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Tomahawk 4 Switch First to 25.6Tbps

December 17, 2019

Author: Bob Wheeler

Doubling the performance of a leading data-center chip usually requires breaking some limit, and that’s what Broadcom did in developing its Tomahawk 4 switch. The new chip integrates an unprecedented 512 serdes, doubling the port density of its predecessor. Like Tomahawk 3, it uses 50Gbps PAM4 interfaces for compatibility with available 400Gbps Ethernet (400GbE) optical modules. The result is the industry’s first 25.6Tbps switch chip, sampling almost exactly two years after its 12.8Tbps predecessor.

To help achieve this feat, Broadcom was still able to take advantage of Moore’s Law, as Tomahawk 3 employs 16nm technology. By moving to 7nm, it delivered Tomahawk 4 within the power budget required to maintain an air-cooled system design. For maximum efficiency, the new chip also remains monolithic, avoiding the power penalty of chiplet-based architectures. Although it doubles port density, Tomahawk 4 uses the same top-level architecture as the prior generation.

The new chip’s port count and speeds are, as expected, double those of Tomahawk 3: the BCM56990 implements up to 64x400GbE, 128x200GbE, and 256x100GbE ports. The lattermost figure is important to hyperscale-data-center operators, as it represents the maximum radix available for their tiered-network fabrics. As with other 400GbE chips, the serdes handle 25Gbps NRZ for backward compatibility with four-lane 100GbE as well as 25GbE. Owing to its narrow target of hyperscale customers, Broadcom is guarded about feature-enhancement details, but we view them as evolutionary.

Tomahawk 4 isn’t Broadcom’s first 7nm switch chip—that honor goes to Trident 4, which sampled in 2Q19. Although Trident 4 offers half the bandwidth of Tomahawk 4, it uses the same PAM4 serdes core. Reusing that serdes gives the company confidence it can qualify its newest chip for production in 2020. That’s bad news for competitors, only one of which is shipping a 12.8Tbps switch chip. Barring unforeseen design or manufacturing problems, Broadcom is positioned to hold its share at the world’s largest switch customers.

Subscribers can view the full article in the Microprocessor Report.

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