diff --git a/core/lib/Drupal/Core/Routing/RouteSubscriberBase.php b/core/lib/Drupal/Core/Routing/RouteSubscriberBase.php
index 8860b11..6edb825 100644
--- a/core/lib/Drupal/Core/Routing/RouteSubscriberBase.php
+++ b/core/lib/Drupal/Core/Routing/RouteSubscriberBase.php
@@ -25,13 +25,26 @@
    * @param \Symfony\Component\Routing\RouteCollection $collection
    *   The route collection for adding routes.
    */
-  abstract public function routes(RouteCollection $collection);
+  public function routes(RouteCollection $collection) {
+  }
+
+  /**
+   * Alters existing routes for a specific collection.
+   *
+   * @param \Symfony\Component\Routing\RouteCollection $collection
+   *   The route collection for adding routes.
+   * @param string $module
+   *   The module these routes belong to.
+   */
+  public function alterRoutes(RouteCollection $collection, $module) {
+  }
 
   /**
    * {@inheritdoc}
    */
   public static function getSubscribedEvents() {
     $events[RoutingEvents::DYNAMIC] = 'getRoutes';
+    $events[RoutingEvents::ALTER] = 'alterAllRoutes';
     return $events;
   }
 
@@ -46,4 +59,15 @@ public function getRoutes(RouteBuildEvent $event) {
     $this->routes($collection);
   }
 
+  /**
+   * Delegates the route altering to self::alterRoutes().
+   *
+   * @param \Drupal\Core\Routing\RouteBuildEvent $event
+   *   The route build event.
+   */
+  public function alterAllRoutes(RouteBuildEvent $event) {
+    $collection = $event->getRouteCollection();
+    $this->alterRoutes($collection, $event->getModule());
+  }
+
 }