diff --git a/charms_ceph/utils.py b/charms_ceph/utils.py
index de917a0..1894cf4 100644
--- a/charms_ceph/utils.py
+++ b/charms_ceph/utils.py
@@ -2186,6 +2186,20 @@ def roll_monitor_cluster(new_version, upgrade_key):
             wait_for_all_monitors_to_upgrade(new_version=new_version,
                                              upgrade_key=upgrade_key)
             bootstrap_manager()
+
+        # NOTE(jmcvaughn):
+        # Nautilus and later binaries use msgr2 by default, but existing
+        # clusters that have been upgraded from pre-Nautilus will not
+        # automatically have msgr2 enabled. Without this, Ceph will show
+        # a warning only (with no impact to operations), but newly added units
+        # will not be able to join the cluster. Therefore, we ensure it is
+        # enabled on upgrade for all versions including and after Nautilus
+        # (to cater for previous charm versions that will not have done this).
+        nautilus_or_later = cmp_pkgrevno('ceph-common', '14.0.0') >= 0
+        if nautilus_or_later:
+            wait_for_all_monitors_to_upgrade(new_version=new_version,
+                                             upgrade_key=upgrade_key)
+            enable_msgr2()
     except ValueError:
         log("Failed to find {} in list {}.".format(
             my_name, mon_sorted_list))
@@ -3332,6 +3346,16 @@ def bootstrap_manager():
         service_restart(unit)
 
 
+def enable_msgr2():
+    """
+    Enables msgr2
+
+    :raises: subprocess.CalledProcessError if the command fails
+    """
+    cmd = ['ceph', 'mon', 'enable-msgr2']
+    subprocess.check_call(cmd)
+
+
 def osd_noout(enable):
     """Sets or unsets 'noout'
 
diff --git a/unit_tests/test_mon_upgrade_roll.py b/unit_tests/test_mon_upgrade_roll.py
index bc37850..9d8f00e 100644
--- a/unit_tests/test_mon_upgrade_roll.py
+++ b/unit_tests/test_mon_upgrade_roll.py
@@ -299,12 +299,13 @@ class UpgradeRollingTestCase(unittest.TestCase):
                                          service='mon',
                                          upgrade_key='admin',
                                          version=new_version)
-        if new_version == 'luminous':
+        if new_version in ['luminous', 'mimic']:
             wait_for_all_monitors_to_upgrade.assert_called_with(
                 new_version=new_version,
                 upgrade_key='admin',
             )
-            bootstrap_manager.assert_called_once_with()
+            if new_version == 'luminous':
+                bootstrap_manager.assert_called_once_with()
         else:
             wait_for_all_monitors_to_upgrade.assert_not_called()
             bootstrap_manager.assert_not_called()
@@ -312,12 +313,23 @@ class UpgradeRollingTestCase(unittest.TestCase):
         upgrade_monitor.assert_has_calls([
             call(new_version, restart_daemons=False)])
 
-    def test_roll_monitor_cluster_luminous(self):
-        self._test_roll_monitor_cluster(new_version='luminous')
+    @patch.object(charms_ceph.utils, 'cmp_pkgrevno', lambda _x, _y: 1)
+    @patch.object(charms_ceph.utils, 'enable_msgr2')
+    def test_roll_monitor_cluster_mimic(self, mock_enable_msgr2):
+        self._test_roll_monitor_cluster(new_version='mimic')
+        mock_enable_msgr2.assert_called_once()
 
+    @patch.object(charms_ceph.utils, 'cmp_pkgrevno', lambda _x, _y: 1)
+    @patch.object(charms_ceph.utils, 'enable_msgr2')
+    def test_roll_monitor_cluster_luminous(self, mock_enable_msgr2):
+        self._test_roll_monitor_cluster(new_version='luminous')
+        mock_enable_msgr2.assert_called_once()
+
+    @patch.object(charms_ceph.utils, 'cmp_pkgrevno', lambda _x, _y: -1)
     def test_roll_monitor_cluster_jewel(self):
         self._test_roll_monitor_cluster(new_version='jewel')
 
+    @patch.object(charms_ceph.utils, 'cmp_pkgrevno', lambda _x, _y: -1)
     def test_roll_monitor_cluster_hammer(self):
         self._test_roll_monitor_cluster(new_version='hammer')