For all EX and QFX switches supported in Mist.

To take a pcap in a switch port in Mist, select the port where the pcap needs to be taken, and select the option "Packet capture":

This will open the packet capture page, and you can configure the number of packets, number of bytes per packet, and the duration in seconds.

On "Add port filter", a new port can be added and an advance filter can be configured.

The option "use expression builder" can be selected, to modify the "TCPDUMP expression"

If no filter needs to be applied, you can click on "start capture" and it will capture all traffic passing through.

The captures can be downloaded on the "captured files" option.