diff --git a/prog/weaver/main.go b/prog/weaver/main.go
index 50bf68e297..53aa8d26da 100644
--- a/prog/weaver/main.go
+++ b/prog/weaver/main.go
@@ -65,7 +65,6 @@ func main() {
 		peers              []string
 		noDNS              bool
 		dnsConfig          dnsConfig
-		iface              *net.Interface
 		datapathName       string
 		trustedSubnetStr   string
 
@@ -131,43 +130,16 @@ func main() {
 	}
 	config.ProtocolMinVersion = byte(protocolMinVersion)
 
-	overlays := weave.NewOverlaySwitch()
-	switch {
-	case datapathName != "" && ifaceName != "":
-		Log.Fatal("At most one of --datapath and --iface must be specified.")
-	case datapathName != "":
-		fastdp, err := weave.NewFastDatapath(datapathName, config.Port)
-		checkFatal(err)
-		networkConfig.Bridge = fastdp.Bridge()
-		overlays.Add("fastdp", fastdp.Overlay())
-	case ifaceName != "":
-		var err error
-		iface, err = weavenet.EnsureInterface(ifaceName)
-		checkFatal(err)
-		networkConfig.Bridge, err = weave.NewPcap(iface, bufSzMB*1024*1024) // bufsz flag is in MB
-		checkFatal(err)
-	}
-	sleeve := weave.NewSleeveOverlay(config.Port)
-	overlays.Add("sleeve", sleeve)
-	overlays.SetCompatOverlay(sleeve)
-
-	if password == "" {
-		password = os.Getenv("WEAVE_PASSWORD")
-	}
-	if password == "" {
-		Log.Println("Communication between peers is unencrypted.")
-	} else {
-		config.Password = []byte(password)
-		Log.Println("Communication between peers via untrusted networks is encrypted.")
-	}
+	overlay, bridge := createOverlay(datapathName, ifaceName, config.Port, bufSzMB)
+	networkConfig.Bridge = bridge
 
 	if routerName == "" {
+		iface := bridge.Interface()
 		if iface == nil {
-			Log.Fatal("Either an interface must be specified with --iface or a name with -name")
+			Log.Fatal("Either an interface must be specified with --datapath or --iface, or a name with --name")
 		}
 		routerName = iface.HardwareAddr.String()
 	}
-
 	name, err := mesh.PeerNameFromUserInput(routerName)
 	checkFatal(err)
 
@@ -176,6 +148,16 @@ func main() {
 		checkFatal(err)
 	}
 
+	if password == "" {
+		password = os.Getenv("WEAVE_PASSWORD")
+	}
+	if password == "" {
+		Log.Println("Communication between peers is unencrypted.")
+	} else {
+		config.Password = []byte(password)
+		Log.Println("Communication between peers via untrusted networks is encrypted.")
+	}
+
 	if prof != "" {
 		p := *profile.CPUProfile
 		p.ProfilePath = prof
@@ -195,7 +177,7 @@ func main() {
 		Log.Fatal("Unable to parse trusted subnets: ", err)
 	}
 
-	router := weave.NewNetworkRouter(config, networkConfig, name, nickName, overlays)
+	router := weave.NewNetworkRouter(config, networkConfig, name, nickName, overlay)
 	Log.Println("Our name is", router.Ourself)
 
 	var dockerCli *docker.Client
@@ -309,6 +291,31 @@ func (nopPacketLogging) LogPacket(string, weave.PacketKey) {
 func (nopPacketLogging) LogForwardPacket(string, weave.ForwardPacketKey) {
 }
 
+func createOverlay(datapathName string, ifaceName string, port int, bufSzMB int) (weave.NetworkOverlay, weave.Bridge) {
+	overlay := weave.NewOverlaySwitch()
+	var bridge weave.Bridge
+	switch {
+	case datapathName != "" && ifaceName != "":
+		Log.Fatal("At most one of --datapath and --iface must be specified.")
+	case datapathName != "":
+		fastdp, err := weave.NewFastDatapath(datapathName, port)
+		checkFatal(err)
+		bridge = fastdp.Bridge()
+		overlay.Add("fastdp", fastdp.Overlay())
+	case ifaceName != "":
+		iface, err := weavenet.EnsureInterface(ifaceName)
+		checkFatal(err)
+		bridge, err = weave.NewPcap(iface, bufSzMB*1024*1024) // bufsz flag is in MB
+		checkFatal(err)
+	default:
+		bridge = weave.NullBridge{}
+	}
+	sleeve := weave.NewSleeveOverlay(port)
+	overlay.Add("sleeve", sleeve)
+	overlay.SetCompatOverlay(sleeve)
+	return overlay, bridge
+}
+
 func parseAndCheckCIDR(cidrStr string) address.CIDR {
 	_, cidr, err := address.ParseCIDR(cidrStr)
 	checkFatal(err)
diff --git a/router/bridge.go b/router/bridge.go
index 00481a0a35..434669bd99 100644
--- a/router/bridge.go
+++ b/router/bridge.go
@@ -1,5 +1,9 @@
 package router
 
+import (
+	"net"
+)
+
 // Interface to packet handling on the local virtual bridge
 type Bridge interface {
 	// Inject a packet to be delivered locally
@@ -9,6 +13,7 @@ type Bridge interface {
 	// should not be included.
 	StartConsumingPackets(BridgeConsumer) error
 
+	Interface() *net.Interface
 	String() string
 	Stats() map[string]int
 }
@@ -26,6 +31,10 @@ func (NullBridge) StartConsumingPackets(BridgeConsumer) error {
 	return nil
 }
 
+func (NullBridge) Interface() *net.Interface {
+	return nil
+}
+
 func (NullBridge) String() string {
 	return "<no bridge networking>"
 }
diff --git a/router/fastdp.go b/router/fastdp.go
index 6a867f9d0d..ad294bfd9b 100644
--- a/router/fastdp.go
+++ b/router/fastdp.go
@@ -32,7 +32,7 @@ type FastDatapath struct {
 	lock             sync.Mutex // guards state and synchronises use of dpif
 	dpif             *odp.Dpif
 	dp               odp.DatapathHandle
-	mtu              int // [1]
+	iface            *net.Interface
 	deleteFlowsCount uint64
 	missCount        uint64
 	missHandlers     map[odp.VportID]missHandler
@@ -59,10 +59,6 @@ type FastDatapath struct {
 
 	// forwarders by remote peer
 	forwarders map[mesh.PeerName]*fastDatapathForwarder
-
-	// [1] The mtu from the datapath netdev (which should match the
-	// mtus on all the veths hooked up to the datapath).  We validate
-	// that we are able to support that mtu.
 }
 
 func NewFastDatapath(dpName string, port int) (*FastDatapath, error) {
@@ -92,7 +88,7 @@ func NewFastDatapath(dpName string, port int) (*FastDatapath, error) {
 		dpname:        dpName,
 		dpif:          dpif,
 		dp:            dp,
-		mtu:           iface.MTU,
+		iface:         iface,
 		missHandlers:  make(map[odp.VportID]missHandler),
 		sendToPort:    nil,
 		sendToMAC:     make(map[MAC]bridgeSender),
@@ -208,6 +204,10 @@ func (fastdp *FastDatapath) Bridge() Bridge {
 	return fastDatapathBridge{fastdp}
 }
 
+func (fastdp fastDatapathBridge) Interface() *net.Interface {
+	return fastdp.iface
+}
+
 func (fastdp fastDatapathBridge) String() string {
 	return fmt.Sprint(fastdp.dpname, " (via ODP)")
 }
@@ -665,7 +665,7 @@ func (fwd *fastDatapathForwarder) sendHeartbeat() {
 
 	// the heartbeat payload consists of the 64-bit connection uid
 	// followed by the 16-bit packet size.
-	buf := make([]byte, EthernetOverhead+fwd.fastdp.mtu)
+	buf := make([]byte, EthernetOverhead+fwd.fastdp.iface.MTU)
 	binary.BigEndian.PutUint64(buf[EthernetOverhead:], fwd.connUID)
 	binary.BigEndian.PutUint16(buf[EthernetOverhead+8:], uint16(len(buf)))
 
diff --git a/router/pcap.go b/router/pcap.go
index 848870be24..781bb81612 100644
--- a/router/pcap.go
+++ b/router/pcap.go
@@ -104,6 +104,10 @@ func newPcapHandle(ifName string, promisc bool, snaplen int, bufSz int) (handle
 	return
 }
 
+func (p *Pcap) Interface() *net.Interface {
+	return p.iface
+}
+
 func (p *Pcap) String() string {
 	return fmt.Sprint(p.iface.Name, " (via pcap)")
 }