tempest vlan test failed

When tempest test is run as:
"tox -e all-plugin -- networking_spp.tests.tempest.scenario --serial"
test_spp_connect_vlan failed as follows:
---
Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):
      File "/usr/local/lib/python2.7/dist-packages/networking_spp/tests/tempest/scenario/test_spp.py", line 135, in test_spp_connect_vlan
        self._test_spp_connect(**net_kwargs)
      File "/usr/local/lib/python2.7/dist-packages/networking_spp/tests/tempest/scenario/test_spp.py", line 53, in _test_spp_connect
        cidr=netaddr.IPNetwork('30.0.0.0/28'))
      File "/opt/stack/neutron-tempest-plugin/neutron_tempest_plugin/api/base.py", line 363, in create_subnet
        raise ValueError(message)
    ValueError: Available CIDR for subnet creation could not be found
---