file: /proc/sys/net/ipv4/fib_multipath_use_neigh
variable: net.ipv4.fib_multipath_use_neigh
Official reference

Use status of existing neighbor entry when determining nexthop for multipath routes. If disabled, neighbor information is not used and packets could be directed to a failed nexthop. Only valid for kernels built with CONFIG_IP_ROUTE_MULTIPATH enabled. Default: 0 (disabled) Possible values: 0 - disabled 1 - enabled