diff --git a/ansible_collections/arista/avd/molecule/eos_designs_unit_tests/inventory/group_vars/MLAG_ISIS_TESTS.yml b/ansible_collections/arista/avd/molecule/eos_designs_unit_tests/inventory/group_vars/MLAG_ISIS_TESTS.yml
index 17f1bb5d007..e45d5785a55 100644
--- a/ansible_collections/arista/avd/molecule/eos_designs_unit_tests/inventory/group_vars/MLAG_ISIS_TESTS.yml
+++ b/ansible_collections/arista/avd/molecule/eos_designs_unit_tests/inventory/group_vars/MLAG_ISIS_TESTS.yml
@@ -3,7 +3,6 @@ underlay_routing_protocol: isis
 underlay_isis_authentication_mode: md5
 underlay_isis_authentication_key: $1c$sTNAlR6rKSw=
 
-
 type: l3leaf
 
 l3leaf:
diff --git a/python-avd/pyavd/_eos_designs/structured_config/mlag/__init__.py b/python-avd/pyavd/_eos_designs/structured_config/mlag/__init__.py
index 0777cc4f154..34e0cba7b66 100644
--- a/python-avd/pyavd/_eos_designs/structured_config/mlag/__init__.py
+++ b/python-avd/pyavd/_eos_designs/structured_config/mlag/__init__.py
@@ -108,16 +108,18 @@ def vlan_interfaces(self) -> list | None:
                     "isis_bfd": get(self._hostvars, "underlay_isis_bfd"),
                     "isis_metric": 50,
                     "isis_network_point_to_point": True,
-                    "isis_authentication": {
-                        "both": {
-                            "mode": get(self._hostvars, "underlay_isis_authentication_mode"),
-                            "key": get(self._hostvars, "underlay_isis_authentication_key"),
-                            "key_type": "7",
-                        }
-                    },
                 }
             )
+            if (isis_authentication_mode := get(self._hostvars, "underlay_isis_authentication_mode")) is not None:
+                l3_cfg.setdefault("isis_authentication", {}).setdefault("both", {})["mode"] = isis_authentication_mode
 
+            if (isis_authentication_key := get(self._hostvars, "underlay_isis_authentication_key")) is not None:
+                l3_cfg.setdefault("isis_authentication", {}).setdefault("both", {}).update(
+                    {
+                        "key": isis_authentication_key,
+                        "key_type": "7",
+                    }
+                )
         if self.shared_utils.underlay_multicast:
             l3_cfg["pim"] = {"ipv4": {"sparse_mode": True}}