Index: head/sys/dev/hyperv/netvsc/hn_nvs.c
===================================================================
--- head/sys/dev/hyperv/netvsc/hn_nvs.c
+++ head/sys/dev/hyperv/netvsc/hn_nvs.c
@@ -43,6 +43,7 @@
 #include <sys/systm.h>
 #include <sys/taskqueue.h>
 
+#include <net/ethernet.h>
 #include <net/if.h>
 #include <net/if_var.h>
 #include <net/if_media.h>
@@ -503,7 +504,7 @@
 
 	memset(&conf, 0, sizeof(conf));
 	conf.nvs_type = HN_NVS_TYPE_NDIS_CONF;
-	conf.nvs_mtu = mtu;
+	conf.nvs_mtu = mtu + ETHER_HDR_LEN;
 	conf.nvs_caps = HN_NVS_NDIS_CONF_VLAN;
 	if (sc->hn_nvs_ver >= HN_NVS_VERSION_5)
 		conf.nvs_caps |= HN_NVS_NDIS_CONF_SRIOV;
Index: head/sys/dev/hyperv/netvsc/hn_rndis.h
===================================================================
--- head/sys/dev/hyperv/netvsc/hn_rndis.h
+++ head/sys/dev/hyperv/netvsc/hn_rndis.h
@@ -41,6 +41,7 @@
 /* link_status: NDIS_MEDIA_STATE_ */
 int		hn_rndis_get_linkstatus(struct hn_softc *sc,
 		    uint32_t *link_status);
+int		hn_rndis_get_mtu(struct hn_softc *sc, uint32_t *mtu);
 /* filter: NDIS_PACKET_TYPE_. */
 int		hn_rndis_set_rxfilter(struct hn_softc *sc, uint32_t filter);
 void		hn_rndis_rx_ctrl(struct hn_softc *sc, const void *data,
Index: head/sys/dev/hyperv/netvsc/hn_rndis.c
===================================================================
--- head/sys/dev/hyperv/netvsc/hn_rndis.c
+++ head/sys/dev/hyperv/netvsc/hn_rndis.c
@@ -188,6 +188,24 @@
 	return (0);
 }
 
+int
+hn_rndis_get_mtu(struct hn_softc *sc, uint32_t *mtu)
+{
+	size_t size;
+	int error;
+
+	size = sizeof(*mtu);
+	error = hn_rndis_query(sc, OID_GEN_MAXIMUM_FRAME_SIZE, NULL, 0,
+	    mtu, &size);
+	if (error)
+		return (error);
+	if (size != sizeof(uint32_t)) {
+		if_printf(sc->hn_ifp, "invalid mtu len %zu\n", size);
+		return (EINVAL);
+	}
+	return (0);
+}
+
 static const void *
 hn_rndis_xact_exec1(struct hn_softc *sc, struct vmbus_xact *xact, size_t reqlen,
     struct hn_nvs_sendctx *sndc, size_t *comp_len)
Index: head/sys/dev/hyperv/netvsc/if_hn.c
===================================================================
--- head/sys/dev/hyperv/netvsc/if_hn.c
+++ head/sys/dev/hyperv/netvsc/if_hn.c
@@ -2003,6 +2003,7 @@
 	uint8_t eaddr[ETHER_ADDR_LEN];
 	struct ifnet *ifp = NULL;
 	int error, ring_cnt, tx_ring_cnt;
+	uint32_t mtu;
 
 	sc->hn_dev = dev;
 	sc->hn_prichan = vmbus_get_channel(dev);
@@ -2159,6 +2160,12 @@
 	if (error)
 		goto failed;
 
+	error = hn_rndis_get_mtu(sc, &mtu);
+	if (error)
+		mtu = ETHERMTU;
+	else if (bootverbose)
+		device_printf(dev, "RNDIS mtu %u\n", mtu);
+
 #if __FreeBSD_version >= 1100099
 	if (sc->hn_rx_ring_inuse > 1) {
 		/*
@@ -2343,6 +2350,10 @@
 		if_printf(ifp, "TSO segcnt %u segsz %u\n",
 		    ifp->if_hw_tsomaxsegcount, ifp->if_hw_tsomaxsegsize);
 	}
+	if (mtu < ETHERMTU) {
+		if_printf(ifp, "fixup mtu %u -> %u\n", ifp->if_mtu, mtu);
+		ifp->if_mtu = mtu;
+	}
 
 	/* Inform the upper layer about the long frame support. */
 	ifp->if_hdrlen = sizeof(struct ether_vlan_header);
@@ -3587,6 +3598,7 @@
 	int mask, error = 0;
 	struct ifrsskey *ifrk;
 	struct ifrsshash *ifrh;
+	uint32_t mtu;
 
 	switch (cmd) {
 	case SIOCSIFMTU:
@@ -3650,11 +3662,23 @@
 			break;
 		}
 
+		error = hn_rndis_get_mtu(sc, &mtu);
+		if (error)
+			mtu = ifr->ifr_mtu;
+		else if (bootverbose)
+			if_printf(ifp, "RNDIS mtu %u\n", mtu);
+
 		/*
 		 * Commit the requested MTU, after the synthetic parts
 		 * have been successfully attached.
 		 */
-		ifp->if_mtu = ifr->ifr_mtu;
+		if (mtu >= ifr->ifr_mtu) {
+			mtu = ifr->ifr_mtu;
+		} else {
+			if_printf(ifp, "fixup mtu %d -> %u\n",
+			    ifr->ifr_mtu, mtu);
+		}
+		ifp->if_mtu = mtu;
 
 		/*
 		 * Synthetic parts' reattach may change the chimney