diff --git a/include/osmocom/netif/osmux.h b/include/osmocom/netif/osmux.h
index f55fb25..3643406 100644
--- a/include/osmocom/netif/osmux.h
+++ b/include/osmocom/netif/osmux.h
@@ -29,6 +29,7 @@ struct osmux_hdr {
 		ft:3;
 #endif
 	uint8_t seq;
+#define OSMUX_CID_MAX		255	/* determined by circuit_id */
 	uint8_t circuit_id;
 #if __BYTE_ORDER == __BIG_ENDIAN
 	uint8_t amr_ft:4,