diff --git a/tests/cts/net/src/android/net/cts/EthernetManagerTest.kt b/tests/cts/net/src/android/net/cts/EthernetManagerTest.kt
index db24b44..e8d7115 100644
--- a/tests/cts/net/src/android/net/cts/EthernetManagerTest.kt
+++ b/tests/cts/net/src/android/net/cts/EthernetManagerTest.kt
@@ -122,7 +122,7 @@
                 val tnm = context.getSystemService(TestNetworkManager::class.java)
                 tnm.createTapInterface(false /* bringUp */)
             }
-            val mtu = 1500
+            val mtu = tapInterface.mtu
             packetReader = TapPacketReader(handler, tapInterface.fileDescriptor.fileDescriptor, mtu)
             raResponder = RouterAdvertisementResponder(packetReader)
             raResponder.addRouterEntry(MacAddress.fromString("01:23:45:67:89:ab"),