diff --git a/api/envoy/api/v2/cds.proto b/api/envoy/api/v2/cds.proto
index f3c4118145dd..faadc20226ad 100644
--- a/api/envoy/api/v2/cds.proto
+++ b/api/envoy/api/v2/cds.proto
@@ -472,6 +472,9 @@ message Cluster {
     // or metadata updates. By default, this is not configured and updates apply immediately. Also,
     // the first set of updates to be seen apply immediately as well (e.g.: a new cluster).
     //
+    // If this is not set, we default to a merge window of 1000ms. To disable it, set the merge
+    // window to 0.
+    //
     // Note: merging does not apply to cluster membership changes (e.g.: adds/removes); this is
     // because merging those updates isn't currently safe. See
     // https://github.com/envoyproxy/envoy/pull/3941.
diff --git a/docs/root/intro/version_history.rst b/docs/root/intro/version_history.rst
index d7d154a235a5..3e390da02982 100644
--- a/docs/root/intro/version_history.rst
+++ b/docs/root/intro/version_history.rst
@@ -9,6 +9,7 @@ Version history
 * admin: :http:get:`/server_info` now responds with a JSON object instead of a single string.
 * circuit-breaker: added cx_open, rq_pending_open, rq_open and rq_retry_open gauges to expose live
   state via :ref:`circuit breakers statistics <config_cluster_manager_cluster_stats_circuit_breakers>`.
+* cluster: set a default of 1s for :ref:`option <envoy_api_field_Cluster.CommonLbConfig.update_merge_window>`.
 * config: removed support for the v1 API.
 * config: added support for :ref:`rate limiting<envoy_api_msg_core.RateLimitSettings>` discovery request calls.
 * cors: added :ref: `invalid/valid stats <cors-statistics>` to filter.
diff --git a/source/common/upstream/cluster_manager_impl.cc b/source/common/upstream/cluster_manager_impl.cc
index 06ee71363572..a1b809d61989 100644
--- a/source/common/upstream/cluster_manager_impl.cc
+++ b/source/common/upstream/cluster_manager_impl.cc
@@ -317,6 +317,15 @@ ClusterManagerStats ClusterManagerImpl::generateStats(Stats::Scope& scope) {
                                     POOL_GAUGE_PREFIX(scope, final_prefix))};
 }
 
+uint64_t ClusterManagerImpl::mergeTimeout(const Cluster& cluster) const {
+  if (cluster.info()->lbConfig().has_update_merge_window()) {
+    const auto& update_merge_window = cluster.info()->lbConfig().update_merge_window();
+    return DurationUtil::durationToMilliseconds(update_merge_window);
+  }
+
+  return 1000;
+}
+
 void ClusterManagerImpl::onClusterInit(Cluster& cluster) {
   // This routine is called when a cluster has finished initializing. The cluster has not yet
   // been setup for cross-thread updates to avoid needless updates during initialization. The order
@@ -347,14 +356,14 @@ void ClusterManagerImpl::onClusterInit(Cluster& cluster) {
     //
     // See https://github.com/envoyproxy/envoy/pull/3941 for more context.
     bool scheduled = false;
-    const bool merging_enabled = cluster.info()->lbConfig().has_update_merge_window();
+    const auto merge_timeout = mergeTimeout(cluster);
     // Remember: we only merge updates with no adds/removes — just hc/weight/metadata changes.
     const bool is_mergeable = !hosts_added.size() && !hosts_removed.size();
 
-    if (merging_enabled) {
+    if (merge_timeout > 0) {
       // If this is not mergeable, we should cancel any scheduled updates since
       // we'll deliver it immediately.
-      scheduled = scheduleUpdate(cluster, priority, is_mergeable);
+      scheduled = scheduleUpdate(cluster, priority, is_mergeable, merge_timeout);
     }
 
     // If an update was not scheduled for later, deliver it immediately.
@@ -374,10 +383,8 @@ void ClusterManagerImpl::onClusterInit(Cluster& cluster) {
   }
 }
 
-bool ClusterManagerImpl::scheduleUpdate(const Cluster& cluster, uint32_t priority, bool mergeable) {
-  const auto& update_merge_window = cluster.info()->lbConfig().update_merge_window();
-  const auto timeout = DurationUtil::durationToMilliseconds(update_merge_window);
-
+bool ClusterManagerImpl::scheduleUpdate(const Cluster& cluster, uint32_t priority, bool mergeable,
+                                        const uint64_t timeout) {
   // Find pending updates for this cluster.
   auto& updates_by_prio = updates_map_[cluster.info()->name()];
   if (!updates_by_prio) {
diff --git a/source/common/upstream/cluster_manager_impl.h b/source/common/upstream/cluster_manager_impl.h
index 6e1087c7b410..2dd2650b81eb 100644
--- a/source/common/upstream/cluster_manager_impl.h
+++ b/source/common/upstream/cluster_manager_impl.h
@@ -403,7 +403,9 @@ class ClusterManagerImpl : public ClusterManager, Logger::Loggable<Logger::Id::u
   using ClusterUpdatesMap = std::unordered_map<std::string, PendingUpdatesByPriorityMapPtr>;
 
   void applyUpdates(const Cluster& cluster, uint32_t priority, PendingUpdates& updates);
-  bool scheduleUpdate(const Cluster& cluster, uint32_t priority, bool mergeable);
+  bool scheduleUpdate(const Cluster& cluster, uint32_t priority, bool mergeable,
+                      const uint64_t timeout);
+  uint64_t mergeTimeout(const Cluster& cluster) const;
   void createOrUpdateThreadLocalCluster(ClusterData& cluster);
   ProtobufTypes::MessagePtr dumpClusterConfigs();
   static ClusterManagerStats generateStats(Stats::Scope& scope);
diff --git a/test/common/upstream/cluster_manager_impl_test.cc b/test/common/upstream/cluster_manager_impl_test.cc
index d7ad715c59e2..e61cb38bbde5 100644
--- a/test/common/upstream/cluster_manager_impl_test.cc
+++ b/test/common/upstream/cluster_manager_impl_test.cc
@@ -188,14 +188,16 @@ class ClusterManagerImplTest : public testing::Test {
           address: "127.0.0.1"
           port_value: 11002
   )EOF";
-    const std::string merge_window = R"EOF(
+    const std::string merge_window_enabled = R"EOF(
       common_lb_config:
         update_merge_window: 3s
   )EOF";
+    const std::string merge_window_disabled = R"EOF(
+      common_lb_config:
+        update_merge_window: 0s
+  )EOF";
 
-    if (enable_merge_window) {
-      yaml += merge_window;
-    }
+    yaml += enable_merge_window ? merge_window_enabled : merge_window_disabled;
 
     const auto& bootstrap = parseBootstrapFromV2Yaml(yaml);