With switch_heartbeat enabled, no traceback is logged when the ssh key to the Nexus switch is absent.

It is a prerequisite that user has to ssh from the Controller to the Nexus switch to establish the ssh keys. If this is not fulfilled, traceback is logged when the nexus ML2 driver attempts to configure the switch as shown below:

2015-03-24 09:09:20.832 ERROR neutron.plugins.ml2.managers [req-cc1eb1f1-ccd0-4388-bcf0-e223b94be8bc None None] Mechanism driver 'cisco_nexus' failed in update_port_postcommit
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers Traceback (most recent call last):
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers File "/opt/stack/neutron/neutron/plugins/ml2/managers.py", line 299, in _call_on_drivers
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers getattr(driver.obj, method_name)(context)
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers File "/opt/stack/neutron/neutron/openstack/common/lockutils.py", line 272, in inner
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers return f(*args, **kwargs)
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers File "/opt/stack/neutron/neutron/plugins/ml2/drivers/cisco/nexus/mech_cisco_nexus.py", line 742, in update_port_postcommit
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers self._configure_host_entries, vni)
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers File "/opt/stack/neutron/neutron/plugins/ml2/drivers/cisco/nexus/mech_cisco_nexus.py", line 651, in _port_action_vlan
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers func(vlan_id, device_id, host_id, vni, is_provider)
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers File "/opt/stack/neutron/neutron/plugins/ml2/drivers/cisco/nexus/mech_cisco_nexus.py", line 480, in _configure_host_entries
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers '_configure_port_binding'))
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers File "/opt/stack/neutron/neutron/openstack/common/excutils.py", line 82, in __exit__
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers six.reraise(self.type_, self.value, self.tb)
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers File "/opt/stack/neutron/neutron/plugins/ml2/drivers/cisco/nexus/mech_cisco_nexus.py", line 475, in _configure_host_entries
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers vni)
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers File "/opt/stack/neutron/neutron/plugins/ml2/drivers/cisco/nexus/mech_cisco_nexus.py", line 428, in _configure_port_binding
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers intf_type, nexus_port)
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers File "/opt/stack/neutron/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py", line 292, in enable_vlan_on_trunk_int
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers intf_type, interface)
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers File "/opt/stack/neutron/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py", line 183, in get_interface_switch_trunk_allowed
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers response = self._get_config(nexus_host, confstr)
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers File "/opt/stack/neutron/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py", line 76, in _get_config
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers exc=e)
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers NexusConfigFailed: Failed to configure Nexus switch: 172.22.191.67 XML:
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers <cmd>show running-config interface ethernet 1/2/1</cmd>
2015-03-24 09:09:20.832 TRACE neutron.plugins.ml2.managers . Reason: Unable to connect to Nexus 172.22.191.67. Reason: Unknown host key [0c:5a:5a:69:6d:db:83:a8:bc:72:0b:1a:db:57:6a:34] for [172.22.191.67]..

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With switch_heartbeat enabled, this traceback is not logged at all. In fact, there is no ERROR or WARNING logged neither, making it very hard to troubleshoot.