diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 221cee3..167d741 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -425,8 +425,14 @@
 
 /* The value of the CID extension is still TBD as of
  * draft-ietf-tls-dtls-connection-id-05
- * (https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-05) */
+ * (https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-05).
+ *
+ * A future minor revision of Mbed TLS may change the default value of
+ * this option to match evolving standards and usage.
+ */
+#if !defined(MBEDTLS_TLS_EXT_CID)
 #define MBEDTLS_TLS_EXT_CID                        254 /* TBD */
+#endif
 
 #define MBEDTLS_TLS_EXT_ECJPAKE_KKPP               256 /* experimental */