+ /*
+ * If the host is directly attached to the src node, then the node
+ * should have been assigned the "Access" tier in
+ * notifyHostLearnedOrRemoved. If not, it would be assigned
+ * "Unknown" tier. Thus the tier of host attached node cannot be
+ * null. If the src node here, is the next node in the hierarchy of
+ * the nodes, then its tier cannot be null
+ */
+