If this object is set to true, the reachability of the next-hops of
routes in this address-family are checked. If this object is set to
false, the next-hops if routes in this address-families are always
considered to be reachable. This attribute only applies to address-
families with SAFI 128 (labeled-vpn-unicast). For all other address-
families this attributes is meaningless and attempting to set the
attribute will result in an error.