Packet Buffer Reference

Broadcom Tomahawk 5 Architecture

The Broadcom Tomahawk 5 (BCM78900 / BCM78902) is the fifth generation of Broadcom's Tomahawk merchant silicon line, targeting 800G-era hyperscale spine and AI/ML fabric deployments. It represents the shift from 400G QSFP-DD to 800G OSFP as the dominant high-density port format.

Variants

Chip Capacity Ports Buffer Process
BCM78900 51.2 Tbps 64x 800G (OSFP) 165.2 MB TSMC N5
BCM78902 25.6 Tbps 32x 800G (OSFP) ~83 MB (inferred) TSMC N5

The BCM78902 is the half-bandwidth Tomahawk 5 variant. Its buffer capacity has not been confirmed from an official Broadcom datasheet; the ~83 MB figure is inferred from the established proportional relationship between the full and half-bandwidth variants in prior generations (BCM56990 = 113.66 MB; BCM56993 = exactly 56.83 MB).

Buffer Architecture

Tomahawk 5 continues the fully unified shared-buffer model introduced with Tomahawk 2. All 165.2 MB of on-chip SRAM in the BCM78900 is accessible as a single pool across all 64 800G ports simultaneously. There is no per-port or per-pipeline partitioning — the Memory Management Unit (MMU) dynamically allocates the entire pool based on instantaneous demand using a WRED/dynamic thresholding scheme.

The 165.2 MB on-chip figure represents approximately a 46% increase over the Tomahawk 4's 113.66 MB, enabled by process node advancement from TSMC 7 nm (N7HPC) to TSMC 5 nm (N5). No HBM or external DRAM is used — the entire buffer is embedded on-die.

Comparison to prior Tomahawk generations

Generation ASIC Capacity Buffer Process
Tomahawk BCM56960 6.4 Tbps 16 MB (4 × 4 MB, isolated pipes) 16 nm
Tomahawk 2 BCM56970 6.4 Tbps 42 MB (unified) 16 nm
Tomahawk 3 BCM56980 12.8 Tbps 64 MB (unified) 7 nm
Tomahawk 4 BCM56990 25.6 Tbps 113.66 MB (unified) 7 nm
Tomahawk 5 BCM78900 51.2 Tbps 165.2 MB (unified) 5 nm

Port Architecture

The Tomahawk 5 introduces OSFP (Octal Small Form-factor Pluggable) as the primary 800G port format, replacing QSFP-DD from Tomahawk 4. OSFP supports 800G via:

This enables single-port 800G connectivity without external gearboxes when paired with 800G SR8/DR8/FR8 or 2x 400G optics.

Queue Model

Like Tomahawk 4, Tomahawk 5 provides 8 traffic classes (queues) per port with MMU-controlled dynamic thresholding. The unified pool architecture means a port with 8 active queues can draw from the full 165.2 MB if all other ports are idle, providing maximum burst absorption capacity.

Forwarding Capabilities

Tomahawk 5 extends the routing capabilities introduced in Tomahawk 4 (which added L3 IP unicast/multicast), adding enhanced support for:

Known Deployments

References

See Also