diff --git a/sys/net/if_epair.c b/sys/net/if_epair.c
--- a/sys/net/if_epair.c
+++ b/sys/net/if_epair.c
@@ -351,6 +351,10 @@
 		return (E2BIG);
 	}
 
+	if ((ifp->if_capenable & IFCAP_MEXTPG) == 0) {
+		M_ASSERTMAPPED(m);
+	}
+
 	/*
 	 * We are not going to use the interface en/dequeue mechanism
 	 * on the TX side. We are called from ether_output_frame()
@@ -632,7 +636,8 @@
 	ifp->if_capabilities =
 	    IFCAP_VLAN_MTU | IFCAP_VLAN_HWTAGGING |
 	    IFCAP_TXCSUM | IFCAP_RXCSUM |
-	    IFCAP_TXCSUM_IPV6 | IFCAP_RXCSUM_IPV6;
+	    IFCAP_TXCSUM_IPV6 | IFCAP_RXCSUM_IPV6 |
+	    IFCAP_MEXTPG;
 	ifp->if_capenable = ifp->if_capabilities;
 	epair_caps_changed(ifp);
 	ifp->if_transmit = epair_transmit;
diff --git a/tests/sys/common/vnet.subr b/tests/sys/common/vnet.subr
--- a/tests/sys/common/vnet.subr
+++ b/tests/sys/common/vnet.subr
@@ -46,7 +46,9 @@
 	# compute checksums, it just marks packets that this computation still
 	# needs to be done. However, some test cases verify the checksum.
 	# Therefore disable this for IPv4 and IPv6.
-	ifconfig ${ifname} -txcsum -txcsum6
+	# Also disable MEXTPG support, so the tests can verify that the network
+	# stack don't pass MEXTPG mbufs to interfaces that don't support it.
+	ifconfig ${ifname} -txcsum -txcsum6 -mextpg
 	list_interface $ifname
 	list_interface ${ifname%a}b
 	echo ${ifname%a}