--- a/sys/netgraph/ng_sample.c	2026-09-14 20:30:13.466637150 +0000
+++ b/sys/netgraph/ng_sample.c	2026-09-14 20:30:22.586522136 +0000
@@ -233,6 +233,7 @@
 	} else if (strcmp(name, NG_XXX_HOOK_DOWNSTREAM) == 0) {
 		/* Example of simple predefined hooks. */
 		/* do something specific to the downstream connection */
+		xxxp->downstream_hook.dlci = -1;	/* sentinel: data from mux link */
 		xxxp->downstream_hook.hook = hook;
 		hook->private = &xxxp->downstream_hook;
 	} else if (strcmp(name, NG_XXX_HOOK_DEBUG) == 0) {