If you mean 802.1Q, then by looking into the RTCS source code there is some logic implemented around 8021Q priority tags. By default this option is off. Set the RTCSCFG_LINKOPT_8021Q_PRIO to 1 in user_config.h:
#define RTCSCFG_LINKOPT_8021Q_PRIO 1
to have RTCS send and receive Ethernet 802.1Q priority tags.
