Dragonfly interconnect
A dragonfly stacks two fabrics: routers form a full mesh inside each group, and every group connects directly to every other group with at least one global link. Aurora, Frontier, and most post-2020 supercomputers use this shape. Drag to rotate; the cyan path traces a minimal 3-hop route between two routers in different groups.
Why a dragonfly?
Two properties matter for an HPC fabric: diameter (worst-case hop count) and cost (number of cables, switches, optics). A dragonfly hits a sweet spot: every pair of routers is reachable in three hops regardless of how many groups you bolt on, and the global cabling scales as O(G²) rather than the O(N²) of a flat all-to-all.
The trick is the hierarchy. Inside a group, full mesh — short electrical links, cheap. Between groups, one (or a few) long optical links per pair. Long links are the expensive ones, so minimising them while preserving 3-hop diameter is the design win.
The 3-hop minimal route
For source (g₀, r_src) reaching destination (g₁, r_dst) in a different group:
- Intra-group hop from r_src to the router in g₀ that holds the global link to g₁.
- Global hop across that link into g₁.
- Intra-group hop to r_dst.
Adversarial traffic patterns (everyone in g₀ hammering the same global link) can collapse this; production deployments use UGAL (Universal Globally Adaptive Load-balanced) routing that occasionally takes a non-minimal 5-hop detour through a third group to spread load.
Where it shows up
HPE Slingshot is the canonical commercial dragonfly today — Aurora (Argonne), Frontier (ORNL), and most of the post-2020 Top500 are wired this way. NVIDIA's Quantum InfiniBand also supports dragonfly configurations at large scale. The torus (see the 3D torus explorer) is the other common shape, favoured by Google's TPU pods because its bandwidth profile better matches all-reduce-heavy training workloads.
Rule of thumb: if a system is doing mostly all-reduce on dense matmuls, torus wins. If it is doing irregular MPI traffic with mixed point-to-point and collectives, dragonfly wins.