diff --git a/consul/client.go b/consul/client.go
index d10db8550..c41cc78a2 100644
--- a/consul/client.go
+++ b/consul/client.go
@@ -3,8 +3,6 @@ package consul
 import (
 	"crypto/tls"
 	"fmt"
-	"github.com/hashicorp/consul/consul/structs"
-	"github.com/hashicorp/serf/serf"
 	"log"
 	"math/rand"
 	"os"
@@ -13,6 +11,9 @@ import (
 	"strings"
 	"sync"
 	"time"
+
+	"github.com/hashicorp/consul/consul/structs"
+	"github.com/hashicorp/serf/serf"
 )
 
 const (
@@ -138,6 +139,7 @@ func (c *Client) setupSerf(conf *serf.Config, ch chan serf.Event, path string) (
 	conf.SnapshotPath = filepath.Join(c.config.DataDir, path)
 	conf.ProtocolVersion = protocolVersionMap[c.config.ProtocolVersion]
 	conf.RejoinAfterLeave = c.config.RejoinAfterLeave
+	conf.Merge = &lanMergeDelegate{logger: c.logger, dc: c.config.Datacenter}
 	if err := ensurePath(conf.SnapshotPath, false); err != nil {
 		return nil, err
 	}
diff --git a/consul/client_test.go b/consul/client_test.go
index 33425cdf7..c799696ab 100644
--- a/consul/client_test.go
+++ b/consul/client_test.go
@@ -99,6 +99,56 @@ func TestClient_JoinLAN(t *testing.T) {
 	})
 }
 
+func TestClient_JoinLAN_Invalid(t *testing.T) {
+	dir1, s1 := testServer(t)
+	defer os.RemoveAll(dir1)
+	defer s1.Shutdown()
+
+	dir2, c1 := testClientDC(t, "other")
+	defer os.RemoveAll(dir2)
+	defer c1.Shutdown()
+
+	// Try to join
+	addr := fmt.Sprintf("127.0.0.1:%d",
+		s1.config.SerfLANConfig.MemberlistConfig.BindPort)
+	if _, err := c1.JoinLAN([]string{addr}); err == nil {
+		t.Fatalf("should error")
+	}
+
+	time.Sleep(50 * time.Millisecond)
+	if len(s1.LANMembers()) != 1 {
+		t.Fatalf("should not join")
+	}
+	if len(c1.LANMembers()) != 1 {
+		t.Fatalf("should not join")
+	}
+}
+
+func TestClient_JoinWAN_Invalid(t *testing.T) {
+	dir1, s1 := testServer(t)
+	defer os.RemoveAll(dir1)
+	defer s1.Shutdown()
+
+	dir2, c1 := testClientDC(t, "dc2")
+	defer os.RemoveAll(dir2)
+	defer c1.Shutdown()
+
+	// Try to join
+	addr := fmt.Sprintf("127.0.0.1:%d",
+		s1.config.SerfWANConfig.MemberlistConfig.BindPort)
+	if _, err := c1.JoinLAN([]string{addr}); err == nil {
+		t.Fatalf("should error")
+	}
+
+	time.Sleep(50 * time.Millisecond)
+	if len(s1.WANMembers()) != 1 {
+		t.Fatalf("should not join")
+	}
+	if len(c1.LANMembers()) != 1 {
+		t.Fatalf("should not join")
+	}
+}
+
 func TestClient_RPC(t *testing.T) {
 	dir1, s1 := testServer(t)
 	defer os.RemoveAll(dir1)
diff --git a/consul/merge.go b/consul/merge.go
new file mode 100644
index 000000000..0efcb0cd3
--- /dev/null
+++ b/consul/merge.go
@@ -0,0 +1,56 @@
+package consul
+
+import (
+	"log"
+
+	"github.com/hashicorp/serf/serf"
+)
+
+// lanMergeDelegate is used to handle a cluster merge on the LAN gossip
+// ring. We check that the peers are in the same datacenter and abort the
+// merge if there is a mis-match.
+type lanMergeDelegate struct {
+	logger *log.Logger
+	dc     string
+}
+
+func (md *lanMergeDelegate) NotifyMerge(members []*serf.Member) (cancel bool) {
+	for _, m := range members {
+		ok, dc := isConsulNode(*m)
+		if ok {
+			if dc != md.dc {
+				md.logger.Printf("[WARN] consul: Canceling cluster merge, member '%s' part of wrong datacenter '%s'",
+					m.Name, dc)
+				return true
+			}
+			continue
+		}
+
+		ok, parts := isConsulServer(*m)
+		if ok && parts.Datacenter != md.dc {
+			md.logger.Printf("[WARN] consul: Canceling cluster merge, member '%s' part of wrong datacenter '%s'",
+				m.Name, parts.Datacenter)
+			return true
+		}
+	}
+	return false
+}
+
+// wanMergeDelegate is used to handle a cluster merge on the WAN gossip
+// ring. We check that the peers are server nodes and abort the merge
+// otherwise.
+type wanMergeDelegate struct {
+	logger *log.Logger
+}
+
+func (md *wanMergeDelegate) NotifyMerge(members []*serf.Member) (cancel bool) {
+	for _, m := range members {
+		ok, _ := isConsulServer(*m)
+		if !ok {
+			md.logger.Printf("[WARN] consul: Canceling cluster merge, member '%s' is not a server",
+				m.Name)
+			return true
+		}
+	}
+	return false
+}
diff --git a/consul/server.go b/consul/server.go
index 88096a175..6a267025e 100644
--- a/consul/server.go
+++ b/consul/server.go
@@ -302,6 +302,11 @@ func (s *Server) setupSerf(conf *serf.Config, ch chan serf.Event, path string, w
 	conf.SnapshotPath = filepath.Join(s.config.DataDir, path)
 	conf.ProtocolVersion = protocolVersionMap[s.config.ProtocolVersion]
 	conf.RejoinAfterLeave = s.config.RejoinAfterLeave
+	if wan {
+		conf.Merge = &wanMergeDelegate{logger: s.logger}
+	} else {
+		conf.Merge = &lanMergeDelegate{logger: s.logger, dc: s.config.Datacenter}
+	}
 
 	// Until Consul supports this fully, we disable automatic resolution.
 	// When enabled, the Serf gossip may just turn off if we are the minority