Index: head/sys/netinet6/nd6.h
===================================================================
--- head/sys/netinet6/nd6.h
+++ head/sys/netinet6/nd6.h
@@ -476,6 +476,7 @@
 /* nd6_rtr.c */
 void nd6_rs_input(struct mbuf *, int, int);
 void nd6_ra_input(struct mbuf *, int, int);
+void nd6_ifnet_link_event(void *, struct ifnet *, int);
 void defrouter_reset(void);
 void defrouter_select_fib(int fibnum);
 void defrouter_select(void);
Index: head/sys/netinet6/nd6.c
===================================================================
--- head/sys/netinet6/nd6.c
+++ head/sys/netinet6/nd6.c
@@ -113,7 +113,7 @@
 VNET_DEFINE(int, nd6_debug) = 0;
 #endif
 
-static eventhandler_tag lle_event_eh, iflladdr_event_eh;
+static eventhandler_tag lle_event_eh, iflladdr_event_eh, ifnet_link_event_eh;
 
 VNET_DEFINE(struct nd_drhead, nd_defrouter);
 VNET_DEFINE(struct nd_prhead, nd_prefix);
@@ -233,6 +233,8 @@
 		    NULL, EVENTHANDLER_PRI_ANY);
 		iflladdr_event_eh = EVENTHANDLER_REGISTER(iflladdr_event,
 		    nd6_iflladdr, NULL, EVENTHANDLER_PRI_ANY);
+		ifnet_link_event_eh = EVENTHANDLER_REGISTER(ifnet_link_event,
+		    nd6_ifnet_link_event, NULL, EVENTHANDLER_PRI_ANY);
 	}
 }
 
@@ -244,6 +246,7 @@
 	callout_drain(&V_nd6_slowtimo_ch);
 	callout_drain(&V_nd6_timer_ch);
 	if (IS_DEFAULT_VNET(curvnet)) {
+		EVENTHANDLER_DEREGISTER(ifnet_link_event, ifnet_link_event_eh);
 		EVENTHANDLER_DEREGISTER(lle_event, lle_event_eh);
 		EVENTHANDLER_DEREGISTER(iflladdr_event, iflladdr_event_eh);
 	}
Index: head/sys/netinet6/nd6_rtr.c
===================================================================
--- head/sys/netinet6/nd6_rtr.c
+++ head/sys/netinet6/nd6_rtr.c
@@ -285,7 +285,32 @@
 	/* Send notification of flag change. */
 #endif
 }
+
+static void
+defrtr_ipv6_only_ipf_down(struct ifnet *ifp)
+{
+
+	IF_AFDATA_WLOCK(ifp);
+	ND_IFINFO(ifp)->flags &= ~ND6_IFF_IPV6_ONLY;
+	IF_AFDATA_WUNLOCK(ifp);
+}
 #endif	/* EXPERIMENTAL */
+
+void
+nd6_ifnet_link_event(void *arg __unused, struct ifnet *ifp, int linkstate)
+{
+
+	/*
+	 * XXX-BZ we might want to trigger re-evaluation of our default router
+	 * availability. E.g., on link down the default router might be
+	 * unreachable but a different interface might still have connectivity.
+	 */
+
+#ifdef EXPERIMENTAL
+	if (linkstate == LINK_STATE_DOWN)
+		defrtr_ipv6_only_ipf_down(ifp);
+#endif
+}
 
 /*
  * Receive Router Advertisement Message.