file: /proc/sys/net/ipv6/conf/interface/ndisc_tclass
variable: net.ipv6.conf.interface.ndisc_tclass
Official reference

The IPv6 Traffic Class to use by default when sending IPv6 Neighbor Discovery (Router Solicitation, Router Advertisement, Neighbor Solicitation, Neighbor Advertisement, Redirect) messages. These 8 bits can be interpreted as 6 high order bits holding the DSCP value and 2 low order bits representing ECN (which you probably want to leave cleared). 0 - (default)

Nb: per interface setting (where “interface” is the name of your network interface); “all” is a special interface: changes the settings for all interfaces.