[SEBA-992] Retrieve old s_tag for BNGPortMapping

Change-Id: I44fe8e7e0c818259e863915627bfa6244aff4469
diff --git a/xos/synchronizer/models/fabric-crossconnect.xproto b/xos/synchronizer/models/fabric-crossconnect.xproto
index 18f0a26..5844106 100644
--- a/xos/synchronizer/models/fabric-crossconnect.xproto
+++ b/xos/synchronizer/models/fabric-crossconnect.xproto
@@ -32,4 +32,7 @@
         unique = True];
     required int32 switch_port = 2 [
         help_text = "Switch port where BNG or other egress point is connected"];
+    optional string old_s_tag = 3 [
+        help_text = "Field for tracking old s-tag of bngportmapping instance",
+        max_length = 1024];
 }