Index: sys/netinet/in.c
===================================================================
--- sys/netinet/in.c
+++ sys/netinet/in.c
@@ -78,6 +78,8 @@
 static void	in_socktrim(struct sockaddr_in *);
 static void	in_purgemaddrs(struct ifnet *);
 
+static void	in_rtrequest(int req, struct rtentry *rt, struct rt_addrinfo *info);
+
 static VNET_DEFINE(int, nosameprefix);
 #define	V_nosameprefix			VNET(nosameprefix)
 SYSCTL_INT(_net_inet_ip, OID_AUTO, no_same_prefix, CTLFLAG_VNET | CTLFLAG_RW,
@@ -401,6 +403,7 @@
 	ifa->ifa_netmask = (struct sockaddr *)&ia->ia_sockmask;
 	callout_init_rw(&ia->ia_garp_timer, &ifp->if_addr_lock,
 	    CALLOUT_RETURNUNLOCKED);
+	ifa->ifa_rtrequest = in_rtrequest;
 
 	ia->ia_ifp = ifp;
 	ia->ia_addr = *addr;
@@ -1497,3 +1500,56 @@
 	lltable_free(ii->ii_llt);
 	free(ii, M_IFADDR);
 }
+
+/*
+ * Return true if the route described by info is the local subnet route for
+ * ifa, or false otherwise.
+ */
+static int
+in_is_subnet_route(struct ifaddr *ifa, struct rt_addrinfo *info)
+{
+	struct sockaddr *tmp_sa;
+	struct sockaddr_storage tmp_storage;
+
+	if (info->rti_info[RTAX_DST] == NULL || info->rti_info[RTAX_NETMASK] == NULL)
+		return (0);
+
+	/* First test that the ifaddr falls into the subnet described by the route. */
+	tmp_sa = (struct sockaddr*)&tmp_storage;
+	rt_maskedcopy(ifa->ifa_addr, tmp_sa, ifa->ifa_netmask);
+
+	if (!sa_equal(tmp_sa, info->rti_info[RTAX_DST]))
+		return (0);
+
+	/*
+	 * Now test that the subnet mask is the same for the address and
+	 * the route.  If they are different then the route has a different
+	 * prefix size and ifa just happens to fall within that route.
+	 *
+	 * For some reason ifa->ifa_netmask sa_len field is not
+	 * sizeof(struct sockaddr_in).  This causes attempts to directly
+	 * compare against info->rti_info[RTAX_NETMASK] to incorrectly
+	 * fail even if the netmasks are the same.  Fix this by making a
+	 * copy of ifa->ifa_netmask with the length field correct.
+	 */
+	bzero(tmp_sa, sizeof(struct sockaddr_in));
+	memcpy(tmp_sa, ifa->ifa_netmask, ifa->ifa_netmask->sa_len);
+	tmp_sa->sa_len = sizeof(struct sockaddr_in);
+
+	return (sa_equal(info->rti_info[RTAX_NETMASK], tmp_sa));
+}
+
+static void
+in_rtrequest(int req, struct rtentry *rt, struct rt_addrinfo *info)
+{
+
+	/*
+	 * If we are modifying a route to a subnet, we need to migrate the IFA_ROUTE
+	 * flag to the new source ifa for the route.
+	 */
+	if (req == RTM_DELETE && rt->rt_ifa->ifa_flags & IFA_ROUTE &&
+	    info->rti_ifa != NULL && in_is_subnet_route(rt->rt_ifa, info)) {
+		rt->rt_ifa->ifa_flags &= ~IFA_ROUTE;
+		info->rti_ifa->ifa_flags |= IFA_ROUTE;
+	}
+}
Index: tests/sys/netinet/fibs_test.sh
===================================================================
--- tests/sys/netinet/fibs_test.sh
+++ tests/sys/netinet/fibs_test.sh
@@ -720,6 +720,83 @@
 	cleanup_ifaces
 }
 
+atf_test_case ipv4_move_subnet_route cleanup
+ipv4_move_subnet_route_head()
+{
+	atf_set "descr" "moving a subnet route to different ifa should be possible"
+	atf_set "require.user" "root"
+	atf_set "require.config" "fibs"
+}
+
+ipv4_move_subnet_route_body()
+{
+	# Configure the TAP interfaces to use a RFC5737 nonrouteable addresses
+	# and a non-default fib
+	SUBNET_PREFIX="192.0.2"
+	SUBNET="${SUBNET_PREFIX}.0"
+	ADDR0="${SUBNET_PREFIX}.1"
+	ADDR1="${SUBNET_PREFIX}.2"
+
+	MASK="24"
+
+	# Check system configuration
+	if [ 0 != `sysctl -n net.add_addr_allfibs` ]; then
+		atf_skip "This test requires net.add_addr_allfibs=0"
+	fi
+	get_fibs 1
+
+	get_epair
+	setup_iface "$EPAIRA" "$FIB0" inet ${ADDR0} $MASK
+	setup_iface "$EPAIRB" "$FIB0" inet ${ADDR1} $MASK
+
+	setfib $FIB0 route change ${SUBNET}/${MASK} -ifp "$EPAIRB"
+	ifconfig "$EPAIRA" inet ${ADDR0}/${MASK} fib "$FIB0" -alias
+	atf_check -s exit:0 ifconfig "$EPAIRB" inet ${ADDR0}/$MASK alias fib "$FIB0"
+}
+
+ipv4_move_subnet_route_cleanup()
+{
+	cleanup_ifaces
+}
+
+atf_test_case ipv6_move_subnet_route cleanup
+ipv6_move_subnet_route_head()
+{
+	atf_set "descr" "moving a subnet route to different ifa should be possible"
+	atf_set "require.user" "root"
+	atf_set "require.config" "fibs"
+}
+
+ipv6_move_subnet_route_body()
+{
+	# Configure the TAP interfaces to use a RFC5737 nonrouteable addresses
+	# and a non-default fib
+	SUBNET_PREFIX="2001:db8:"
+	SUBNET="${SUBNET_PREFIX}:0"
+	ADDR0="${SUBNET_PREFIX}:1"
+	ADDR1="${SUBNET_PREFIX}:2"
+
+	MASK="64"
+
+	# Check system configuration
+	if [ 0 != `sysctl -n net.add_addr_allfibs` ]; then
+		atf_skip "This test requires net.add_addr_allfibs=0"
+	fi
+	get_fibs 1
+
+	get_epair
+	setup_iface "$EPAIRA" "$FIB0" inet6 ${ADDR0} $MASK
+	setup_iface "$EPAIRB" "$FIB0" inet6 ${ADDR1} $MASK
+
+	setfib $FIB0 route -6 change ${SUBNET}/${MASK} -ifp "$EPAIRB"
+	ifconfig "$EPAIRA" inet6 ${ADDR0}/${MASK} fib "$FIB0" -alias
+	atf_check -s exit:0 setfib 1 ifconfig "$EPAIRB" inet6 ${ADDR0}/$MASK fib "$FIB0" alias
+}
+
+ipv6_move_subnet_route_cleanup()
+{
+	cleanup_ifaces
+}
 
 atf_init_test_cases()
 {
@@ -736,6 +813,8 @@
 	atf_add_test_case subnet_route_with_multiple_fibs_on_same_subnet_inet6
 	atf_add_test_case udp_dontroute
 	atf_add_test_case udp_dontroute6
+	atf_add_test_case ipv4_move_subnet_route
+	atf_add_test_case ipv6_move_subnet_route
 }
 
 # Looks up one or more fibs from the configuration data and validates them.