diff --git a/neutron_tempest_plugin/api/base.py b/neutron_tempest_plugin/api/base.py
index d63dec8..4833c71 100644
--- a/neutron_tempest_plugin/api/base.py
+++ b/neutron_tempest_plugin/api/base.py
@@ -140,6 +140,17 @@
         cls.conntrack_helpers = []
 
     @classmethod
+    def reserve_external_subnet_cidrs(cls):
+        client = cls.os_admin.network_client
+        ext_nets = client.list_networks(
+            **{"router:external": True})['networks']
+        for ext_net in ext_nets:
+            ext_subnets = client.list_subnets(
+                network_id=ext_net['id'])['subnets']
+            for ext_subnet in ext_subnets:
+                cls.reserve_subnet_cidr(ext_subnet['cidr'])
+
+    @classmethod
     def resource_cleanup(cls):
         if CONF.service_available.neutron:
             # Clean up trunks
diff --git a/neutron_tempest_plugin/scenario/test_ipv6.py b/neutron_tempest_plugin/scenario/test_ipv6.py
index 732c96d..4237d4f 100644
--- a/neutron_tempest_plugin/scenario/test_ipv6.py
+++ b/neutron_tempest_plugin/scenario/test_ipv6.py
@@ -84,6 +84,7 @@
     @tempest_utils.requires_ext(extension="router", service="network")
     def resource_setup(cls):
         super(IPv6Test, cls).resource_setup()
+        cls.reserve_external_subnet_cidrs()
         cls._setup_basic_resources()
 
     @classmethod
diff --git a/neutron_tempest_plugin/scenario/test_metadata.py b/neutron_tempest_plugin/scenario/test_metadata.py
index 3897183..c6d7505 100644
--- a/neutron_tempest_plugin/scenario/test_metadata.py
+++ b/neutron_tempest_plugin/scenario/test_metadata.py
@@ -49,6 +49,7 @@
         super(MetadataTest, cls).resource_setup()
         cls.rand_name = data_utils.rand_name(
             cls.__name__.rsplit('.', 1)[-1])
+        cls.reserve_external_subnet_cidrs()
         cls.network = cls.create_network(name=cls.rand_name)
         cls.subnet_v4 = cls.create_subnet(
             network=cls.network, name=cls.rand_name)