diff --git a/cloudshell/networking/cisco/command_actions/iface_actions.py b/cloudshell/networking/cisco/command_actions/iface_actions.py
index 7639b7f..71d032b 100644
--- a/cloudshell/networking/cisco/command_actions/iface_actions.py
+++ b/cloudshell/networking/cisco/command_actions/iface_actions.py
@@ -12,6 +12,9 @@
     iface,
 )
 
+# Pattern to match VLAN 1 configuration (default state that should be preserved)
+VLAN_1_PATTERN = r"^\s*switchport\s+trunk\s+allowed\s+vlan\s+1\s*$"
+
 
 class IFaceActions:
     def __init__(self, cli_service, logger):
@@ -114,6 +117,9 @@ def clean_interface_switchport_config(
 
         for line in current_config.splitlines():
             if line.strip(" ").startswith("switchport "):
+                # Skip removing "switchport trunk allowed vlan 1" as it's the default state
+                if re.match(VLAN_1_PATTERN, line, re.IGNORECASE):
+                    continue
                 line_to_remove = re.sub(r"\s+\d+[-\d+,]+", "", line).strip(" ")
                 CommandTemplateExecutor(
                     self._cli_service,
diff --git a/tests/networking/cisco/command_actions/test_iface_actions.py b/tests/networking/cisco/command_actions/test_iface_actions.py
index 996013d..bbcf294 100644
--- a/tests/networking/cisco/command_actions/test_iface_actions.py
+++ b/tests/networking/cisco/command_actions/test_iface_actions.py
@@ -58,3 +58,103 @@ def test_get_no_l2_protocol_tunnel_cmd(self, vlan_templates_mock, cte_mock):
             error_map=None,
         )
         self.assertEqual(result, cte_mock.return_value)
+
+    @patch(
+        "cloudshell.networking.cisco.command_actions.iface_actions"
+        ".CommandTemplateExecutor"
+    )
+    def test_clean_interface_switchport_config_preserves_vlan_1(self, cte_mock):
+        """Test that switchport trunk allowed vlan 1 is not removed (default state)."""
+        current_config = """Building configuration...
+
+Current configuration : 144 bytes
+!
+interface GigabitEthernet110/1/0/6
+ description KG-255X-06-PT
+ switchport
+ switchport trunk allowed vlan 1
+ switchport mode dynamic auto
+end
+"""
+        executor_mock = MagicMock()
+        cte_mock.return_value = executor_mock
+
+        self._handler.clean_interface_switchport_config(current_config)
+
+        # Verify that execute_command was called for other switchport lines but not for vlan 1
+        calls = executor_mock.execute_command.call_args_list
+        # Should be called once for "switchport mode dynamic auto"
+        # but NOT for "switchport trunk allowed vlan 1"
+        # Note: "switchport" alone (without trailing space) is not matched by the pattern
+        self.assertEqual(len(calls), 1)
+
+        # Verify the command that was issued
+        called_commands = [call[1]["command"] for call in calls]
+        self.assertIn("switchport mode dynamic auto", called_commands)
+        # Ensure vlan 1 line was NOT processed (would appear as "switchport trunk allowed vlan" after regex)
+        for cmd in called_commands:
+            self.assertNotIn("trunk allowed vlan", cmd)
+
+    @patch(
+        "cloudshell.networking.cisco.command_actions.iface_actions"
+        ".CommandTemplateExecutor"
+    )
+    def test_clean_interface_switchport_config_removes_other_vlans(self, cte_mock):
+        """Test that switchport trunk allowed vlan with other VLANs are removed."""
+        current_config = """Building configuration...
+
+Current configuration : 144 bytes
+!
+interface GigabitEthernet110/1/0/6
+ description KG-255X-06-PT
+ switchport
+ switchport trunk allowed vlan 100
+ switchport mode trunk
+end
+"""
+        executor_mock = MagicMock()
+        cte_mock.return_value = executor_mock
+
+        self._handler.clean_interface_switchport_config(current_config)
+
+        # Verify that execute_command was called for all switchport lines including vlan 100
+        calls = executor_mock.execute_command.call_args_list
+        # Should be called twice: "switchport trunk allowed vlan", "switchport mode trunk"
+        # Note: "switchport" alone (without trailing space) is not matched by the pattern
+        self.assertEqual(len(calls), 2)
+
+        # Verify the commands that were issued
+        called_commands = [call[1]["command"] for call in calls]
+        self.assertIn("switchport trunk allowed vlan", called_commands)
+        self.assertIn("switchport mode trunk", called_commands)
+
+    @patch(
+        "cloudshell.networking.cisco.command_actions.iface_actions"
+        ".CommandTemplateExecutor"
+    )
+    def test_clean_interface_switchport_config_removes_vlan_1_in_range(self, cte_mock):
+        """Test that VLAN 1 in a range or list is still removed (not default state)."""
+        current_config = """Building configuration...
+
+Current configuration : 144 bytes
+!
+interface GigabitEthernet110/1/0/6
+ description Test Port
+ switchport trunk allowed vlan 1,2,3
+ switchport mode trunk
+end
+"""
+        executor_mock = MagicMock()
+        cte_mock.return_value = executor_mock
+
+        self._handler.clean_interface_switchport_config(current_config)
+
+        # Verify that execute_command was called for VLAN range including vlan 1
+        calls = executor_mock.execute_command.call_args_list
+        # Should be called twice: "switchport trunk allowed vlan", "switchport mode trunk"
+        self.assertEqual(len(calls), 2)
+
+        # Verify the commands that were issued - vlan 1,2,3 should be removed
+        called_commands = [call[1]["command"] for call in calls]
+        self.assertIn("switchport trunk allowed vlan", called_commands)
+        self.assertIn("switchport mode trunk", called_commands)