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

Define mode for notification of address and device changes. 0 - (default): do nothing 1 - Generate unsolicited neighbour advertisements when device is brought up or hardware address changes.

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