How to set-up capture filter by DSCP value

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And see "What if you need to use DSCP in a capture filter?" on that page.

You might want to request that the libpcap filter language (which is the language of Wireshark capture filters) have primitives added to allow filtering on DSCP values etc. You would do this by reporting an issue on the libpcap issue list (do not file it as a Wireshark issue, as the capture filter syntax is defined by libpcap, not by Wireshark).