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

Maximum prefix length of Route Information in RA.

Route Information w/ prefix larger than this variable shall be ignored.

Functional default: 0 if accept_ra_rtr_pref is enabled. -1 if accept_ra_rtr_pref is disabled.

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