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ESUN (Ethernet for Scale-Up Networking)

ESUN (Ethernet for Scale-Up Networking) is an OCP initiative Meta founded in 2025 to advance open specifications for using Ethernet in the scale-up domain — the high-bandwidth, ultra-low-latency interconnect between accelerators within a rack/pod. Its deliberate scope is to prove and standardize that Ethernet can serve scale-up, a domain historically owned by proprietary interconnects.

What ESUN specifies

Working alongside AMD, Arista, ARM, Broadcom, Cisco, HPE, Marvell, Microsoft, and NVIDIA, ESUN advances open specifications for:

  • Protocol headers and framing optimized for scale-up topologies.
  • Error-handling mechanisms for lossless multi-hop fabrics.
  • Alignment with UEC (Ultra Ethernet Consortium) and IEEE standards.

ESUN 1.0 — fabric-scoped by design

The ESUN 1.0 Network Operator Requirements Base Specification (released earlier in 2026) makes two deliberate design choices:

  1. Focuses on the fabric. It defines what the network must deliver while leaving implementation flexibility to GPU and accelerator vendors — a fabric-centric scope that preserves accelerator-side freedom and thereby broadens adoption.
  2. Coordinates with existing standards bodies. It leverages work underway in IEEE and the Ultra Ethernet Consortium (UEC) rather than duplicating it.

ESUN 1.1 — in progress

The group is close to ESUN 1.1, tackling switch profiles and inter-packet-gap reduction that further optimize Ethernet for low-latency, scale-up use cases.

Where it fits

ESUN is the open scale-up data-plane standard that Meta's Tahan scale-up switch line is moving toward: a future Tahan-generation blade (216×400G on 102.4T silicon, 72 accelerators/rack) is planned to adopt ESUN as its scale-up data plane. ESUN is also the open-fabric philosophy that MetaRoCE extends from the fabric into the transport layer — MetaRoCE describes itself as extending "OCP's Ethernet Scalable Unified Network (ESUN) open, multi-vendor philosophy from the fabric into the transport." Together with OSPA (platform layer) and SAI (data-plane control), ESUN is a pillar of the fully-open scale-up stack.

Why it matters

  • Open standard for the last proprietary interconnect. ESUN is the governance vehicle that makes "Ethernet for scale-up" a multi-vendor reality rather than a Meta-only design.
  • Fabric-scoped-not-accelerator-scoped is the adoption trick. By specifying only what the fabric must deliver and leaving the accelerator side free, ESUN avoids forcing GPU vendors into a single implementation — the same broaden-adoption instinct behind SAI.

Caveats

ESUN 1.0 is released; 1.1 is in progress. Tahan's adoption of ESUN as a data plane is a future-chip roadmap item, not shipping.

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