diff --git a/README.md b/README.md
index 79a7042..02ed57f 100644
--- a/README.md
+++ b/README.md
@@ -17,6 +17,8 @@ sudo pip3 install argparse
 sudo pip3 install yamale
 sudo pip3 install pyyaml
 sudo pip3 install pyinstaller
+sudo pip3 install netaddr
+sudo pip3 install ipaddress
 
 ## Ensure all unittests pass.
 ./tests.py -d -t unittest/yaml/*.yaml
diff --git a/config/interface.py b/config/interface.py
index 7e79a62..de8b35f 100644
--- a/config/interface.py
+++ b/config/interface.py
@@ -18,6 +18,7 @@ import config.loopback as loopback
 import config.vxlan_tunnel as vxlan_tunnel
 import config.lcp as lcp
 import config.address as address
+import config.mac as mac
 
 def get_qinx_parent_by_name(yaml, ifname):
     """ Returns the sub-interface which matches a QinAD or QinQ outer tag, or None,None
@@ -424,6 +425,10 @@ def validate_interfaces(yaml):
         if not 'state' in iface:
             iface['state'] = 'up'
 
+        if 'mac' in iface and mac.is_multicast(iface['mac']):
+            msgs.append("interface %s MAC address %s cannot be multicast" % (ifname, iface['mac']))
+            result = False
+
         iface_mtu = get_mtu(yaml, ifname)
         iface_lcp = get_lcp(yaml, ifname)
         iface_address = has_address(yaml, ifname)
diff --git a/config/mac.py b/config/mac.py
new file mode 100644
index 0000000..ba7a05b
--- /dev/null
+++ b/config/mac.py
@@ -0,0 +1,48 @@
+#
+# Copyright (c) 2022 Pim van Pelt
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at:
+#     http://www.apache.org/licenses/LICENSE-2.0
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+import logging
+import netaddr
+
+def is_valid(mac):
+    """ Return True if the string given in `mac` is a valid (6-byte) MAC address,
+        as defined by netaddr.EUI """
+    try:
+        addr = netaddr.EUI(mac)
+    except:
+        return False
+    return True
+
+def is_local(mac):
+    """ Return True if a MAC address is a valid locally administered one. """
+    try:
+        addr = netaddr.EUI(mac)
+    except:
+        return False
+    return bool(addr.words[0] & 0b10)
+
+def is_multicast(mac):
+    """ Return True if a MAC address is a valid multicast one. """
+    try:
+        addr = netaddr.EUI(mac)
+    except:
+        return False
+    return bool(addr.words[0] & 0b01)
+
+def is_unicast(mac):
+    """ Return True if a MAC address is a valid unicast one. """
+    try:
+        addr = netaddr.EUI(mac)
+    except:
+        return False
+    return not bool(addr.words[0] & 0b01)
diff --git a/config/test_mac.py b/config/test_mac.py
new file mode 100644
index 0000000..f9b194a
--- /dev/null
+++ b/config/test_mac.py
@@ -0,0 +1,21 @@
+import unittest
+import config.mac as mac
+
+class TestMACMethods(unittest.TestCase):
+    def test_is_valid(self):
+        self.assertTrue(mac.is_valid("00:01:02:03:04:05"))
+        self.assertTrue(mac.is_valid("00-01-02-03-04-05"))
+        self.assertTrue(mac.is_valid("0001.0203.0405"))
+        self.assertFalse(mac.is_valid("hoi"))
+
+    def test_is_local(self):
+        self.assertTrue(mac.is_local("02:00:00:00:00:00"))
+        self.assertFalse(mac.is_local("00:00:00:00:00:00"))
+
+    def test_is_multicast(self):
+        self.assertTrue(mac.is_multicast("01:00:00:00:00:00"))
+        self.assertFalse(mac.is_multicast("00:00:00:00:00:00"))
+
+    def test_is_unicast(self):
+        self.assertFalse(mac.is_unicast("01:00:00:00:00:00"))
+        self.assertTrue(mac.is_unicast("00:00:00:00:00:00"))
diff --git a/unittest/yaml/error-interface1.yaml b/unittest/yaml/error-interface1.yaml
new file mode 100644
index 0000000..15e5cbd
--- /dev/null
+++ b/unittest/yaml/error-interface1.yaml
@@ -0,0 +1,17 @@
+test:
+  description: "Interface mac addresses cannot be multicast"
+  errors:
+    expected:
+     - "interface .* MAC address .* cannot be multicast"
+    count: 1
+---
+interfaces:
+  GigabitEthernet3/0/0:
+    description: "Cool, local MACs are fine"
+    mac: 02:00:00:00:00:00
+  GigabitEthernet3/0/1:
+    description: "Cool, global unicast MACs are fine"
+    mac: 04:00:00:00:00:00
+  GigabitEthernet3/0/2:
+    description: "Not cool, multicast MACs"
+    mac: 01:00:00:00:00:00