What is the syntax of a display filter that exclusively shows double-tagged frames whose outer tag has VID 129. I want to filter the S-tag. I do not want to filter the C-tag.
I think you can use
ieee8021ad.id == 129, but in case that's not the field you're looking for, the full list of ieee8021ad fields is listed on the Wireshark Display Filter Reference page for IEEE 802.1ad fields here. Note that using these fields requires that you're running at least version 1.0.0 of Wireshark and that the Ethertype is in fact 0x88a8 for IEEE 802.1ad.