Similar failures:
(1) https://04f7ac194e853c778b5c-637f8ccb892d615b1b6b9e58fd553696.ssl.cf2.rackcdn.com/787474/1/check/neutron-tempest-plugin-scenario-linuxbridge-train/9358492/testr_results.html

Traceback (most recent call last):
  File "/opt/stack/tempest/.tox/tempest/lib/python3.6/site-packages/neutron_tempest_plugin/scenario/test_multicast.py", line 275, in test_multicast_between_vms_on_same_network
    unregistered=unregistered)
  File "/opt/stack/tempest/.tox/tempest/lib/python3.6/site-packages/neutron_tempest_plugin/scenario/test_multicast.py", line 306, in _check_multicast_conectivity
    self._prepare_unregistered(unregistered, mcast_address)
  File "/opt/stack/tempest/.tox/tempest/lib/python3.6/site-packages/neutron_tempest_plugin/scenario/test_multicast.py", line 251, in _prepare_unregistered
    addresses = ip_command.list_addresses(port=server['port'])
  File "/opt/stack/tempest/.tox/tempest/lib/python3.6/site-packages/neutron_tempest_plugin/common/ip.py", line 120, in list_addresses
    output = self.execute('address', *command)
  File "/opt/stack/tempest/.tox/tempest/lib/python3.6/site-packages/neutron_tempest_plugin/common/ip.py", line 58, in execute
    timeout=self.timeout).stdout
  File "/opt/stack/tempest/.tox/tempest/lib/python3.6/site-packages/neutron_tempest_plugin/common/shell.py", line 69, in execute
    ssh_client=ssh_client)
  File "/opt/stack/tempest/.tox/tempest/lib/python3.6/site-packages/neutron_tempest_plugin/common/shell.py", line 103, in execute_remote_command
    stdout = ssh_client.exec_command(command, timeout=timeout)
  File "/opt/stack/tempest/.tox/tempest/lib/python3.6/site-packages/tenacity/__init__.py", line 333, in wrapped_f
    return self(f, *args, **kw)
  File "/opt/stack/tempest/.tox/tempest/lib/python3.6/site-packages/tenacity/__init__.py", line 423, in __call__
    do = self.iter(retry_state=retry_state)
  File "/opt/stack/tempest/.tox/tempest/lib/python3.6/site-packages/tenacity/__init__.py", line 360, in iter
    return fut.result()
  File "/usr/lib/python3.6/concurrent/futures/_base.py", line 425, in result
    return self.__get_result()
  File "/usr/lib/python3.6/concurrent/futures/_base.py", line 384, in __get_result
    raise self._exception
  File "/opt/stack/tempest/.tox/tempest/lib/python3.6/site-packages/tenacity/__init__.py", line 426, in __call__
    result = fn(*args, **kwargs)
  File "/opt/stack/tempest/.tox/tempest/lib/python3.6/site-packages/neutron_tempest_plugin/common/ssh.py", line 171, in exec_command
    return super(Client, self).exec_command(cmd=cmd, encoding=encoding)
  File "/opt/stack/tempest/tempest/lib/common/ssh.py", line 160, in exec_command
    with transport.open_session() as channel:
  File "/opt/stack/tempest/.tox/tempest/lib/python3.6/site-packages/paramiko/transport.py", line 879, in open_session
    timeout=timeout,
  File "/opt/stack/tempest/.tox/tempest/lib/python3.6/site-packages/paramiko/transport.py", line 1001, in open_channel
    event.wait(0.1)
  File "/usr/lib/python3.6/threading.py", line 551, in wait
    signaled = self._cond.wait(timeout)
  File "/usr/lib/python3.6/threading.py", line 299, in wait
    gotit = waiter.acquire(True, timeout)
  File "/opt/stack/tempest/.tox/tempest/lib/python3.6/site-packages/fixtures/_fixtures/timeout.py", line 52, in signal_handler
    raise TimeoutException()
fixtures._fixtures.timeout.TimeoutException

(2) https://bf5f3a9a289e8407aabf-57cbbae9688bff7a3084998f0691628a.ssl.cf1.rackcdn.com/787474/1/check/neutron-tempest-plugin-scenario-linuxbridge/34cbee0/testr_results.html

Traceback (most recent call last):
  File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/neutron_tempest_plugin/scenario/test_multicast.py", line 274, in test_multicast_between_vms_on_same_network
    self._check_multicast_conectivity(sender=sender, receivers=receivers,
  File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/neutron_tempest_plugin/scenario/test_multicast.py", line 381, in _check_multicast_conectivity
    utils.wait_until_true(
  File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/neutron_tempest_plugin/common/utils.py", line 79, in wait_until_true
    while not predicate():
  File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/neutron_tempest_plugin/scenario/test_multicast.py", line 351, in check_unregistered_host
    unregistered_result = unregistered['ssh_client'].execute_script(
  File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/neutron_tempest_plugin/common/ssh.py", line 224, in execute_script
    channel = self.open_session()
  File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/neutron_tempest_plugin/common/ssh.py", line 152, in open_session
    return client.get_transport().open_session()
  File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/paramiko/transport.py", line 875, in open_session
    return self.open_channel(
  File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/paramiko/transport.py", line 1001, in open_channel
    event.wait(0.1)
  File "/usr/lib/python3.8/threading.py", line 558, in wait
    signaled = self._cond.wait(timeout)
  File "/usr/lib/python3.8/threading.py", line 306, in wait
    gotit = waiter.acquire(True, timeout)
  File "/opt/stack/tempest/.tox/tempest/lib/python3.8/site-packages/fixtures/_fixtures/timeout.py", line 52, in signal_handler
    raise TimeoutException()
fixtures._fixtures.timeout.TimeoutException