Allow IPv4 link-local nexthops (sonic-net#1903)

*Orchagent (NeighOrch) currently ignores IPv4 link-local neighbours. Any route pointing to such a nexthop is not added to the ASIC DB. Such routes are expected and should be supported.
Signed-off-by: Utpal Kant Pintoo utpal.pintoo@broadcom.com

orchagent/neighorch.cpp

@@ -685,19 +685,6 @@ void NeighOrch::doTask(Consumer &consumer)
 
         IpAddress ip_address(key.substr(found+1));
 
-        /* Verify Ipv4 LinkLocal and skip neighbor entry added for RFC5549 */
-        if ((ip_address.getAddrScope() == IpAddress::LINK_SCOPE) && (ip_address.isV4()))
-        {
-            /* Check if this prefix is not a configured ip, if so allow */
-            IpPrefix ipll_prefix(ip_address.getV4Addr(), 16);
-            if (!m_intfsOrch->isPrefixSubnet (ipll_prefix, alias))
-            {
-                SWSS_LOG_NOTICE("Skip IPv4LL neighbor %s, Intf:%s op: %s ", ip_address.to_string().c_str(), alias.c_str(), op.c_str());
-                it = consumer.m_toSync.erase(it);
-                continue;
-            }
-        }
-
         NeighborEntry neighbor_entry = { ip_address, alias };
 
         if (op == SET_COMMAND)
@@ -807,6 +794,18 @@ bool NeighOrch::addNeighbor(const NeighborEntry &neighborEntry, const MacAddress
     memcpy(neighbor_attr.value.mac, macAddress.getMac(), 6);
     neighbor_attrs.push_back(neighbor_attr);
 
+    if ((ip_address.getAddrScope() == IpAddress::LINK_SCOPE) && (ip_address.isV4()))
+    {
+        /* Check if this prefix is a configured ip, if not allow */
+        IpPrefix ipll_prefix(ip_address.getV4Addr(), 16);
+        if (!m_intfsOrch->isPrefixSubnet (ipll_prefix, alias))
+        {
+            neighbor_attr.id = SAI_NEIGHBOR_ENTRY_ATTR_NO_HOST_ROUTE;
+            neighbor_attr.value.booldata = 1;
+            neighbor_attrs.push_back(neighbor_attr);
+        }
+    }
+
     MuxOrch* mux_orch = gDirectory.get<MuxOrch*>();
     bool hw_config = isHwConfigured(neighborEntry);