diff --git a/bsc/MSC_ConnectionHandler.ttcn b/bsc/MSC_ConnectionHandler.ttcn
index b3e6af6..fa7cd84 100644
--- a/bsc/MSC_ConnectionHandler.ttcn
+++ b/bsc/MSC_ConnectionHandler.ttcn
@@ -198,6 +198,28 @@
 	return -1;
 }
 
+/* Verify that CSD CRCX/MDCX has the RSL_IE_IPAC_RTP_CSD_FMT IE, and that
+ * inside it the D value is set to RSL_IPA_RTP_CSD_TRAU_BTS. */
+private function f_ipacc_crcx_mdcx_check_rtp_pt_csd(RSL_Message rsl) runs on MSC_ConnHdlr {
+	var SDP_FIELD_PayloadType pt_csd := PT_CSD;
+	var RSL_IE_Body ie;
+
+	if (g_media.bts.rtp_pt != enum2int(pt_csd)) {
+		return;
+	}
+
+	if (f_rsl_find_ie(rsl, RSL_IE_IPAC_RTP_CSD_FMT, ie)) {
+		if (ie.ipa_rtp_csd_fmt.d != RSL_IPA_RTP_CSD_TRAU_BTS) {
+			Misc_Helpers.f_shutdown(__BFILE__, __LINE__, fail,
+						"Rx unexpected IPAC CRCX for CSD with RTP_CSD_FMT IE");
+		}
+		return;
+	}
+
+	Misc_Helpers.f_shutdown(__BFILE__, __LINE__, fail,
+				"Rx unexpected IPAC CRCX for CSD without RTP_CSD_FMT IE");
+}
+
 /* altstep for handling of IPACC media related commands. Activated by as_Media() to test
  * RSL level media handling */
 altstep as_Media_ipacc(RSL_DCHAN_PT rsl_pt := RSL, RSL_DCHAN_PT rsl_pt_ho_target := RSL1) runs on MSC_ConnHdlr {
@@ -224,6 +246,7 @@
 			g_media.bts.local_osmux_cid := omit;
 			g_media.bts.remote_osmux_cid := omit;
 		}
+		f_ipacc_crcx_mdcx_check_rtp_pt_csd(rsl);
 		rsl_pt.send(ts_RSL_IPA_CRCX_ACK(g_chan_nr, g_media.bts.conn_id,
 						f_inet_addr(g_media.bts.bts.host),
 						g_media.bts.bts.port_nr,
@@ -262,6 +285,7 @@
 			g_media.bts.local_osmux_cid := omit;
 			g_media.bts.remote_osmux_cid := omit;
 		}
+		f_ipacc_crcx_mdcx_check_rtp_pt_csd(rsl);
 		rsl_pt.send(ts_RSL_IPA_MDCX_ACK(g_chan_nr, g_media.bts.conn_id,
 						f_inet_addr(g_media.bts.peer.host),
 						g_media.bts.peer.port_nr,