diff -Nur a/system-dummy.c netifd-2019-08-05-5e02f944/system-dummy.c
--- a/system-dummy.c	2020-05-04 10:18:55.245933547 -0700
+++ netifd-2019-08-05-5e02f944/system-dummy.c	2020-05-04 10:19:21.505807825 -0700
@@ -130,6 +130,12 @@
 }
 
 struct device *
+system_if_apply_vlan_real(struct device *dev, int mtu)
+{
+	return NULL;
+}
+
+struct device *
 system_if_get_parent(struct device *dev)
 {
 	return NULL;
diff -Nur a/system.h netifd-2019-08-05-5e02f944/system.h
--- a/system.h	2020-05-04 10:18:55.245933547 -0700
+++ netifd-2019-08-05-5e02f944/system.h	2020-05-04 10:19:21.509807807 -0700
@@ -221,6 +221,7 @@
 int system_if_dump_info(struct device *dev, struct blob_buf *b);
 int system_if_dump_stats(struct device *dev, struct blob_buf *b);
 struct device *system_if_get_parent(struct device *dev);
+struct device *system_if_apply_vlan_real(struct device *dev, int mtu);
 bool system_if_force_external(const char *ifname);
 void system_if_apply_settings(struct device *dev, struct device_settings *s,
 			      unsigned int apply_mask);
diff -Nur a/system-linux.c netifd-2019-08-05-5e02f944/system-linux.c
--- a/system-linux.c	2020-05-04 10:18:55.245933547 -0700
+++ netifd-2019-08-05-5e02f944/system-linux.c	2020-05-04 10:19:33.277751264 -0700
@@ -1834,6 +1834,39 @@
 	return device_get(devname, true);
 }
 
+struct device *
+system_if_apply_vlan_real(struct device *dev, int mtu)
+{
+	char buf[64];
+	int len, pmtu;
+	struct device *parentdev;
+	FILE *f;
+
+	/* Check if it is a vlan device. */
+	snprintf(buf, sizeof(buf), "/proc/net/vlan/%s", dev->ifname);
+	if (access(buf, F_OK) == -1)
+		return NULL;
+
+	/* Get the vlan real device. */
+	parentdev = system_if_get_parent(dev);
+
+	/* Check if real device's mtu is less than new mtu. */
+	snprintf(buf, sizeof(buf), "/sys/class/net/%s/mtu", parentdev->ifname);
+	f = fopen(buf, "r");
+	if (!f)
+		return NULL;
+
+	len = fread(buf, 1, sizeof(buf) - 1, f);
+	fclose(f);
+
+	buf[len] = 0;
+	pmtu= strtoul(buf, NULL, 0);
+	if (!pmtu || pmtu >= (mtu + 4))
+		return NULL;
+
+	return parentdev;
+}
+
 static bool
 read_string_file(int dir_fd, const char *file, char *buf, int len)
 {
@@ -3384,9 +3417,9 @@
 		if (ret == 0)
 			ret = ifr.ifr_mtu;
 	} else {
-		struct device * parent = system_if_get_parent(dev);
+		struct device * parent = system_if_apply_vlan_real(dev, mtu);
 		if (parent)
-			system_update_ipv4_mtu(parent, mtu);
+			system_update_ipv4_mtu(parent, mtu + 4);
 
 		ifr.ifr_mtu = mtu;
 		ret = ioctl(sock_ioctl, SIOCSIFMTU, &ifr);