Cisco Nexus K35-S FPGA SmartNIC Data Sheet 

Data Sheet

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Updated:July 9, 2021

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Table of Contents



Ultra-low latency network interface card

The Cisco Nexus K35-S FPGA SmartNIC is a 10Gbps PCI Express network interface card specifically optimized for low latency environments.

While initially conceived and built for use in latency-sensitive financial applications such as high-frequency trading, the Cisco Nexus K35-S FPGA SmartNIC has appeal that extends to any environment where latency is key. On an Intel® Ivy Bridge test system, median latency from application to network to application is 780 nanoseconds for small packets, which is significantly better than competing network cards on the same hardware. Half-roundtrip TCP latencies are as low as 930 nanoseconds for small payloads.

Easy to use

In addition to a standard Linux driver, a transparent TCP and UDP acceleration library is included, as well as a library for low-level access.

A transparent socket acceleration library allows applications to benefit from the low latency of a kernel bypass, in most cases without modifications to the applications. For the most latency-sensitive applications, a library called “libexanic” allows direct low-level access to the Cisco Nexus K35-S FPGA SmartNIC hardware and includes simple functions for sending and receiving Ethernet frames. With the optional firmware development kit, it is even possible to extend the Cisco Nexus K35-S FPGA SmartNIC firmware and add your own logic to the onboard FPGA.

Advanced capture and timestamping

Built-in timestamping functionality records each frame’s arrival time to within 6.2ns.

These timestamps are available through a direct API and through Exact Capture, our free and open-source, high-rate capture system. Exact Capture can write tcpdump-compatible captures at line-rates to disk.

Additionally, the Cisco Nexus K35-S FPGA SmartNIC has Pulse-Per-Second (PPS) input and output and supports hardware-accelerated PTP. These can be used to synchronize the ExaNIC clock to external time references (such as a GPS receivers) allowing users to meaningfully compare captured timestamps across multiple servers and geographic locations. Finally, flow steering can be used to deliver packets to the right application’s receive buffer, while flow hashing can be used to distribute packets across multiple CPU cores for demanding capture applications.

Built-in bridging

The Cisco Nexus K35-S FPGA SmartNIC includes integrated support for bridging, which can further reduce latency by hundreds of nanoseconds.

Normally, sharing an upstream connection between multiple servers would necessitate introducing a switch. The bridging feature allows the most latency-critical server to be directly connected to the upstream connection.

Packets not destined for the local server can be bridged to the second port, transparently and with low latency. This port could be connected to a backup server or to a downstream switch.

Cisco Nexus K35-S FPGA SmartNIC

Figure 1.            

Cisco Nexus K35-S FPGA SmartNIC


Typical latency, raw frames:1

      64 bytes: 780 ns

      256 bytes: 1 µs

Typical latency, raw frames with preloaded TX buffer:1

      64 bytes: 710 ns

      256 bytes: 930 ns

Typical latency, UDP:2

      14 bytes: 880 ns

      256 bytes: 1.2 µs

Typical Latency, TCP:2

      14 bytes: 930 ns

      256 bytes: 1.2 µs


Timestamp resolution:


Timestamp availability:

      All received frames, most recent transmitted frame

Time synchronization:

      Host, hardware assisted PTP, optional PPS

PPS input/output:

      3.3V CMOS, selectable 50ohm termination

Other features


      Optional forwarding between ports 1 and 2, latency <110ns


      Line-rate capture to disk

Flow steering:

      128 IP rules per port

      64 MAC rules per port

FPGA Development Kit:

      Add custom user logic to FPGA

      Xilinx Ultrascale XCKU035-2

      Fully integrated with drivers, utilities, and TCP/UDP stack


Form factor:

      Low-profile PCI Express Card

      117x68mm (4.65x2.67in)


      Operating temperature: 0 °C to 55 °C

      Storage temperature: -40 °C to 70 °C

      Operating Relative Humidity: 5% to 90% (non-condensing)

      Storage Relative Humidity: 5% to 95% (non-condensing)


      2x SFP+

      SMA for PPS in/out

Data rates:

      10GbE, 1GbE, 100M Fast Ethernet

Supported media:

      Fiber (10GBASE-SR, 10GBASE-LR, 1000BASE-SX), SFP+ Direct Attach

Host interface:

      PCIe x8 Gen 3 @ 8.0 GT/s per lane

Operating systems:

      Linux x86_64 (all distributions)


1. Latencies are median latencies for raw frames from wire-userspace-wire via the libexanic library, on a 3.5Ghz Intel Ivy Bridge processor.
2. Latencies are median half-roundtrip time latencies for the sockperf benchmark using the exasock socket.

Cisco environmental sustainability

Information about Cisco’s environmental sustainability policies and initiatives for our products, solutions, operations, and extended operations or supply chain is provided in the “Environment Sustainability” section of Cisco’s Corporate Social Responsibility (CSR) Report.

Reference links to information about key environmental sustainability topics (mentioned in the “Environment Sustainability” section of the CSR Report) are provided in the following table:

Sustainability topic


Information on product material content laws and regulations


Information on electronic waste laws and regulations, including products, batteries, and packaging

WEEE compliance

Cisco makes the packaging data available for informational purposes only. It may not reflect the most current legal developments, and Cisco does not represent, warrant, or guarantee that it is complete, accurate, or up to date. This information is subject to change without notice.

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