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Cisco Nexus X40 SmartNIC K35-Q Data Sheet

Data Sheet

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Updated:August 26, 2020

Available Languages

Download Options

  • PDF
    (332.8 KB)
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Updated:August 26, 2020


Ultra-low latency network interface card

High port density

Dual QSFP+ ports in a half-height form factor provide eight 10GbE interfaces.

The Cisco Nexus® X40 SmartNIC K35-Q is a 10/40Gbps interface card specifically optimized for low-latency environments. It can connect to eight SFP+ ports QSFP+ breakout cables and/or Layer 1 switches. This enables a range of high-performance applications such as packet capture across multiple connections, or market data line arbitration across many different feeds.

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 X40 SmartNIC K35-Q hardware and includes simple functions for sending and receiving Ethernet frames. With the optional firmware development kit, it is even possible to extend the SmartNIC firmware and add your own logic to the onboard Field Programmable Gate Array (FPGA).

Advanced capture

Flow steering delivers packets to the right application’s receive buffer.

Filters can be defined over Ethernet frame components such as SRC/DST MAC, SRC/DST IP, etc., and a receive buffer associated with that filter. As packets come off the wire, the Cisco Nexus X40 SmartNIC K35-Q will analyze the traffic and deliver packets that the match filters directly to the correct receive buffer. Nonmatched packets are delivered to the default buffer. This flow steering is done inline at line rate, adding no additional latency. Flow hashing distributes packets evenly across multiple buffers, allowing CPU load to be spread for demanding capture and analysis applications.


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-rate to disk.

Additionally, the Cisco Nexus X40 SmartNIC K35-Q features Pulse-Per-Second (PPS) input and output and supports hardware accelerated PTP. These can be used to synchronize the SmartNIC clock to external time references (such as a GPS receivers), allowing users to meaningfully compare captured timestamps across multiple servers and geographic locations.

Nexus X40 SmartNIC K35-Q

Figure 1.            

Cisco Nexus X40 SmartNIC K35-Q


Typical latency, raw frames:1

      64 bytes: 800 ns

      256 bytes: 1.02 µs

Typical latency, raw frames with preloaded TX buffer:1

      64 bytes: 730 ns

      256 bytes: 950 ns

Typical latency, UDP:2

      14 bytes: 900 ns

      256 bytes: 1.22 µs

Typical latency, TCP:2

      14 bytes: 950 ns

      256 bytes: 1.22 µs


Timestamp resolution:


Timestamp availability:

      All received frames, most recently transmitted frame

Time synchronization:

      Host, hardware assisted PTP, optional PPS

PPS input/output:

      3.3V CMOS, selectable 50ohm termination


Form factor:

      Low-profile PCI Express Card

      150x68mm (5.9x2.67in)


      2 QSFP+

      SMA for PPS in/out

Data rates:

      40GbE, 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)


Other features

Flow steering:

      128 IP rules per port

      64 MAC rules per port


      Line-rate capture to disk

FPGA Development Kit:

      Add custom user logic to FPGA

      Xilinx UltraScale XCKU035-2

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


1Latencies are median latencies for raw frames from wire-userspace-wire via the libexanic library, on a 3.5Ghz Intel® Ivy Bridge processor.
2Latencies are median half round trip time latencies for the sockperf benchmark using the exasock socket Acceleration Library. More information about benchmarking methodology is available on request.

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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:

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