diff --git a/neutron/agent/l3/l3_agent_extensions_manager.py b/neutron/agent/l3/l3_agent_extensions_manager.py
index b3362613e45..9e5bf8452a1 100644
--- a/neutron/agent/l3/l3_agent_extensions_manager.py
+++ b/neutron/agent/l3/l3_agent_extensions_manager.py
@@ -14,10 +14,9 @@
 #    under the License.
 
 from neutron_lib.agent import l3_extension
-from neutron_lib import exceptions
+from neutron_lib.exceptions import l3 as l3_exc
 from oslo_log import log
 
-from neutron._i18n import _
 from neutron.agent import agent_extensions_manager as agent_ext_manager
 from neutron.conf.agent import agent_extensions_manager as agent_ext_mgr_config
 
@@ -27,13 +26,6 @@ LOG = log.getLogger(__name__)
 L3_AGENT_EXT_MANAGER_NAMESPACE = 'neutron.agent.l3.extensions'
 
 
-# TODO(ralonsoh): rehome to neutron_lib.
-class L3ExtensionException(exceptions.NeutronException):
-    message = _('The following L3 agent extensions do not inherit from '
-                '``neutron_lib.agent.l3_extension.L3AgentExtension``: '
-                '%(extensions)s.')
-
-
 def register_opts(conf):
     agent_ext_mgr_config.register_agent_ext_manager_opts(conf)
 
@@ -49,7 +41,7 @@ class L3AgentExtensionsManager(agent_ext_manager.AgentExtensionsManager):
             if not isinstance(extension.obj, (l3_extension.L3AgentExtension,)):
                 extensions.append(extension.attr)
         if extensions:
-            raise L3ExtensionException(extensions=extensions)
+            raise l3_exc.L3ExtensionException(extensions=extensions)
 
     def add_router(self, context, data):
         """Notify all agent extensions to add router."""
diff --git a/neutron/agent/linux/dhcp.py b/neutron/agent/linux/dhcp.py
index 030d07e707e..b08fdcd4d01 100644
--- a/neutron/agent/linux/dhcp.py
+++ b/neutron/agent/linux/dhcp.py
@@ -41,7 +41,6 @@ from neutron.agent.linux import external_process
 from neutron.agent.linux import ip_lib
 from neutron.agent.linux import iptables_manager
 from neutron.cmd import runtime_checks as checks
-from neutron.common import _constants as common_constants
 from neutron.common.ovn import constants as ovn_constants
 from neutron.common.ovn import utils as ovn_utils
 from neutron.common import utils as common_utils
@@ -1881,7 +1880,7 @@ class DeviceManager(object):
         if self.conf.force_metadata or self.conf.enable_isolated_metadata:
             ip_cidrs.append(constants.METADATA_CIDR)
             if netutils.is_ipv6_enabled():
-                ip_cidrs.append(common_constants.METADATA_V6_CIDR)
+                ip_cidrs.append(constants.METADATA_V6_CIDR)
 
         self.driver.init_l3(interface_name, ip_cidrs,
                             namespace=network.namespace)
diff --git a/neutron/agent/metadata/driver.py b/neutron/agent/metadata/driver.py
index b7b4d859942..2f715bb743b 100644
--- a/neutron/agent/metadata/driver.py
+++ b/neutron/agent/metadata/driver.py
@@ -33,7 +33,6 @@ from neutron.agent.l3 import namespaces
 from neutron.agent.linux import external_process
 from neutron.agent.linux import ip_lib
 from neutron.agent.linux import utils as linux_utils
-from neutron.common import _constants as common_constants
 from neutron.common import coordination
 from neutron.common import metadata as comm_meta
 from neutron.common import utils as common_utils
@@ -377,6 +376,6 @@ def apply_metadata_nat_rules(router, proxy):
     if netutils.is_ipv6_enabled():
         for c, r in proxy.metadata_nat_rules(
                 proxy.metadata_port,
-                metadata_address=(common_constants.METADATA_V6_CIDR)):
+                metadata_address=(constants.METADATA_V6_CIDR)):
             router.iptables_manager.ipv6['nat'].add_rule(c, r)
     router.iptables_manager.apply()
diff --git a/neutron/common/_constants.py b/neutron/common/_constants.py
index 40745ecc86a..b9dbe6e958d 100644
--- a/neutron/common/_constants.py
+++ b/neutron/common/_constants.py
@@ -87,8 +87,5 @@ TRAIT_NETWORK_TUNNEL = 'CUSTOM_NETWORK_TUNNEL_PROVIDER'
 # The lowest binding index for L3 agents and DHCP agents.
 LOWEST_AGENT_BINDING_INDEX = 1
 
-# Neutron-lib defines this with a /64 but it should be /128
-METADATA_V6_CIDR = constants.METADATA_V6_IP + '/128'
-
 # TODO(haleyb): move this constant to neutron_lib.constants
 IPV4_MIN_MTU = 68
diff --git a/neutron/db/l3_db.py b/neutron/db/l3_db.py
index 4460c65291f..224a766ac42 100644
--- a/neutron/db/l3_db.py
+++ b/neutron/db/l3_db.py
@@ -83,13 +83,6 @@ FIP_ASSOC_MSG = ('Floating IP %(fip_id)s %(assoc)s. External IP: %(ext_ip)s, '
                  'port: %(port_id)s.')
 
 
-# TODO(froyo): Move this exception to neutron-lib as soon as possible, and when
-# a new release is created and pointed to in the requirements remove this code.
-class FipAssociated(n_exc.InUse):
-    message = _('Unable to complete the operation on port "%(port_id)s" '
-                'because the port still has an associated floating IP.')
-
-
 @registry.has_registry_receivers
 class L3_NAT_dbonly_mixin(l3.RouterPluginBase,
                           base_services.WorkerBase,
@@ -1832,7 +1825,7 @@ class L3_NAT_dbonly_mixin(l3.RouterPluginBase,
                 floating_ip_objs_admin = l3_obj.FloatingIP.get_objects(
                     context.elevated(), fixed_port_id=port_id)
                 if floating_ip_objs_admin != floating_ip_objs:
-                    raise FipAssociated(port_id=port_id)
+                    raise l3_exc.FipAssociated(port_id=port_id)
 
             router_ids = {fip.router_id for fip in floating_ip_objs}
             old_fips = {fip.id: self._make_floatingip_dict(fip)
diff --git a/neutron/exceptions/qos.py b/neutron/exceptions/qos.py
deleted file mode 100644
index 73efa5c2eb7..00000000000
--- a/neutron/exceptions/qos.py
+++ /dev/null
@@ -1,24 +0,0 @@
-# Copyright (c) 2021 Ericsson Software Technology
-#
-#    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.
-
-from neutron_lib import exceptions as e
-
-from neutron._i18n import _
-
-
-# TODO(przszc): Move to n-lib
-class QosPlacementAllocationUpdateConflict(e.Conflict):
-    message = _("Updating placement allocation with %(alloc_diff)s for "
-                "consumer %(consumer)s failed. The requested resources would "
-                "exceed the capacity available.")
diff --git a/neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/ovn_client.py b/neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/ovn_client.py
index c47c00a2ca7..f960135a6d6 100644
--- a/neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/ovn_client.py
+++ b/neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/ovn_client.py
@@ -2474,7 +2474,6 @@ class OVNClient(object):
                          'fixed_ips': fixed_ips,
                          }
                 }
-        # TODO(boden): rehome create_port into neutron-lib
         return p_utils.create_port(self._plugin, context, port)
 
     def update_metadata_port(self, context, network, subnet=None):
diff --git a/neutron/services/qos/qos_plugin.py b/neutron/services/qos/qos_plugin.py
index 75e652e92d7..659a133fe61 100644
--- a/neutron/services/qos/qos_plugin.py
+++ b/neutron/services/qos/qos_plugin.py
@@ -51,7 +51,6 @@ from oslo_log import log as logging
 from neutron._i18n import _
 from neutron.common import _constants as n_const
 from neutron.db import db_base_plugin_common
-from neutron.exceptions import qos as neutron_qos_exc
 from neutron.extensions import qos
 from neutron.objects import base as base_obj
 from neutron.objects import network as network_object
@@ -570,7 +569,7 @@ class QoSPlugin(qos.QoSPluginBase):
                 self._placement_client.update_qos_allocation(
                     consumer_uuid=device_id, alloc_diff=alloc_diff)
             except ks_exc.Conflict:
-                raise neutron_qos_exc.QosPlacementAllocationUpdateConflict(
+                raise qos_exc.QosPlacementAllocationUpdateConflict(
                     alloc_diff=alloc_diff, consumer=device_id)
 
         # NOTE(przszc): Upon successful allocation update in Placement we can
diff --git a/neutron/tests/functional/services/conntrack_helper/test_conntrack_helper.py b/neutron/tests/functional/services/conntrack_helper/test_conntrack_helper.py
index e57f0dd15d1..16847b18adb 100644
--- a/neutron/tests/functional/services/conntrack_helper/test_conntrack_helper.py
+++ b/neutron/tests/functional/services/conntrack_helper/test_conntrack_helper.py
@@ -17,6 +17,7 @@ from unittest import mock
 from neutron_lib.api.definitions import l3_conntrack_helper as apidef
 from neutron_lib import exceptions as lib_exc
 from neutron_lib.exceptions import l3 as lib_l3_exc
+from neutron_lib.plugins import constants as plugin_consts
 from neutron_lib.plugins import directory
 from oslo_utils import uuidutils
 
@@ -34,7 +35,7 @@ class ConntrackHelperTestCase(ml2_test_base.ML2TestFramework,
     def setUp(self):
         super(ConntrackHelperTestCase, self).setUp()
         self.cth_plugin = cth_plugin.Plugin()
-        directory.add_plugin("CONNTRACKHELPER", self.cth_plugin)
+        directory.add_plugin(plugin_consts.CONNTRACKHELPER, self.cth_plugin)
         self.router = self._create_router(distributed=True)
         self.conntack_helper = {
             apidef.RESOURCE_NAME:
diff --git a/neutron/tests/unit/agent/dhcp/test_agent.py b/neutron/tests/unit/agent/dhcp/test_agent.py
index 8bc55e7d1ab..191a465d7a0 100644
--- a/neutron/tests/unit/agent/dhcp/test_agent.py
+++ b/neutron/tests/unit/agent/dhcp/test_agent.py
@@ -37,7 +37,6 @@ from neutron.agent.linux import dhcp
 from neutron.agent.linux import interface
 from neutron.agent.linux import utils as linux_utils
 from neutron.agent.metadata import driver as metadata_driver
-from neutron.common import _constants as common_constants
 from neutron.common import config as common_config
 from neutron.common.ovn import constants as ovn_const
 from neutron.common import utils
@@ -2000,7 +1999,7 @@ class TestDeviceManager(base.BaseTestCase):
             expected_ips = ['172.9.9.9/24', const.METADATA_CIDR]
 
         if ipv6_enabled:
-            expected_ips.append(common_constants.METADATA_V6_CIDR)
+            expected_ips.append(const.METADATA_V6_CIDR)
 
         expected = [mock.call.get_device_name(port)]
 
diff --git a/neutron/tests/unit/agent/linux/test_dhcp.py b/neutron/tests/unit/agent/linux/test_dhcp.py
index 6a8010ea08e..50e82c73f26 100644
--- a/neutron/tests/unit/agent/linux/test_dhcp.py
+++ b/neutron/tests/unit/agent/linux/test_dhcp.py
@@ -33,7 +33,6 @@ import testtools
 from neutron.agent.linux import dhcp
 from neutron.agent.linux import ip_lib
 from neutron.cmd import runtime_checks as checks
-from neutron.common import _constants as common_constants
 from neutron.common.ovn import constants as ovn_const
 from neutron.common import utils as common_utils
 from neutron.conf.agent import common as config
@@ -3378,7 +3377,7 @@ class TestDeviceManager(TestConfBase):
             if enable_isolated_metadata or force_metadata:
                 expect_ips.extend([
                     constants.METADATA_CIDR,
-                    common_constants.METADATA_V6_CIDR])
+                    constants.METADATA_V6_CIDR])
             mgr.driver.init_l3.assert_called_with('ns-XXX',
                                                   expect_ips,
                                                   namespace='qdhcp-ns')
diff --git a/neutron/tests/unit/db/test_l3_db.py b/neutron/tests/unit/db/test_l3_db.py
index 58a0a5ffde7..5d230b9ad72 100644
--- a/neutron/tests/unit/db/test_l3_db.py
+++ b/neutron/tests/unit/db/test_l3_db.py
@@ -27,6 +27,7 @@ from neutron_lib import context
 from neutron_lib.db import api as db_api
 from neutron_lib import exceptions as n_exc
 from neutron_lib.exceptions import extraroute as xroute_exc
+from neutron_lib.exceptions import l3 as l3_exc
 from neutron_lib.plugins import constants as plugin_constants
 from neutron_lib.plugins import directory
 from neutron_lib.plugins import utils as plugin_utils
@@ -414,7 +415,7 @@ class TestL3_NAT_dbonly_mixin(
         get_objects.side_effect = [
             [],
             [{'id': 'floating_ip1', 'port_id': 'port_id'}]]
-        self.assertRaises(l3_db.FipAssociated,
+        self.assertRaises(l3_exc.FipAssociated,
                           self.db.disassociate_floatingips,
                           context_tenant,
                           'port_id')
diff --git a/neutron/tests/unit/services/conntrack_helper/test_plugin.py b/neutron/tests/unit/services/conntrack_helper/test_plugin.py
index 673802d6348..86310611dc9 100644
--- a/neutron/tests/unit/services/conntrack_helper/test_plugin.py
+++ b/neutron/tests/unit/services/conntrack_helper/test_plugin.py
@@ -17,6 +17,7 @@ from unittest import mock
 from neutron_lib import context
 from neutron_lib import exceptions as lib_exc
 from neutron_lib.objects import exceptions as obj_exc
+from neutron_lib.plugins import constants as plugin_consts
 from neutron_lib.plugins import directory
 from oslo_config import cfg
 
@@ -69,9 +70,7 @@ class TestConntrackHelperPlugin(testlib_api.SqlTestCase):
         cfg.CONF.set_override("service_plugins", ["router",
                                                   "conntrack_helper"])
         manager.init()
-        # TODO(hjensas): Add CONNTRACKHELPER to neutron-lib Well-known
-        #  service type constants.
-        self.cth_plugin = directory.get_plugin("CONNTRACKHELPER")
+        self.cth_plugin = directory.get_plugin(plugin_consts.CONNTRACKHELPER)
         self.ctxt = context.Context('admin', 'fake_tenant')
         mock.patch.object(self.ctxt.session, 'refresh').start()
         mock.patch.object(self.ctxt.session, 'expunge').start()
diff --git a/neutron/tests/unit/services/qos/test_qos_plugin.py b/neutron/tests/unit/services/qos/test_qos_plugin.py
index ecdea9987fa..0aa377f378d 100644
--- a/neutron/tests/unit/services/qos/test_qos_plugin.py
+++ b/neutron/tests/unit/services/qos/test_qos_plugin.py
@@ -35,7 +35,6 @@ from oslo_utils import uuidutils
 import webob.exc
 
 from neutron.common import _constants as n_const
-from neutron.exceptions import qos as neutron_qos_exc
 from neutron.extensions import qos_pps_minimum_rule_alias
 from neutron.extensions import qos_rules_alias
 from neutron import manager
@@ -2802,7 +2801,7 @@ class TestQosPluginDB(base.BaseQosTestCase):
                 response={'errors': [{'code': 'placement.concurrent_update'}]}
             )
             self.assertRaises(
-                neutron_qos_exc.QosPlacementAllocationUpdateConflict,
+                qos_exc.QosPlacementAllocationUpdateConflict,
                 self.qos_plugin._change_placement_allocation,
                 qos1, qos2, orig_port, port)