diff --git a/sys/dev/virtio/network/if_vtnet.c b/sys/dev/virtio/network/if_vtnet.c
--- a/sys/dev/virtio/network/if_vtnet.c
+++ b/sys/dev/virtio/network/if_vtnet.c
@@ -1223,6 +1223,15 @@
 	} else
 		framesz = sizeof(struct vtnet_rx_header);
 	framesz += sizeof(struct ether_vlan_header) + mtu;
+#ifndef __NO_STRICT_ALIGNMENT
+	/*
+	 * Account for the offsetting we'll do elsewhere so we allocate the
+	 * right size for the mtu.
+	 */
+	if (sc->vtnet_hdr_size % 4 == 0) {
+		framesz += ETHER_ALIGN;
+	}
+#endif
 
 	if (framesz <= MCLBYTES)
 		return (MCLBYTES);
diff --git a/sys/dev/virtio/network/if_vtnetvar.h b/sys/dev/virtio/network/if_vtnetvar.h
--- a/sys/dev/virtio/network/if_vtnetvar.h
+++ b/sys/dev/virtio/network/if_vtnetvar.h
@@ -236,6 +236,10 @@
 CTASSERT(sizeof(struct virtio_net_hdr_mrg_rxbuf) ==
     sizeof(struct virtio_net_hdr_v1));
 
+/* We depend on both of these structures being even, and matching std */
+CTASSERT(sizeof(struct virtio_net_hdr_v1) == 12);
+CTASSERT(sizeof(struct virtio_net_hdr) == 10);
+
 /*
  * In legacy VirtIO when mergeable buffers are not negotiated, this structure
  * is placed at the beginning of the mbuf data. Use 4 bytes of pad to keep