diff --git a/core/net/src/main/java/org/onlab/onos/net/link/impl/LinkManager.java b/core/net/src/main/java/org/onlab/onos/net/link/impl/LinkManager.java
index 493580d..779e1ee 100644
--- a/core/net/src/main/java/org/onlab/onos/net/link/impl/LinkManager.java
+++ b/core/net/src/main/java/org/onlab/onos/net/link/impl/LinkManager.java
@@ -53,7 +53,7 @@
     protected final AbstractListenerRegistry<LinkEvent, LinkListener>
             listenerRegistry = new AbstractListenerRegistry<>();
 
-    private LinkStoreDelegate delegate = new InternalStoreDelegate();
+    private final LinkStoreDelegate delegate = new InternalStoreDelegate();
 
     private final DeviceListener deviceListener = new InternalDeviceListener();
 
diff --git a/core/net/src/main/java/org/onlab/onos/net/proxyarp/impl/ProxyArpManager.java b/core/net/src/main/java/org/onlab/onos/net/proxyarp/impl/ProxyArpManager.java
new file mode 100644
index 0000000..e7e08fc
--- /dev/null
+++ b/core/net/src/main/java/org/onlab/onos/net/proxyarp/impl/ProxyArpManager.java
@@ -0,0 +1,325 @@
+package org.onlab.onos.net.proxyarp.impl;
+
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Preconditions.checkNotNull;
+import static org.slf4j.LoggerFactory.getLogger;
+
+import java.nio.ByteBuffer;
+import java.util.List;
+import java.util.Map.Entry;
+import java.util.Set;
+
+import org.apache.felix.scr.annotations.Activate;
+import org.apache.felix.scr.annotations.Component;
+import org.apache.felix.scr.annotations.Deactivate;
+import org.apache.felix.scr.annotations.Reference;
+import org.apache.felix.scr.annotations.ReferenceCardinality;
+import org.apache.felix.scr.annotations.Service;
+import org.onlab.onos.net.Device;
+import org.onlab.onos.net.Host;
+import org.onlab.onos.net.HostId;
+import org.onlab.onos.net.Link;
+import org.onlab.onos.net.Port;
+import org.onlab.onos.net.PortNumber;
+import org.onlab.onos.net.device.DeviceEvent;
+import org.onlab.onos.net.device.DeviceListener;
+import org.onlab.onos.net.device.DeviceService;
+import org.onlab.onos.net.flow.DefaultTrafficTreatment;
+import org.onlab.onos.net.flow.TrafficTreatment;
+import org.onlab.onos.net.host.HostService;
+import org.onlab.onos.net.link.LinkEvent;
+import org.onlab.onos.net.link.LinkListener;
+import org.onlab.onos.net.link.LinkService;
+import org.onlab.onos.net.packet.DefaultOutboundPacket;
+import org.onlab.onos.net.packet.PacketService;
+import org.onlab.onos.net.proxyarp.ProxyArpService;
+import org.onlab.packet.ARP;
+import org.onlab.packet.Ethernet;
+import org.onlab.packet.IpPrefix;
+import org.onlab.packet.VlanId;
+import org.slf4j.Logger;
+
+import com.google.common.collect.HashMultimap;
+import com.google.common.collect.Lists;
+import com.google.common.collect.Multimap;
+
+
+@Component(immediate = true)
+@Service
+public class ProxyArpManager implements ProxyArpService {
+
+    private final Logger log = getLogger(getClass());
+
+    private static final String MAC_ADDR_NULL = "Mac address cannot be null.";
+    private static final String REQUEST_NULL = "Arp request cannot be null.";
+    private static final String REQUEST_NOT_ARP = "Ethernet frame does not contain ARP request.";
+    private static final String NOT_ARP_REQUEST = "ARP is not a request.";
+
+    @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
+    protected HostService hostService;
+
+    @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
+    protected PacketService packetService;
+
+    @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
+    protected LinkService linkService;
+
+    @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
+    protected DeviceService deviceService;
+
+    private final Multimap<Device, PortNumber> internalPorts =
+            HashMultimap.<Device, PortNumber>create();
+
+    private final Multimap<Device, PortNumber> externalPorts =
+            HashMultimap.<Device, PortNumber>create();
+
+    /**
+     * Listens to both device service and link service to determine
+     * whether a port is internal or external.
+     */
+    @Activate
+    public void activate() {
+        deviceService.addListener(new InternalDeviceListener());
+        linkService.addListener(new InternalLinkListener());
+        determinePortLocations();
+        log.info("Started");
+    }
+
+
+    @Deactivate
+    public void deactivate() {
+        log.info("Stopped");
+    }
+
+    @Override
+    public boolean known(IpPrefix addr) {
+        checkNotNull(MAC_ADDR_NULL, addr);
+        Set<Host> hosts = hostService.getHostsByIp(addr);
+        return !hosts.isEmpty();
+    }
+
+    @Override
+    public void reply(Ethernet eth) {
+        checkNotNull(REQUEST_NULL, eth);
+        checkArgument(eth.getEtherType() == Ethernet.TYPE_ARP,
+                REQUEST_NOT_ARP);
+        ARP arp = (ARP) eth.getPayload();
+        checkArgument(arp.getOpCode() == ARP.OP_REQUEST, NOT_ARP_REQUEST);
+
+        VlanId vlan = VlanId.vlanId(eth.getVlanID());
+        Set<Host> hosts = hostService.getHostsByIp(IpPrefix.valueOf(arp
+                .getTargetProtocolAddress()));
+
+        Host dst = null;
+        Host src = hostService.getHost(HostId.hostId(eth.getSourceMAC(),
+                VlanId.vlanId(eth.getVlanID())));
+
+        for (Host host : hosts) {
+            if (host.vlan().equals(vlan)) {
+                dst = host;
+                break;
+            }
+        }
+
+        if (src == null || dst == null) {
+            flood(eth);
+            return;
+        }
+
+        Ethernet arpReply = buildArpReply(dst, eth);
+        // TODO: check send status with host service.
+        TrafficTreatment.Builder builder = new DefaultTrafficTreatment.Builder();
+        builder.setOutput(src.location().port());
+        packetService.emit(new DefaultOutboundPacket(src.location().deviceId(),
+                builder.build(), ByteBuffer.wrap(arpReply.serialize())));
+    }
+
+    @Override
+    public void forward(Ethernet eth) {
+        checkNotNull(REQUEST_NULL, eth);
+        checkArgument(eth.getEtherType() == Ethernet.TYPE_ARP,
+                REQUEST_NOT_ARP);
+        ARP arp = (ARP) eth.getPayload();
+        checkArgument(arp.getOpCode() == ARP.OP_REPLY, NOT_ARP_REQUEST);
+
+        Host h = hostService.getHost(HostId.hostId(eth.getDestinationMAC(),
+                VlanId.vlanId(eth.getVlanID())));
+
+        if (h == null) {
+            flood(eth);
+        } else {
+            TrafficTreatment.Builder builder = new DefaultTrafficTreatment.Builder();
+            builder.setOutput(h.location().port());
+            packetService.emit(new DefaultOutboundPacket(h.location().deviceId(),
+                    builder.build(), ByteBuffer.wrap(eth.serialize())));
+        }
+
+    }
+
+    /**
+     * Flood the arp request at all edges in the network.
+     * @param request the arp request.
+     */
+    private void flood(Ethernet request) {
+        TrafficTreatment.Builder builder = null;
+        ByteBuffer buf = ByteBuffer.wrap(request.serialize());
+
+        synchronized (externalPorts) {
+            for (Entry<Device, PortNumber> entry : externalPorts.entries()) {
+                builder = new DefaultTrafficTreatment.Builder();
+                builder.setOutput(entry.getValue());
+                packetService.emit(new DefaultOutboundPacket(entry.getKey().id(),
+                        builder.build(), buf));
+            }
+
+        }
+    }
+
+    /**
+     * Determines the location of all known ports in the system.
+     */
+    private void determinePortLocations() {
+        Iterable<Device> devices = deviceService.getDevices();
+        Iterable<Link> links = null;
+        List<PortNumber> ports = null;
+        for (Device d : devices) {
+            ports = buildPortNumberList(deviceService.getPorts(d.id()));
+            links = linkService.getLinks();
+            for (Link l : links) {
+                // for each link, mark the concerned ports as internal
+                // and the remaining ports are therefore external.
+                if (l.src().deviceId().equals(d)
+                        && ports.contains(l.src().port())) {
+                    ports.remove(l.src().port());
+                    internalPorts.put(d, l.src().port());
+                }
+                if (l.dst().deviceId().equals(d)
+                        && ports.contains(l.dst().port())) {
+                    ports.remove(l.dst().port());
+                    internalPorts.put(d, l.dst().port());
+                }
+            }
+            synchronized (externalPorts) {
+                externalPorts.putAll(d, ports);
+            }
+        }
+
+    }
+
+    private List<PortNumber> buildPortNumberList(List<Port> ports) {
+        List<PortNumber> portNumbers = Lists.newLinkedList();
+        for (Port p : ports) {
+            portNumbers.add(p.number());
+        }
+        return portNumbers;
+    }
+
+    /**
+     * Builds an arp reply based on a request.
+     * @param h the host we want to send to
+     * @param request the arp request we got
+     * @return an ethernet frame containing the arp reply
+     */
+    private Ethernet buildArpReply(Host h, Ethernet request) {
+        Ethernet eth = new Ethernet();
+        eth.setDestinationMACAddress(request.getSourceMACAddress());
+        eth.setSourceMACAddress(h.mac().getAddress());
+        eth.setEtherType(Ethernet.TYPE_ARP);
+        eth.setVlanID(request.getVlanID());
+
+        ARP arp = new ARP();
+        arp.setOpCode(ARP.OP_REPLY);
+        arp.setProtocolType(ARP.PROTO_TYPE_IP);
+        arp.setHardwareType(ARP.HW_TYPE_ETHERNET);
+        arp.setProtocolAddressLength((byte) IpPrefix.INET_LEN);
+        arp.setHardwareAddressLength((byte) Ethernet.DATALAYER_ADDRESS_LENGTH);
+        arp.setSenderHardwareAddress(h.mac().getAddress());
+        arp.setTargetHardwareAddress(request.getSourceMACAddress());
+
+        arp.setTargetProtocolAddress(((ARP) request.getPayload())
+                .getSenderProtocolAddress());
+        arp.setSenderProtocolAddress(h.ipAddresses().iterator().next().toInt());
+        eth.setPayload(arp);
+        return eth;
+    }
+
+    public class InternalLinkListener implements LinkListener {
+
+        @Override
+        public void event(LinkEvent event) {
+            Link link = event.subject();
+            Device src = deviceService.getDevice(link.src().deviceId());
+            Device dst = deviceService.getDevice(link.dst().deviceId());
+            switch (event.type()) {
+                case LINK_ADDED:
+                    synchronized (externalPorts) {
+                        externalPorts.remove(src, link.src().port());
+                        externalPorts.remove(dst, link.dst().port());
+                        internalPorts.put(src, link.src().port());
+                        internalPorts.put(dst, link.dst().port());
+                    }
+
+                    break;
+                case LINK_REMOVED:
+                    synchronized (externalPorts) {
+                        externalPorts.put(src, link.src().port());
+                        externalPorts.put(dst, link.dst().port());
+                        internalPorts.remove(src, link.src().port());
+                        internalPorts.remove(dst, link.dst().port());
+                    }
+
+                    break;
+                case LINK_UPDATED:
+                    // don't care about links being updated.
+                    break;
+                default:
+                    break;
+            }
+
+        }
+
+    }
+
+    public class InternalDeviceListener implements DeviceListener {
+
+        @Override
+        public void event(DeviceEvent event) {
+            Device device = event.subject();
+            switch (event.type()) {
+                case DEVICE_ADDED:
+                case DEVICE_AVAILABILITY_CHANGED:
+                case DEVICE_MASTERSHIP_CHANGED:
+                case DEVICE_SUSPENDED:
+                case DEVICE_UPDATED:
+                case PORT_UPDATED:
+                 // nothing to do in these cases; handled when links get reported
+                    break;
+                case DEVICE_REMOVED:
+                    synchronized (externalPorts) {
+                        externalPorts.removeAll(device);
+                        internalPorts.removeAll(device);
+                    }
+                    break;
+                case PORT_ADDED:
+                    synchronized (externalPorts) {
+                        externalPorts.put(device, event.port().number());
+                        internalPorts.remove(device, event.port().number());
+                    }
+                    break;
+                case PORT_REMOVED:
+                    synchronized (externalPorts) {
+                        externalPorts.remove(device, event.port().number());
+                        internalPorts.remove(device, event.port().number());
+                    }
+                    break;
+                default:
+                    break;
+
+            }
+
+        }
+
+}
+
+
+}
diff --git a/core/net/src/main/java/org/onlab/onos/net/proxyarp/impl/package-info.java b/core/net/src/main/java/org/onlab/onos/net/proxyarp/impl/package-info.java
new file mode 100644
index 0000000..53830bc
--- /dev/null
+++ b/core/net/src/main/java/org/onlab/onos/net/proxyarp/impl/package-info.java
@@ -0,0 +1,4 @@
+/**
+ * Core subsystem for responding to arp requests.
+ */
+package org.onlab.onos.net.proxyarp.impl;
\ No newline at end of file
diff --git a/core/net/src/main/java/org/onlab/onos/proxyarp/impl/ProxyArpManager.java b/core/net/src/main/java/org/onlab/onos/proxyarp/impl/ProxyArpManager.java
deleted file mode 100644
index f267f68..0000000
--- a/core/net/src/main/java/org/onlab/onos/proxyarp/impl/ProxyArpManager.java
+++ /dev/null
@@ -1,100 +0,0 @@
-package org.onlab.onos.proxyarp.impl;
-
-import static com.google.common.base.Preconditions.checkArgument;
-import static com.google.common.base.Preconditions.checkNotNull;
-
-import java.nio.ByteBuffer;
-import java.util.Set;
-
-import org.apache.felix.scr.annotations.Reference;
-import org.apache.felix.scr.annotations.ReferenceCardinality;
-import org.onlab.onos.net.Host;
-import org.onlab.onos.net.flow.DefaultTrafficTreatment;
-import org.onlab.onos.net.flow.TrafficTreatment;
-import org.onlab.onos.net.host.HostService;
-import org.onlab.onos.net.packet.DefaultOutboundPacket;
-import org.onlab.onos.net.packet.PacketService;
-import org.onlab.onos.net.proxyarp.ProxyArpService;
-import org.onlab.onos.net.topology.TopologyService;
-import org.onlab.packet.ARP;
-import org.onlab.packet.Ethernet;
-import org.onlab.packet.IpPrefix;
-import org.onlab.packet.VlanId;
-
-public class ProxyArpManager implements ProxyArpService {
-
-    private static final String MAC_ADDR_NULL = "Mac address cannot be null.";
-    private static final String REQUEST_NULL = "Arp request cannot be null.";
-    private static final String REQUEST_NOT_ARP = "Ethernet frame does not contain ARP request.";
-    private static final String NOT_ARP_REQUEST = "ARP is not a request.";
-
-    @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
-    protected HostService hostService;
-
-    @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
-    protected PacketService packetService;
-
-    @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
-    protected TopologyService topologyService;
-
-    @Override
-    public boolean known(IpPrefix addr) {
-        checkNotNull(MAC_ADDR_NULL, addr);
-        Set<Host> hosts = hostService.getHostsByIp(addr);
-        return !hosts.isEmpty();
-    }
-
-    @Override
-    public void reply(Ethernet request) {
-        checkNotNull(REQUEST_NULL, request);
-        checkArgument(request.getEtherType() == Ethernet.TYPE_ARP,
-                REQUEST_NOT_ARP);
-        ARP arp = (ARP) request.getPayload();
-        checkArgument(arp.getOpCode() == ARP.OP_REQUEST, NOT_ARP_REQUEST);
-
-        VlanId vlan = VlanId.vlanId(request.getVlanID());
-        Set<Host> hosts = hostService.getHostsByIp(IpPrefix.valueOf(arp
-                .getTargetProtocolAddress()));
-
-        Host h = null;
-        for (Host host : hosts) {
-            if (host.vlan().equals(vlan)) {
-                h = host;
-                break;
-            }
-        }
-
-        if (h == null) {
-            flood(request);
-            return;
-        }
-
-        Ethernet arpReply = buildArpReply(h, request);
-        // TODO: check send status with host service.
-        TrafficTreatment.Builder builder = new DefaultTrafficTreatment.Builder();
-        builder.setOutput(h.location().port());
-        packetService.emit(new DefaultOutboundPacket(h.location().deviceId(),
-                builder.build(), ByteBuffer.wrap(arpReply.serialize())));
-    }
-
-    private void flood(Ethernet request) {
-        // TODO: flood on all edge ports.
-    }
-
-    private Ethernet buildArpReply(Host h, Ethernet request) {
-        Ethernet eth = new Ethernet();
-        eth.setDestinationMACAddress(request.getSourceMACAddress());
-        eth.setSourceMACAddress(h.mac().getAddress());
-        eth.setEtherType(Ethernet.TYPE_ARP);
-        ARP arp = new ARP();
-        arp.setOpCode(ARP.OP_REPLY);
-        arp.setSenderHardwareAddress(h.mac().getAddress());
-        arp.setTargetHardwareAddress(request.getSourceMACAddress());
-
-        arp.setTargetProtocolAddress(((ARP) request.getPayload())
-                .getSenderProtocolAddress());
-        arp.setSenderProtocolAddress(h.ipAddresses().iterator().next().toInt());
-        eth.setPayload(arp);
-        return eth;
-    }
-}
diff --git a/core/net/src/main/java/org/onlab/onos/proxyarp/impl/package-info.java b/core/net/src/main/java/org/onlab/onos/proxyarp/impl/package-info.java
deleted file mode 100644
index a9ad72a..0000000
--- a/core/net/src/main/java/org/onlab/onos/proxyarp/impl/package-info.java
+++ /dev/null
@@ -1,4 +0,0 @@
-/**
- * Core subsystem for responding to arp requests.
- */
-package org.onlab.onos.proxyarp.impl;
\ No newline at end of file