diff --git a/proxy/proxy.go b/proxy/proxy.go
index 2de0161d01..97143f6575 100644
--- a/proxy/proxy.go
+++ b/proxy/proxy.go
@@ -169,10 +169,11 @@ func InitPassiveHealthChecker(o map[string]string) (bool, *PassiveHealthCheck, e
 	}
 
 	result := &PassiveHealthCheck{}
-	keysInitialized := make(map[string]struct{})
+	requiredKeysInitialized := make(map[string]struct{})
 
 	for key, value := range o {
 		switch key {
+		/* required parameters */
 		case "period":
 			period, err := time.ParseDuration(value)
 			if err != nil {
@@ -182,6 +183,7 @@ func InitPassiveHealthChecker(o map[string]string) (bool, *PassiveHealthCheck, e
 				return false, nil, fmt.Errorf("passive health check: invalid period value: %s", value)
 			}
 			result.Period = period
+			requiredKeysInitialized[key] = struct{}{}
 		case "min-requests":
 			minRequests, err := strconv.Atoi(value)
 			if err != nil {
@@ -191,15 +193,7 @@ func InitPassiveHealthChecker(o map[string]string) (bool, *PassiveHealthCheck, e
 				return false, nil, fmt.Errorf("passive health check: invalid minRequests value: %s", value)
 			}
 			result.MinRequests = int64(minRequests)
-		case "min-drop-probability":
-			minDropProbability, err := strconv.ParseFloat(value, 64)
-			if err != nil {
-				return false, nil, fmt.Errorf("passive health check: invalid minDropProbability value: %s", value)
-			}
-			if minDropProbability < 0 || minDropProbability > 1 {
-				return false, nil, fmt.Errorf("passive health check: invalid minDropProbability value: %s", value)
-			}
-			result.MinDropProbability = minDropProbability
+			requiredKeysInitialized[key] = struct{}{}
 		case "max-drop-probability":
 			maxDropProbability, err := strconv.ParseFloat(value, 64)
 			if err != nil {
@@ -209,14 +203,24 @@ func InitPassiveHealthChecker(o map[string]string) (bool, *PassiveHealthCheck, e
 				return false, nil, fmt.Errorf("passive health check: invalid maxDropProbability value: %s", value)
 			}
 			result.MaxDropProbability = maxDropProbability
+			requiredKeysInitialized[key] = struct{}{}
+
+		/* optional parameters */
+		case "min-drop-probability":
+			minDropProbability, err := strconv.ParseFloat(value, 64)
+			if err != nil {
+				return false, nil, fmt.Errorf("passive health check: invalid minDropProbability value: %s", value)
+			}
+			if minDropProbability < 0 || minDropProbability > 1 {
+				return false, nil, fmt.Errorf("passive health check: invalid minDropProbability value: %s", value)
+			}
+			result.MinDropProbability = minDropProbability
 		default:
 			return false, nil, fmt.Errorf("passive health check: invalid parameter: key=%s,value=%s", key, value)
 		}
-
-		keysInitialized[key] = struct{}{}
 	}
 
-	if len(keysInitialized) != 4 {
+	if len(requiredKeysInitialized) != 3 {
 		return false, nil, fmt.Errorf("passive health check: missing required parameters")
 	}
 	if result.MinDropProbability >= result.MaxDropProbability {
diff --git a/proxy/proxy_test.go b/proxy/proxy_test.go
index ea3cc79463..f4bbf6ad52 100644
--- a/proxy/proxy_test.go
+++ b/proxy/proxy_test.go
@@ -2454,6 +2454,22 @@ func TestInitPassiveHealthChecker(t *testing.T) {
 			expectedParams:  nil,
 			expectedError:   fmt.Errorf("passive health check: missing required parameters"),
 		},
+		{
+			inputArg: map[string]string{
+				"period":               "1m",
+				"min-requests":         "10",
+				"max-drop-probability": "0.9",
+				/* using default max-drop-probability */
+			},
+			expectedEnabled: true,
+			expectedParams: &PassiveHealthCheck{
+				Period:             1 * time.Minute,
+				MinRequests:        10,
+				MaxDropProbability: 0.9,
+				MinDropProbability: 0.0,
+			},
+			expectedError: nil,
+		},
 	} {
 		t.Run(fmt.Sprintf("case_%d", i), func(t *testing.T) {
 			enabled, params, err := InitPassiveHealthChecker(ti.inputArg)