diff --git a/ggsn_tests/GGSN_Tests.ttcn b/ggsn_tests/GGSN_Tests.ttcn
index 634534f..63e0696 100644
--- a/ggsn_tests/GGSN_Tests.ttcn
+++ b/ggsn_tests/GGSN_Tests.ttcn
@@ -35,6 +35,11 @@
 	const integer GTP1C_PORT := 2123;
 	const integer GTP1U_PORT := 2152;
 
+	type enumerated GGSN_Impl {
+		GGSN_IMPL_OSMOCOM,
+		GGSN_IMPL_OPEN5GS
+	};
+
 	modulepar {
 		/* Default IP addresses. May be overridden by GGSN_Tests configuration files. */
 
@@ -65,6 +70,8 @@
 		 * The tests expect to be able to send ping packets between any two simulated MS within the same
 		 * address range. This requires IP forwarding to be enabled on the corresponding tun interfaces.
 		 */
+
+		GGSN_Impl m_ggsn_impl := GGSN_IMPL_OSMOCOM;
 	}
 
 	type set PdpContext {
@@ -197,9 +204,11 @@
 		g_c_seq_nr := f_rnd_int(65535);
 		g_d_seq_nr := f_rnd_int(65535);
 
-		f_init_vty();
-		f_vty_set_gpdu_txseq(use_gtpu_txseq);
-		f_vty_enable_echo_interval(g_use_echo);
+		if (m_ggsn_impl == GGSN_IMPL_OSMOCOM) {
+			f_init_vty();
+			f_vty_set_gpdu_txseq(use_gtpu_txseq);
+			f_vty_enable_echo_interval(g_use_echo);
+		}
 	}
 
 	/* Altstep implementing responses to any incoming echo requests */