Meta’s Backend Aggregation (BAG) connects the building-level network fabrics in its Prometheus AI supercluster. In a technical post published October 19, 2026, Meta said BAG uses Arista 7800-series chassis containing deep-buffered Broadcom Jericho3 silicon, and connects building fabrics across distances from 2 km to more than 100 km. The layer matters because it links networks between buildings, while separate fabrics handle connections within them.
In this article
- What Backend Aggregation does
- How BAG differs from the network inside a building
- The reach and bandwidth Meta describes
What Backend Aggregation does
A network fabric is a coordinated system of switches that connects equipment into a network. Meta describes BAG as a multilayer, hub-and-spoke network joining Prometheus’s building-level fabrics across the cluster. Think of each building fabric as a local network and BAG as the connections that let those separate networks communicate across longer distances.
Meta identifies Arista 7800-series chassis with Broadcom Jericho3 silicon for BAG. That hardware description applies to this aggregation layer. It does not describe every switch in Prometheus.
How BAG differs from the network inside a building
Each building-level Ethernet fabric can connect up to 20,736 GPUs, according to Meta. That maximum belongs to an individual building fabric, not to BAG or to the whole cluster. In its October 19 post, Meta said Prometheus then comprised hundreds of thousands of GPUs across interconnected buildings and structures, with roughly one million GPUs as its full-scale target.
Meta says the initial building-level design used Disaggregated Scheduled Fabric (DSF), then advanced Ethernet network interface cards enabled a move to Non-Scheduled Fabric (NSF). The NSF design uses shallow-buffer 51.2T Ethernet switches, including Minipack3 and Minipack3N. Meta identifies NVIDIA Spectrum-4 switch ASICs in Minipack3N and describes Dynamic Load Balancing and NVIDIA Adaptive Routing on ConnectX network interface cards. Both DSF and NSF use Meta’s FBOSS networking stack.
The reach and bandwidth Meta describes
Meta describes BAG bandwidth as multiple petabits per second per region pair. That is a broad capacity description, not a single exact rate. The stated 2 km to more than 100 km span applies to the connections between building-level fabrics.
For links inside buildings, Meta describes short-reach 2×400G FR4 LITE pluggable optics. For links beyond approximately 10 km, it describes 800G-ZR coherent optics. Meta characterizes the wider network as involving millions of miles of fiber strand, tens of thousands of switches and millions of optical transceivers.
Meta’s October 19 account puts Prometheus’s network in context: BAG ties together the building fabrics, while each building fabric connects its own GPU network.
