[core] remove children when attached after receiving MLE Announce Messages (#3093)

src/core/thread/mle.hpp

@@ -1578,6 +1578,14 @@ protected:
      */
     void InformPreviousChannel(void);
 
+    /**
+     * This method indicates whether or not in announce attach process.
+     *
+     * @retval true if attaching/attached on the announced parameters, false otherwise.
+     *
+     */
+    bool IsAnnounceAttach(void) const { return mAlternatePanId != Mac::kPanIdBroadcast; }
+
 #if (OPENTHREAD_CONFIG_LOG_LEVEL >= OT_LOG_LEVEL_NOTE) && (OPENTHREAD_CONFIG_LOG_MLE == 1)
     /**
      * This method converts an `AttachMode` enumeration value into a human-readable string.

src/core/thread/mle_router.cpp

@@ -268,6 +268,21 @@ otError MleRouter::HandleChildStart(AttachMode aMode)
         break;
 
     case kAttachAny:
+        // if attach was started due to receiving MLE Annouce Messageas, all rx-on-when-idle devices would
+        // start attach immediately when receiving such Announce message as in Thread 1.1 specification,
+        // Section 4.8.1,
+        // "If the received value is newer and the channel and/or PAN ID in the Announce message differ
+        //  from those currently in use, the receiving device attempts to attach using the channel and
+        //  PAN ID received from the Announce message."
+        //
+        // That is, Parent-child relationship is highly unlikely to be kept in the new partition, so here
+        // removes all children, leaving whether to become router according to the new partition status.
+        if (IsAnnounceAttach() && HasChildren())
+        {
+            RemoveChildren();
+        }
+
+        // fall through
     case kAttachBetter:
         if (HasChildren() && mPreviousPartitionIdRouter != mLeaderData.GetPartitionId())
         {