Add method to reserve the external subnet cidrs

Related-Bug: #1903433
Change-Id: Ic2333d1529d7749207bf81fc842963a5e5208306
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