diff --git a/src/knx/cemi_frame.cpp b/src/knx/cemi_frame.cpp
index 13e7c79..58b1385 100644
--- a/src/knx/cemi_frame.cpp
+++ b/src/knx/cemi_frame.cpp
@@ -380,3 +380,14 @@ bool CemiFrame::valid() const
 
     return true;
 }
+
+void CemiFrame::sourceInterface(uint8_t index)
+{
+    _sourceInterfaceIndex = index;
+}
+
+uint8_t CemiFrame::sourceInterface()
+{
+    return _sourceInterfaceIndex;
+}
+
diff --git a/src/knx/cemi_frame.h b/src/knx/cemi_frame.h
index fc21501..b394d4b 100644
--- a/src/knx/cemi_frame.h
+++ b/src/knx/cemi_frame.h
@@ -57,6 +57,9 @@ class CemiFrame
     uint16_t destinationAddress() const;
     void destinationAddress(uint16_t value);
 
+    void sourceInterface(uint8_t index);
+    uint8_t sourceInterface();
+
 #ifdef USE_RF
     // only for RF medium
     uint8_t* rfSerialOrDoA() const;
@@ -89,4 +92,6 @@ class CemiFrame
     uint8_t  _rfInfo = 0;
     uint8_t  _rfLfn = 0xFF; // RF Data Link layer frame number
  #endif
-};
\ No newline at end of file
+
+    uint8_t _sourceInterfaceIndex;
+};
diff --git a/src/knx/network_layer.cpp b/src/knx/network_layer.cpp
index 02455b2..01b509b 100644
--- a/src/knx/network_layer.cpp
+++ b/src/knx/network_layer.cpp
@@ -5,7 +5,7 @@
 #include "bits.h"
 
 NetworkLayer::NetworkLayer(DeviceObject &deviceObj, TransportLayer& layer) :
-    _netLayerEntities {*this, *this},
+    _netLayerEntities { {*this, 0}, {*this, 1} },
     _transportLayer(layer),
     _deviceObj(deviceObj)
 {
diff --git a/src/knx/network_layer.h b/src/knx/network_layer.h
index 42e1287..d120b4a 100644
--- a/src/knx/network_layer.h
+++ b/src/knx/network_layer.h
@@ -38,6 +38,7 @@ class NetworkLayer
   private:
     uint8_t _hopCount = 6;
 
+    // Support a maximum of two physical interfaces
     NetworkLayerEntity _netLayerEntities[2];
 
     TransportLayer& _transportLayer;
diff --git a/src/knx/network_layer_entity.cpp b/src/knx/network_layer_entity.cpp
index ebdc5bf..c812cc8 100644
--- a/src/knx/network_layer_entity.cpp
+++ b/src/knx/network_layer_entity.cpp
@@ -5,7 +5,7 @@
 #include "data_link_layer.h"
 #include "bits.h"
 
-NetworkLayerEntity::NetworkLayerEntity(NetworkLayer &netLayer) : _netLayer(netLayer)
+NetworkLayerEntity::NetworkLayerEntity(NetworkLayer &netLayer, uint8_t entityIndex) : _netLayer(netLayer), _entityIndex(entityIndex)
 {
 }
 
@@ -16,31 +16,37 @@ void NetworkLayerEntity::dataLinkLayer(DataLinkLayer& layer)
 
 void NetworkLayerEntity::dataIndication(AckType ack, AddressType addrType, uint16_t destination, FrameFormat format, NPDU& npdu, Priority priority, uint16_t source)
 {
+    npdu.frame().sourceInterface(_entityIndex);
     _netLayer.dataIndication(ack, addrType, destination, format, npdu, priority, source);
 }
 
 void NetworkLayerEntity::dataConfirm(AckType ack, AddressType addressType, uint16_t destination, FrameFormat format, Priority priority, uint16_t source, NPDU& npdu, bool status)
 {
+    npdu.frame().sourceInterface(_entityIndex);
     _netLayer.dataConfirm(ack, addressType, destination, format, priority, source, npdu, status);
 }
 
 void NetworkLayerEntity::broadcastIndication(AckType ack, FrameFormat format, NPDU& npdu, Priority priority, uint16_t source)
 {
+    npdu.frame().sourceInterface(_entityIndex);
     _netLayer.broadcastIndication(ack, format, npdu, priority, source);
 }
 
 void NetworkLayerEntity::broadcastConfirm(AckType ack, FrameFormat format, Priority priority, uint16_t source, NPDU& npdu, bool status)
 {
+    npdu.frame().sourceInterface(_entityIndex);
     _netLayer.broadcastConfirm(ack, format, priority, source, npdu, status);
 }
 
 void NetworkLayerEntity::systemBroadcastIndication(AckType ack, FrameFormat format, NPDU& npdu, Priority priority, uint16_t source)
 {
+    npdu.frame().sourceInterface(_entityIndex);
     _netLayer.systemBroadcastIndication(ack, format, npdu, priority, source);
 }
 
 void NetworkLayerEntity::systemBroadcastConfirm(AckType ack, FrameFormat format, Priority priority, uint16_t source, NPDU& npdu, bool status)
 {
+    npdu.frame().sourceInterface(_entityIndex);
     _netLayer.systemBroadcastConfirm(ack, format, priority, source, npdu, status);
 }
 
diff --git a/src/knx/network_layer_entity.h b/src/knx/network_layer_entity.h
index 576883a..ec4b683 100644
--- a/src/knx/network_layer_entity.h
+++ b/src/knx/network_layer_entity.h
@@ -10,7 +10,7 @@ class NetworkLayer;
 class NetworkLayerEntity
 {
   public:
-    NetworkLayerEntity(NetworkLayer &netLayer);
+    NetworkLayerEntity(NetworkLayer &netLayer, uint8_t entityIndex);
 
     void dataLinkLayer(DataLinkLayer& layer);
 
@@ -35,4 +35,5 @@ class NetworkLayerEntity
   private:
     DataLinkLayer* _dataLinkLayer = 0;
     NetworkLayer& _netLayer;
+    uint8_t _entityIndex;
 };