diff --git a/src/kernel/mds_client.c b/src/kernel/mds_client.c
index c34b7afe637..8b839f8a5fb 100644
--- a/src/kernel/mds_client.c
+++ b/src/kernel/mds_client.c
@@ -1627,8 +1627,15 @@ void ceph_mdsc_handle_map(struct ceph_mds_client *mdsc, struct ceph_msg *msg)
 	struct ceph_mdsmap *newmap, *oldmap;
 	int from = le32_to_cpu(msg->hdr.src.name.num);
 	int newstate;
+	struct ceph_fsid fsid;
 
-	ceph_decode_need(&p, end, 2*sizeof(__u32), bad);
+	ceph_decode_need(&p, end, sizeof(fsid)+2*sizeof(__u32), bad);
+	ceph_decode_64(&p, fsid.major);
+	ceph_decode_64(&p, fsid.minor);
+	if (!ceph_fsid_equal(&fsid, &mdsc->client->monc.monmap->fsid)) {
+		derr(0, "got mdsmap with wrong fsid\n");
+		return;
+	}
 	ceph_decode_32(&p, epoch);
 	ceph_decode_32(&p, maplen);
 	dout(2, "handle_map epoch %u len %d\n", epoch, (int)maplen);
diff --git a/src/mds/MDS.cc b/src/mds/MDS.cc
index 67c9242b7eb..d9a21a3fac6 100644
--- a/src/mds/MDS.cc
+++ b/src/mds/MDS.cc
@@ -237,7 +237,7 @@ void MDS::send_message_mds(Message *m, int mds)
 {
   // send mdsmap first?
   if (peer_mdsmap_epoch[mds] < mdsmap->get_epoch()) {
-    messenger->send_message(new MMDSMap(mdsmap), 
+    messenger->send_message(new MMDSMap(monmap->fsid, mdsmap), 
 			    mdsmap->get_inst(mds));
     peer_mdsmap_epoch[mds] = mdsmap->get_epoch();
   }
@@ -659,7 +659,7 @@ void MDS::bcast_mds_map()
   for (set<Session*>::const_iterator p = clients.begin();
        p != clients.end();
        ++p) 
-    messenger->send_message(new MMDSMap(mdsmap), (*p)->inst);
+    messenger->send_message(new MMDSMap(monmap->fsid, mdsmap), (*p)->inst);
   last_client_mdsmap_bcast = mdsmap->get_epoch();
 }
 
diff --git a/src/messages/MMDSMap.h b/src/messages/MMDSMap.h
index 064fa737056..0ec0ae41497 100644
--- a/src/messages/MMDSMap.h
+++ b/src/messages/MMDSMap.h
@@ -44,7 +44,8 @@ class MMDSMap : public Message {
     return e;
   }
   */
-
+  
+  ceph_fsid fsid;
   epoch_t epoch;
   bufferlist encoded;
 
@@ -53,8 +54,9 @@ class MMDSMap : public Message {
 
   MMDSMap() : 
     Message(CEPH_MSG_MDS_MAP) {}
-  MMDSMap(MDSMap *mm) :
-    Message(CEPH_MSG_MDS_MAP) {
+  MMDSMap(ceph_fsid &f, MDSMap *mm) :
+    Message(CEPH_MSG_MDS_MAP),
+    fsid(f) {
     epoch = mm->get_epoch();
     mm->encode(encoded);
   }
@@ -67,10 +69,12 @@ class MMDSMap : public Message {
   // marshalling
   void decode_payload() {
     int off = 0;
+    ::_decode(fsid, payload, off);
     ::_decode(epoch, payload, off);
     ::_decode(encoded, payload, off);
   }
   void encode_payload() {
+    ::_encode(fsid, payload);
     ::_encode(epoch, payload);
     ::_encode(encoded, payload);
   }
diff --git a/src/mon/MDSMonitor.cc b/src/mon/MDSMonitor.cc
index af942681bba..ed6dee85739 100644
--- a/src/mon/MDSMonitor.cc
+++ b/src/mon/MDSMonitor.cc
@@ -604,7 +604,7 @@ void MDSMonitor::bcast_latest_mds()
 void MDSMonitor::send_full(entity_inst_t dest)
 {
   dout(11) << "send_full to " << dest << dendl;
-  mon->messenger->send_message(new MMDSMap(&mdsmap), dest);
+  mon->messenger->send_message(new MMDSMap(mon->monmap->fsid, &mdsmap), dest);
 }
 
 void MDSMonitor::send_to_waiting()