diff --git a/pkg/plugins/gateway/gateway.go b/pkg/plugins/gateway/gateway.go
index 2d4d0124..21db7270 100644
--- a/pkg/plugins/gateway/gateway.go
+++ b/pkg/plugins/gateway/gateway.go
@@ -142,6 +142,7 @@ func (s *Server) HandleRequestHeaders(ctx context.Context, requestID string, req
 	klog.Info("\n\n")
 	klog.Info("-- In RequestHeaders processing ...")
 	var username, routingStrategy string
+	var headerBasedRoutingStrategyEnabled bool
 	var user utils.User
 	var rpm int64
 	var err error
@@ -154,14 +155,11 @@ func (s *Server) HandleRequestHeaders(ctx context.Context, requestID string, req
 		}
 		if strings.ToLower(n.Key) == "routing-strategy" {
 			routingStrategy = string(n.RawValue)
+			headerBasedRoutingStrategyEnabled = true
 		}
 	}
 
-	if routingStrategy == "" {
-		routingStrategy = utils.GetEnv("ROUTING_ALGORITHM", "")
-	}
-
-	if !slices.Contains(routingStrategies, routingStrategy) {
+	if !validateRoutingStrategy(routingStrategy, headerBasedRoutingStrategyEnabled) {
 		return generateErrorResponse(
 			envoyTypePb.StatusCode_BadRequest,
 			[]*configPb.HeaderValueOption{{Header: &configPb.HeaderValue{
@@ -480,6 +478,23 @@ func (s *Server) selectTargetPod(ctx context.Context, routingStrategy string, po
 	return route.Route(ctx, pods)
 }
 
+func validateRoutingStrategy(routingStrategy string, headerBasedRoutingStrategyEnabled bool) bool {
+	routingStrategy = strings.TrimSpace(routingStrategy)
+	if headerBasedRoutingStrategyEnabled && routingStrategy == "" {
+		return false
+	}
+
+	if routingStrategy == "" {
+		routingStrategy = utils.GetEnv("ROUTING_ALGORITHM", "")
+	}
+
+	if routingStrategy != "" && !slices.Contains(routingStrategies, routingStrategy) {
+		return false
+	}
+
+	return true
+}
+
 func generateErrorResponse(statusCode envoyTypePb.StatusCode, headers []*configPb.HeaderValueOption, body string) *extProcPb.ProcessingResponse {
 	return &extProcPb.ProcessingResponse{
 		Response: &extProcPb.ProcessingResponse_ImmediateResponse{
diff --git a/pkg/plugins/gateway/gateway_test.go b/pkg/plugins/gateway/gateway_test.go
new file mode 100644
index 00000000..6a637034
--- /dev/null
+++ b/pkg/plugins/gateway/gateway_test.go
@@ -0,0 +1,106 @@
+/*
+Copyright 2024 The Aibrix Team.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+	http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package gateway
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_ValidateRoutingStrategy(t *testing.T) {
+	var tests = []struct {
+		headerBasedRoutingStrategyEnabled bool
+		routingStrategy                   string
+		setEnvRoutingStrategy             bool
+		message                           string
+		expectedValidation                bool
+	}{
+		{
+			headerBasedRoutingStrategyEnabled: true,
+			routingStrategy:                   "",
+			setEnvRoutingStrategy:             false,
+			message:                           "empty routing strategy in header",
+			expectedValidation:                false,
+		},
+		{
+			headerBasedRoutingStrategyEnabled: true,
+			routingStrategy:                   "  ",
+			setEnvRoutingStrategy:             false,
+			message:                           "spaced routing strategy in header",
+			expectedValidation:                false,
+		},
+		{
+			headerBasedRoutingStrategyEnabled: false,
+			routingStrategy:                   "",
+			setEnvRoutingStrategy:             true,
+			message:                           "empty routing strategy in header with env var set",
+			expectedValidation:                true,
+		},
+		{
+			headerBasedRoutingStrategyEnabled: false,
+			routingStrategy:                   "  ",
+			setEnvRoutingStrategy:             true,
+			message:                           "spaced routing strategy in header with env var set",
+			expectedValidation:                true,
+		},
+		{
+			headerBasedRoutingStrategyEnabled: true,
+			routingStrategy:                   "least-request",
+			setEnvRoutingStrategy:             false,
+			message:                           "header routing strategy least-request",
+			expectedValidation:                true,
+		},
+		{
+			headerBasedRoutingStrategyEnabled: true,
+			routingStrategy:                   "rrandom",
+			setEnvRoutingStrategy:             false,
+			message:                           "header routing strategy invalid",
+			expectedValidation:                false,
+		},
+		{
+			headerBasedRoutingStrategyEnabled: false,
+			routingStrategy:                   "random",
+			setEnvRoutingStrategy:             true,
+			message:                           "env routing strategy",
+			expectedValidation:                true,
+		},
+		{
+			headerBasedRoutingStrategyEnabled: false,
+			routingStrategy:                   "rrandom",
+			setEnvRoutingStrategy:             true,
+			message:                           "incorrect env routing strategy",
+			expectedValidation:                false,
+		},
+		{
+			headerBasedRoutingStrategyEnabled: true,
+			routingStrategy:                   "random",
+			setEnvRoutingStrategy:             true,
+			message:                           "per request overrides env",
+			expectedValidation:                true,
+		},
+	}
+
+	for _, tt := range tests {
+		if tt.setEnvRoutingStrategy {
+			t.Setenv("ROUTING_ALGORITHM", "least-request")
+		}
+
+		currentValidation := validateRoutingStrategy(tt.routingStrategy, tt.headerBasedRoutingStrategyEnabled)
+		assert.Equal(t, tt.expectedValidation, currentValidation, tt.message)
+	}
+}