diff --git a/envoy/api/v2/route/route_components.proto b/envoy/api/v2/route/route_components.proto
index a001eb45..d79dd67d 100644
--- a/envoy/api/v2/route/route_components.proto
+++ b/envoy/api/v2/route/route_components.proto
@@ -548,6 +548,10 @@ message RouteAction {
   //
   // During shadowing, the host/authority header is altered such that *-shadow* is appended. This is
   // useful for logging. For example, *cluster1* becomes *cluster1-shadow*.
+  //
+  // .. note::
+  //
+  //   Shadowing will not be triggered if the primary cluster does not exist.
   message RequestMirrorPolicy {
     // Specifies the cluster that requests will be mirrored to. The cluster must
     // exist in the cluster manager configuration.
diff --git a/envoy/config/route/v3/route_components.proto b/envoy/config/route/v3/route_components.proto
index 856d8cad..26bf3c5d 100644
--- a/envoy/config/route/v3/route_components.proto
+++ b/envoy/config/route/v3/route_components.proto
@@ -512,6 +512,10 @@ message RouteAction {
   //
   // During shadowing, the host/authority header is altered such that *-shadow* is appended. This is
   // useful for logging. For example, *cluster1* becomes *cluster1-shadow*.
+  //
+  // .. note::
+  //
+  //   Shadowing will not be triggered if the primary cluster does not exist.
   message RequestMirrorPolicy {
     option (udpa.annotations.versioning).previous_message_type =
         "envoy.api.v2.route.RouteAction.RequestMirrorPolicy";