Your syntax is not correct, try this
CREATE TABLE `t2` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`ext_id` varchar(128) COLLATE utf8_unicode_ci NOT NULL,
PRIMARY KEY (`id`),
KEY `ext_id` (`ext_id`),
CONSTRAINT `t2_ibfk_1` FOREIGN KEY (`ext_id`) REFERENCES `d1`.`t1` (`int_id`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;