briefOcNOS-DC Datasheet
Quick form. Your PDF opens in a new tab immediately after submit.
✓ Opening your PDF in a new tab…
If it didn't open, use the link below.
OcNOS 800G Lossless AI Fabric
Quick form. Your PDF opens in a new tab immediately after submit.
✓ Opening your PDF in a new tab…
If it didn't open, use the link below.
EVPN-VXLAN Data Center Fabric
Quick form. Your PDF opens in a new tab immediately after submit.
✓ Opening your PDF in a new tab…
If it didn't open, use the link below.
Sizing a multi-thousand-GPU fabric? Let's run the numbers together
Tell us the workload and the GPU scale, and an IP Infusion engineer will size the GLB telemetry plane with you, or start with a first-pass leaf-spine layout in the AI fabric design suite.
Design the whole AI fabric with OcNOS
From the business case to the port-count maths, pick up wherever you are in the build.