Nexus 9k Packet Capture ((free)) Today

N9K# configure terminal N9K(config)# monitor capture cap1 interface ethernet 1/1 both # 'both' is actually ingress only N9K(config)# monitor capture cap1 match ipv4 protocol tcp dst-port 80 N9K(config)# monitor capture cap1 buffer-size 50 N9K(config)# monitor capture cap1 limit duration 60 N9K(config)# monitor capture cap1 start N9K# show monitor capture cap1 buffer N9K# monitor capture cap1 export bootflash:capture.pcap

This allows for "Smart SPAN" or "Streaming Telemetry." Instead of dumping all traffic, the ASIC can filter on-chip and only send metadata or headers to the CPU for analysis, allowing you to monitor 100Gbps links without melting the control plane.

: SPAN sessions on the Nexus 9000 often cannot capture broadcast/multicast packets (like ARP or OSPF Hellos) if the source is the supervisor in-band interface; use physical interfaces as sources instead. 3. Integrated Tool: SPAN-to-CPU nexus 9k packet capture

: It cannot capture hardware-switched data plane traffic unless that traffic is specifically punted to the CPU.

Cisco wrapped the functionality of tcpdump into a native NX-OS command called ethanalyzer . Choosing the right tool depends on whether you

Packet capture on the Cisco Nexus 9000 series is divided into two main categories: traffic (destined to/from the CPU) and data plane traffic (forwarded in hardware) . Choosing the right tool depends on whether you need to analyze routing protocols, verify ASIC-level forwarding decisions, or monitor real-time data flows. 1. Control Plane Capture: Ethanalyzer

: Includes a decode-internal option to troubleshoot how the CPU is processing specific packets. verify ASIC-level forwarding decisions

Ethanalyzer is a built-in Cisco Nexus tool based on Wireshark that captures and decodes packets sent to or generated by the switch's CPU.