With this configuration, VLAN tagging is preserved.
set interfaces xe-0/0/19 description "SPAN-SOURCE" set interfaces xe-0/0/19 unit 0 family ethernet-switching interface-mode trunk set interfaces xe-0/0/19 unit 0 family ethernet-switching vlan members PROD set interfaces xe-0/0/19 unit 0 family ethernet-switching vlan members DEV set interfaces xe-0/1/2:3 description "SPAN-OUTPUT" set interfaces xe-0/1/2:3 unit 0 family ethernet-switching set forwarding-options analyzer SPAN input ingress interface xe-0/0/19.0 set forwarding-options analyzer SPAN output interface xe-0/1/2:3.0