diff --git a/apps/segmentrouting/src/main/java/org/onosproject/segmentrouting/LinkHandler.java b/apps/segmentrouting/src/main/java/org/onosproject/segmentrouting/LinkHandler.java
new file mode 100644
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--- /dev/null
+++ b/apps/segmentrouting/src/main/java/org/onosproject/segmentrouting/LinkHandler.java
@@ -0,0 +1,561 @@
+/*
+ * Copyright 2018-present Open Networking Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.onosproject.segmentrouting;
+
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+import org.onosproject.net.Device;
+import org.onosproject.net.DeviceId;
+import org.onosproject.net.HostLocation;
+import org.onosproject.net.Link;
+import org.onosproject.net.PortNumber;
+import org.onosproject.net.link.LinkService;
+import org.onosproject.segmentrouting.config.DeviceConfigNotFoundException;
+import org.onosproject.segmentrouting.config.DeviceConfiguration;
+import org.onosproject.segmentrouting.grouphandler.DefaultGroupHandler;
+import org.onosproject.store.service.EventuallyConsistentMap;
+import org.onosproject.store.service.EventuallyConsistentMapBuilder;
+import org.onosproject.store.service.WallClockTimestamp;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import com.google.common.collect.Sets;
+
+public class LinkHandler {
+    private static final Logger log = LoggerFactory.getLogger(LinkHandler.class);
+    protected final SegmentRoutingManager srManager;
+    protected LinkService linkService;
+
+    // Local store for all links seen and their present status, used for
+    // optimized routing. The existence of the link in the keys is enough to
+    // know
+    // if the link has been "seen-before" by this instance of the controller.
+    // The boolean value indicates if the link is currently up or not.
+    // XXX Currently the optimized routing logic depends on "forgetting" a link
+    // when a switch goes down, but "remembering" it when only the link goes
+    // down.
+    // Consider changing this logic so we can use the Link Service instead of
+    // a local cache.
+    private Map<Link, Boolean> seenLinks = new ConcurrentHashMap<>();
+
+    private EventuallyConsistentMap<DeviceId, Set<PortNumber>> downedPortStore = null;
+
+    /**
+     * Constructs the LinkHandler.
+     *
+     * @param srManager Segment Routing manager
+     */
+    LinkHandler(SegmentRoutingManager srManager) {
+        this.srManager = srManager;
+        linkService = srManager.linkService;
+        log.debug("Creating EC map downedportstore");
+        EventuallyConsistentMapBuilder<DeviceId, Set<PortNumber>> downedPortsMapBuilder
+                = srManager.storageService.eventuallyConsistentMapBuilder();
+        downedPortStore = downedPortsMapBuilder.withName("downedportstore")
+                .withSerializer(srManager.createSerializer())
+                .withTimestampProvider((k, v) -> new WallClockTimestamp())
+                .build();
+        log.trace("Current size {}", downedPortStore.size());
+    }
+
+    /**
+     * Constructs the LinkHandler for unit-testing.
+     *
+     * @param srManager SegmentRoutingManager
+     * @param linkService LinkService
+     */
+    LinkHandler(SegmentRoutingManager srManager, LinkService linkService) {
+        this.srManager = srManager;
+        this.linkService = linkService;
+    }
+
+    /**
+     * Preprocessing of added link before being sent for route-path handling.
+     * Also performs post processing of link.
+     *
+     * @param link the link to be processed
+     */
+    void processLinkAdded(Link link) {
+        log.info("** LINK ADDED {}", link.toString());
+        if (!isLinkValid(link)) {
+            return;
+        }
+        if (!srManager.deviceConfiguration
+                .isConfigured(link.src().deviceId())) {
+            updateSeenLink(link, true);
+            // XXX revisit - what about devicePortMap
+            log.warn("Source device of this link is not configured.. "
+                    + "not processing further");
+            return;
+        }
+
+        // Irrespective of whether the local is a MASTER or not for this device,
+        // create group handler instance and push default TTP flow rules if
+        // needed,
+        // as in a multi-instance setup, instances can initiate groups for any
+        // device.
+        DefaultGroupHandler groupHandler = srManager.groupHandlerMap
+                .get(link.src().deviceId());
+        if (groupHandler != null) {
+            groupHandler.portUpForLink(link);
+        } else {
+            // XXX revisit/cleanup
+            Device device = srManager.deviceService.getDevice(link.src().deviceId());
+            if (device != null) {
+                log.warn("processLinkAdded: Link Added "
+                        + "Notification without Device Added "
+                        + "event, still handling it");
+                srManager.processDeviceAdded(device);
+                groupHandler = srManager.groupHandlerMap.get(link.src().deviceId());
+                groupHandler.portUpForLink(link);
+            }
+        }
+
+        /*
+         // process link only if it is bidirectional
+         if (!isBidirectional(link)) {
+            log.debug("Link not bidirectional.. waiting for other direction " +
+                      "src {} --> dst {} ", link.dst(), link.src());
+            // note that if we are not processing for routing, it should at least
+            // be considered a seen-link
+            updateSeenLink(link, true); return;
+          }
+         //TODO ensure that rehash is still done correctly even if link is not processed for
+         //rerouting - perhaps rehash in both directions when it ultimately becomes bidi?
+         */
+
+        log.debug("Starting optimized route-path processing for added link "
+                + "{} --> {}", link.src(), link.dst());
+        boolean seenBefore = isSeenLink(link);
+        // seenLink updates will be done after route-path changes
+        srManager.defaultRoutingHandler
+                .populateRoutingRulesForLinkStatusChange(null, link, null);
+
+        if (srManager.mastershipService.isLocalMaster(link.src().deviceId())) {
+            // handle edge-ports for dual-homed hosts
+            updateDualHomedHostPorts(link, true);
+
+            // It's possible that linkUp causes no route-path change as ECMP
+            // graph does
+            // not change if the link is a parallel link (same src-dst as
+            // another link.
+            // However we still need to update ECMP hash groups to include new
+            // buckets
+            // for the link that has come up.
+            if (!seenBefore && isParallelLink(link)) {
+                // if link seen first time, we need to ensure hash-groups have
+                // all ports
+                log.debug("Attempting retryHash for paralled first-time link {}",
+                          link);
+                groupHandler.retryHash(link, false, true);
+            } else {
+                // seen before-link
+                if (isParallelLink(link)) {
+                    log.debug("Attempting retryHash for paralled seen-before "
+                            + "link {}", link);
+                    groupHandler.retryHash(link, false, false);
+                }
+            }
+        }
+
+        srManager.mcastHandler.init();
+    }
+
+    /**
+     * Preprocessing of removed link before being sent for route-path handling.
+     * Also performs post processing of link.
+     *
+     * @param link the link to be processed
+     */
+    void processLinkRemoved(Link link) {
+        log.info("** LINK REMOVED {}", link.toString());
+        if (!isLinkValid(link)) {
+            return;
+        }
+        // when removing links, update seen links first, before doing route-path
+        // changes
+        updateSeenLink(link, false);
+
+        // device availability check helps to ensure that multiple link-removed
+        // events are actually treated as a single switch removed event.
+        // purgeSeenLink is necessary so we do rerouting (instead of rehashing)
+        // when switch comes back.
+        if (link.src().elementId() instanceof DeviceId
+                && !srManager.deviceService.isAvailable(link.src().deviceId())) {
+            purgeSeenLink(link);
+            return;
+        }
+        if (link.dst().elementId() instanceof DeviceId
+                && !srManager.deviceService.isAvailable(link.dst().deviceId())) {
+            purgeSeenLink(link);
+            return;
+        }
+
+        // handle edge-ports for dual-homed hosts
+        if (srManager.mastershipService.isLocalMaster(link.src().deviceId())) {
+            updateDualHomedHostPorts(link, false);
+        }
+
+        log.debug("Starting optimized route-path processing for removed link "
+                + "{} --> {}", link.src(), link.dst());
+        srManager.defaultRoutingHandler
+                .populateRoutingRulesForLinkStatusChange(link, null, null);
+
+        // update local groupHandler stores
+        DefaultGroupHandler groupHandler = srManager.groupHandlerMap
+                .get(link.src().deviceId());
+        if (groupHandler != null) {
+            if (srManager.mastershipService.isLocalMaster(link.src().deviceId())
+                    && isParallelLink(link)) {
+                log.debug("* retrying hash for parallel link removed:{}", link);
+                groupHandler.retryHash(link, true, false);
+            } else {
+                log.debug("Not attempting retry-hash for link removed: {} .. {}",
+                          link,
+                          (srManager.mastershipService.isLocalMaster(link.src()
+                                  .deviceId())) ? "not parallel"
+                                                : "not master");
+            }
+            // ensure local stores are updated
+            groupHandler.portDown(link.src().port());
+        } else {
+            log.warn("group handler not found for dev:{} when removing link: {}",
+                     link.src().deviceId(), link);
+        }
+
+        srManager.mcastHandler.processLinkDown(link);
+    }
+
+    /**
+     * Checks validity of link. Examples of invalid links include
+     * indirect-links, links between ports on the same switch, and more.
+     *
+     * @param link the link to be processed
+     * @return true if valid link
+     */
+    private boolean isLinkValid(Link link) {
+        if (link.type() != Link.Type.DIRECT) {
+            // NOTE: A DIRECT link might be transiently marked as INDIRECT
+            // if BDDP is received before LLDP. We can safely ignore that
+            // until the LLDP is received and the link is marked as DIRECT.
+            log.info("Ignore link {}->{}. Link type is {} instead of DIRECT.",
+                     link.src(), link.dst(), link.type());
+            return false;
+        }
+        DeviceId srcId = link.src().deviceId();
+        DeviceId dstId = link.dst().deviceId();
+        if (srcId.equals(dstId)) {
+            log.warn("Links between ports on the same switch are not "
+                    + "allowed .. ignoring link {}", link);
+            return false;
+        }
+        DeviceConfiguration devConfig = srManager.deviceConfiguration;
+        try {
+            if (!devConfig.isEdgeDevice(srcId)
+                    && !devConfig.isEdgeDevice(dstId)) {
+                // ignore links between spines
+                // XXX revisit when handling multi-stage fabrics
+                log.warn("Links between spines not allowed...ignoring "
+                        + "link {}", link);
+                return false;
+            }
+            if (devConfig.isEdgeDevice(srcId)
+                    && devConfig.isEdgeDevice(dstId)) {
+                // ignore links between leaves if they are not pair-links
+                // XXX revisit if removing pair-link config or allowing more than
+                // one pair-link
+                if (devConfig.getPairDeviceId(srcId).equals(dstId)
+                        && devConfig.getPairLocalPort(srcId)
+                                .equals(link.src().port())
+                        && devConfig.getPairLocalPort(dstId)
+                                .equals(link.dst().port())) {
+                    // found pair link - allow it
+                    return true;
+                } else {
+                    log.warn("Links between leaves other than pair-links are "
+                            + "not allowed...ignoring link {}", link);
+                    return false;
+                }
+            }
+        } catch (DeviceConfigNotFoundException e) {
+            // We still want to count the links in seenLinks even though there
+            // is no config. So we let it return true
+            log.warn("Could not check validity of link {} as subtending devices "
+                    + "are not yet configured", link);
+        }
+        return true;
+    }
+
+    /**
+     * Administratively enables or disables edge ports if the link that was
+     * added or removed was the only uplink port from an edge device. Only edge
+     * ports that belong to dual-homed hosts are considered.
+     *
+     * @param link the link to be processed
+     * @param added true if link was added, false if link was removed
+     */
+    private void updateDualHomedHostPorts(Link link, boolean added) {
+        if (!lastUplink(link)) {
+            return;
+        }
+        if (added) {
+            // re-enable previously disabled ports on this dev
+            Set<PortNumber> p = downedPortStore.remove(link.src().deviceId());
+            if (p != null) {
+                log.warn("Link src {} -->dst {} added is the first uplink, "
+                        + "enabling dual homed ports: {}", link.src().deviceId(),
+                     link.dst().deviceId(), (p.isEmpty()) ? "no ports" : p);
+                p.forEach(pnum -> srManager.deviceAdminService
+                        .changePortState(link.src().deviceId(), pnum, true));
+            }
+        } else {
+            // find dual homed hosts on this dev to disable
+            Set<PortNumber> dhp = srManager.hostHandler
+                    .getDualHomedHostPorts(link.src().deviceId());
+            log.warn("Link src {} -->dst {} removed was the last uplink, "
+                    + "disabling  dual homed ports:  {}", link.src().deviceId(),
+                     link.dst().deviceId(), (dhp.isEmpty()) ? "no ports" : dhp);
+            dhp.forEach(pnum -> srManager.deviceAdminService
+                        .changePortState(link.src().deviceId(), pnum, false));
+            if (!dhp.isEmpty()) {
+                // update global store
+                Set<PortNumber> p = downedPortStore.get(link.src().deviceId());
+                if (p == null) {
+                    p = dhp;
+                } else {
+                    p.addAll(dhp);
+                }
+                downedPortStore.put(link.src().deviceId(), p);
+            }
+        }
+    }
+
+    /**
+     * Returns true if given link is the last active uplink from src-device of
+     * link. An uplink is defined as a unidirectional link with src as
+     * edgeRouter and dst as non-edgeRouter.
+     *
+     * @param link
+     * @return true if given link is-the-first/was-the-last uplink from the src
+     *         device
+     */
+    private boolean lastUplink(Link link) {
+        DeviceConfiguration devConfig = srManager.deviceConfiguration;
+        try {
+            if (!devConfig.isEdgeDevice(link.src().deviceId())) {
+                return false;
+            }
+            Set<Link> devLinks = srManager.linkService
+                    .getDeviceLinks(link.src().deviceId());
+            boolean foundOtherUplink = false;
+            for (Link l : devLinks) {
+                if (devConfig.isEdgeDevice(l.dst().deviceId()) || l.equals(link)
+                        || l.state() == Link.State.INACTIVE) {
+                    continue;
+                }
+                foundOtherUplink = true;
+                break;
+            }
+            if (!foundOtherUplink) {
+                return true;
+            }
+        } catch (DeviceConfigNotFoundException e) {
+            log.warn("Unable to determine if link is last uplink"
+                    + e.getMessage());
+        }
+        return false;
+    }
+
+    /**
+     * Returns true if this controller instance has seen this link before. The
+     * link may not be currently up, but as long as the link had been seen
+     * before this method will return true. The one exception is when the link
+     * was indeed seen before, but this controller instance was forced to forget
+     * it by a call to purgeSeenLink method.
+     *
+     * @param link the infrastructure link being queried
+     * @return true if this controller instance has seen this link before
+     */
+    boolean isSeenLink(Link link) {
+        return seenLinks.containsKey(link);
+    }
+
+    /**
+     * Updates the seen link store. Updates can be for links that are currently
+     * available or not.
+     *
+     * @param link the link to update in the seen-link local store
+     * @param up the status of the link, true if up, false if down
+     */
+    void updateSeenLink(Link link, boolean up) {
+        seenLinks.put(link, up);
+    }
+
+    /**
+     * Returns the status of a seen-link (up or down). If the link has not been
+     * seen-before, a null object is returned.
+     *
+     * @param link the infrastructure link being queried
+     * @return null if the link was not seen-before; true if the seen-link is
+     *         up; false if the seen-link is down
+     */
+    private Boolean isSeenLinkUp(Link link) {
+        return seenLinks.get(link);
+    }
+
+    /**
+     * Makes this controller instance forget a previously seen before link.
+     *
+     * @param link the infrastructure link to purge
+     */
+    private void purgeSeenLink(Link link) {
+        seenLinks.remove(link);
+    }
+
+    /**
+     * Returns the status of a link as parallel link. A parallel link is defined
+     * as a link which has common src and dst switches as another seen-link that
+     * is currently enabled. It is not necessary for the link being queried to
+     * be a seen-link.
+     *
+     * @param link the infrastructure link being queried
+     * @return true if a seen-link exists that is up, and shares the same src
+     *         and dst switches as the link being queried
+     */
+    private boolean isParallelLink(Link link) {
+        for (Entry<Link, Boolean> seen : seenLinks.entrySet()) {
+            Link seenLink = seen.getKey();
+            if (seenLink.equals(link)) {
+                continue;
+            }
+            if (seenLink.src().deviceId().equals(link.src().deviceId())
+                    && seenLink.dst().deviceId().equals(link.dst().deviceId())
+                    && seen.getValue()) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    /**
+     * Returns true if the link being queried is a bidirectional link. A bidi
+     * link is defined as a link, whose reverse link - ie. the link in the
+     * reverse direction - has been seen-before and is up. It is not necessary
+     * for the link being queried to be a seen-link.
+     *
+     * @param link the infrastructure link being queried
+     * @return true if another unidirectional link exists in the reverse
+     *         direction, has been seen-before and is up
+     */
+    boolean isBidirectional(Link link) {
+        Link reverseLink = linkService.getLink(link.dst(), link.src());
+        if (reverseLink == null) {
+            return false;
+        }
+        Boolean result = isSeenLinkUp(reverseLink);
+        if (result == null) {
+            return false;
+        }
+        return result.booleanValue();
+    }
+
+    /**
+     * Determines if the given link should be avoided in routing calculations by
+     * policy or design.
+     *
+     * @param link the infrastructure link being queried
+     * @return true if link should be avoided
+     */
+    boolean avoidLink(Link link) {
+        // XXX currently only avoids all pair-links. In the future can be
+        // extended to avoid any generic link
+        DeviceId src = link.src().deviceId();
+        PortNumber srcPort = link.src().port();
+        DeviceConfiguration devConfig = srManager.deviceConfiguration;
+        if (devConfig == null || !devConfig.isConfigured(src)) {
+            log.warn("Device {} not configured..cannot avoid link {}", src,
+                     link);
+            return false;
+        }
+        DeviceId pairDev;
+        PortNumber pairLocalPort, pairRemotePort = null;
+        try {
+            pairDev = devConfig.getPairDeviceId(src);
+            pairLocalPort = devConfig.getPairLocalPort(src);
+            if (pairDev != null) {
+                pairRemotePort = devConfig
+                        .getPairLocalPort(pairDev);
+            }
+        } catch (DeviceConfigNotFoundException e) {
+            log.warn("Pair dev for dev {} not configured..cannot avoid link {}",
+                     src, link);
+            return false;
+        }
+
+        return srcPort.equals(pairLocalPort)
+                && link.dst().deviceId().equals(pairDev)
+                && link.dst().port().equals(pairRemotePort);
+    }
+
+    /**
+     * Cleans up internal LinkHandler stores.
+     *
+     * @param device the device that has been removed
+     */
+    void processDeviceRemoved(Device device) {
+        seenLinks.keySet()
+                .removeIf(key -> key.src().deviceId().equals(device.id())
+                        || key.dst().deviceId().equals(device.id()));
+    }
+
+    /**
+     * Administratively disables the host location switchport if the edge device
+     * has no viable uplinks.
+     *
+     * @param loc one of the locations of the dual-homed host
+     */
+    void checkUplinksForDualHomedHosts(HostLocation loc) {
+        try {
+            for (Link l : srManager.linkService.getDeviceLinks(loc.deviceId())) {
+                if (srManager.deviceConfiguration.isEdgeDevice(l.dst().deviceId())
+                        || l.state() == Link.State.INACTIVE) {
+                    continue;
+                }
+                // found valid uplink - so, nothing to do
+                return;
+            }
+        } catch (DeviceConfigNotFoundException e) {
+            log.warn("Could not check for valid uplinks due to missing device"
+                    + "config " + e.getMessage());
+            return;
+        }
+        log.warn("Dual homed host location {} has no valid uplinks; "
+                + "disabling  dual homed port", loc);
+        srManager.deviceAdminService.changePortState(loc.deviceId(), loc.port(),
+                                                     false);
+        Set<PortNumber> p = downedPortStore.get(loc.deviceId());
+        if (p == null) {
+            p = Sets.newHashSet(loc.port());
+        } else {
+            p.add(loc.port());
+        }
+        downedPortStore.put(loc.deviceId(), p);
+    }
+
+}