"The information contained in Router Advertisements may change through
the actions of system management. For instance, the lifetime or
preference of advertised routes may change, or new routes could be
added. In such cases, the router MAY transmit up to
MAX_INITIAL_RTR_ADVERTISEMENTS unsolicited advertisements, using the
same rules as in [RFC2461]."
RFC4191 duplicates RFC4861:
"The information contained in Router Advertisements may change through the actions of system management. For instance, the lifetime or preference of advertised routes may change, or new routes could be added. In such cases, the router MAY transmit up to MAX_INITIAL_RTR_ADVERTISEMENTS unsolicited advertisements, using the same rules as in [RFC2461]."