When it fails, it always happen at the first VLAN configuration at the trunk interface.  Then it waits 30 seconds (the heartbeat timeout) and replay the config again from the beginning.

2015-06-25 16:43:53.178 13954 INFO neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus [-] Re-established connection to switch ip 10.86.1.128
2015-06-25 16:43:53.178 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus [-] Replaying config for switch ip 10.86.1.128 replay_config /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/mech_cisco_nexus.py:77
2015-06-25 16:43:53.178 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.nexus_db_v2 [-] get_nve_switch_bindings() called get_nve_switch_bindings /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_db_v2.py:216
2015-06-25 16:43:53.180 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.nexus_db_v2 [-] get_nexusport_switch_bindings() called get_nexusport_switch_bindings /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_db_v2.py:46
2015-06-25 16:43:53.187 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus [-] Nexus: create & trunk vlan neutron-502 _configure_port_binding /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/mech_cisco_nexus.py:431
2015-06-25 16:43:53.187 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.nexus_network_driver [-] NexusDriver:  create_vlan /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py:247
2015-06-25 16:43:53.187 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.nexus_network_driver [-] NexusDriver edit config: 
      <config xmlns:xc="urn:ietf:params:xml:ns:netconf:base:1.0">
        <configure>
          <__XML__MODE__exec_configure>
            <vlan>
              <vlan-id-create-delete>
                <__XML__PARAM_value>502</__XML__PARAM_value>
                <__XML__MODE_vlan>
                  <name>
                    <vlan-name>neutron-502</vlan-name>
                  </name>
                </__XML__MODE_vlan>
              </vlan-id-create-delete>
            </vlan>

          </__XML__MODE__exec_configure>
        </configure>
      </config>
 _edit_config /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py:102
2015-06-25 16:43:53.301 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.nexus_network_driver [-] NexusDriver edit config: 
      <config xmlns:xc="urn:ietf:params:xml:ns:netconf:base:1.0">
        <configure>
          <__XML__MODE__exec_configure>
            <vlan>
              <vlan-id-create-delete>
                <__XML__PARAM_value>502</__XML__PARAM_value>
                <__XML__MODE_vlan>
                  <state>
                    <vstate>active</vstate>
                  </state>
                </__XML__MODE_vlan>
              </vlan-id-create-delete>
            </vlan>

          </__XML__MODE__exec_configure>
        </configure>
      </config>
 _edit_config /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py:102
2015-06-25 16:43:53.414 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.nexus_network_driver [-] NexusDriver edit config: 
      <config xmlns:xc="urn:ietf:params:xml:ns:netconf:base:1.0">
        <configure>
          <__XML__MODE__exec_configure>
            <vlan>
              <vlan-id-create-delete>
                <__XML__PARAM_value>502</__XML__PARAM_value>
                <__XML__MODE_vlan>
                  <no>
                    <shutdown/>
                  </no>
                </__XML__MODE_vlan>
              </vlan-id-create-delete>
            </vlan>

          </__XML__MODE__exec_configure>
        </configure>
      </config>
 _edit_config /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py:102
2015-06-25 16:43:53.528 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.nexus_network_driver [-] NexusDriver created VLAN: 502 create_and_trunk_vlan /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py:331
2015-06-25 16:43:53.818 13954 ERROR neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus [-] Nexus Driver cisco_nexus failed in replay _configure_port_binding
2015-06-25 16:43:53.818 13954 TRACE neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus Traceback (most recent call last):
2015-06-25 16:43:53.818 13954 TRACE neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus   File "/usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/mech_cisco_nexus.py", line 526, in configure_switch_entries
2015-06-25 16:43:53.818 13954 TRACE neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus     port.vni)
2015-06-25 16:43:53.818 13954 TRACE neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus   File "/usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/mech_cisco_nexus.py", line 434, in _configure_port_binding
2015-06-25 16:43:53.818 13954 TRACE neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus     vni)
2015-06-25 16:43:53.818 13954 TRACE neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus   File "/usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py", line 334, in create_and_trunk_vlan
2015-06-25 16:43:53.818 13954 TRACE neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus     nexus_port)
2015-06-25 16:43:53.818 13954 TRACE neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus   File "/usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py", line 297, in enable_vlan_on_trunk_int
2015-06-25 16:43:53.818 13954 TRACE neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus     intf_type, interface)
2015-06-25 16:43:53.818 13954 TRACE neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus   File "/usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py", line 188, in get_interface_switch_trunk_allowed
2015-06-25 16:43:53.818 13954 TRACE neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus     response = self._get_config(nexus_host, confstr)
2015-06-25 16:43:53.818 13954 TRACE neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus   File "/usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py", line 78, in _get_config
2015-06-25 16:43:53.818 13954 TRACE neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus     exc=e)
2015-06-25 16:43:53.818 13954 TRACE neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus NexusConfigFailed: Failed to configure Nexus switch: 10.86.1.128 XML: 
2015-06-25 16:43:53.818 13954 TRACE neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus     <cmd>show running-config interface ethernet 1/22</cmd>
2015-06-25 16:43:53.818 13954 TRACE neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus . Reason: Syntax error while parsing 'show running-config interface ethernet 1/22'
2015-06-25 16:43:53.818 13954 TRACE neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus .
2015-06-25 16:43:53.818 13954 TRACE neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus 
2015-06-25 16:43:53.818 13954 ERROR neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus [-] Failed to configure port binding for switch 10.86.1.128, vlan 502 vni 0, port ethernet:1/22, reason Failed to configure Nexus switch: 10.86.1.128 XML: 
    <cmd>show running-config interface ethernet 1/22</cmd>
. Reason: Syntax error while parsing 'show running-config interface ethernet 1/22'
.
2015-06-25 16:43:53.819 13954 WARNING neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus [-] Replay config failed for ip 10.86.1.128
2015-06-25 16:43:58.424 13954 DEBUG neutron.context [req-ece6d5f3-e6b3-43d3-aad6-3a4dd956e04b None] Arguments dropped when creating context: {u'project_name': None, u'tenant': None} __init__ /usr/lib/python2.7/site-packages/neutron/context.py:84
2015-06-25 16:44:00.014 13954 DEBUG neutron.context [req-4dd1cc74-bd5a-4fbf-8202-b24432d021f0 None] Arguments dropped when creating context: {u'project_name': None, u'tenant': None} __init__ /usr/lib/python2.7/site-packages/neutron/context.py:84
2015-06-25 16:44:00.370 13954 DEBUG neutron.context [req-84707bd6-1066-4822-842f-404344db5bbf None] Arguments dropped when creating context: {u'project_name': None, u'tenant': None} __init__ /usr/lib/python2.7/site-packages/neutron/context.py:84
2015-06-25 16:44:04.922 13954 DEBUG neutron.context [req-c22f8839-e60e-46ea-b665-7b2a83dd2ecc None] Arguments dropped when creating context: {u'project_name': None, u'tenant': None} __init__ /usr/lib/python2.7/site-packages/neutron/context.py:84
2015-06-25 16:44:08.298 13954 DEBUG neutron.context [req-85848d49-6d53-46ec-bb54-9c783ec7605e None] Arguments dropped when creating context: {u'project_name': None, u'tenant': None} __init__ /usr/lib/python2.7/site-packages/neutron/context.py:84
2015-06-25 16:44:20.092 13954 DEBUG neutron.context [req-7b24f7a7-4797-47a6-80f8-402c02b4fc2f None] Arguments dropped when creating context: {u'project_name': None, u'tenant': None} __init__ /usr/lib/python2.7/site-packages/neutron/context.py:84
2015-06-25 16:44:23.820 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus [-] check_connections() switch 10.86.7.59 state 1 check_connections /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/mech_cisco_nexus.py:128
2015-06-25 16:44:23.934 13954 WARNING neutron.plugins.ml2.drivers.cisco.nexus.nexus_network_driver [-] GET call failed to return Nexus type
2015-06-25 16:44:23.934 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus [-] check_connections() switch 10.86.1.128 state 0 check_connections /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/mech_cisco_nexus.py:128
2015-06-25 16:44:24.360 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.nexus_network_driver [-] GET call returned Nexus type 9 get_nexus_type /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py:229
2015-06-25 16:44:24.360 13954 INFO neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus [-] Re-established connection to switch ip 10.86.1.128
2015-06-25 16:44:24.360 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus [-] Replaying config for switch ip 10.86.1.128 replay_config /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/mech_cisco_nexus.py:77
2015-06-25 16:44:24.360 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.nexus_db_v2 [-] get_nve_switch_bindings() called get_nve_switch_bindings /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_db_v2.py:216
2015-06-25 16:44:24.362 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.nexus_db_v2 [-] get_nexusport_switch_bindings() called get_nexusport_switch_bindings /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_db_v2.py:46
2015-06-25 16:44:24.373 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.mech_cisco_nexus [-] Nexus: create & trunk vlan neutron-502 _configure_port_binding /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/mech_cisco_nexus.py:431
2015-06-25 16:44:24.373 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.nexus_network_driver [-] NexusDriver:  create_vlan /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py:247
2015-06-25 16:44:24.373 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.nexus_network_driver [-] NexusDriver edit config: 
      <config xmlns:xc="urn:ietf:params:xml:ns:netconf:base:1.0">
        <configure>
          <__XML__MODE__exec_configure>
            <vlan>
              <vlan-id-create-delete>
                <__XML__PARAM_value>502</__XML__PARAM_value>
                <__XML__MODE_vlan>
                  <name>
                    <vlan-name>neutron-502</vlan-name>
                  </name>
                </__XML__MODE_vlan>
              </vlan-id-create-delete>
            </vlan>

          </__XML__MODE__exec_configure>
        </configure>
      </config>
 _edit_config /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py:102
2015-06-25 16:44:24.437 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.nexus_network_driver [-] NexusDriver edit config: 
      <config xmlns:xc="urn:ietf:params:xml:ns:netconf:base:1.0">
        <configure>
          <__XML__MODE__exec_configure>
            <vlan>
              <vlan-id-create-delete>
                <__XML__PARAM_value>502</__XML__PARAM_value>
                <__XML__MODE_vlan>
                  <state>
                    <vstate>active</vstate>
                  </state>
                </__XML__MODE_vlan>
              </vlan-id-create-delete>
            </vlan>

          </__XML__MODE__exec_configure>
        </configure>
      </config>
 _edit_config /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py:102
2015-06-25 16:44:24.551 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.nexus_network_driver [-] NexusDriver edit config: 
      <config xmlns:xc="urn:ietf:params:xml:ns:netconf:base:1.0">
        <configure>
          <__XML__MODE__exec_configure>
            <vlan>
              <vlan-id-create-delete>
                <__XML__PARAM_value>502</__XML__PARAM_value>
                <__XML__MODE_vlan>
                  <no>
                    <shutdown/>
                  </no>
                </__XML__MODE_vlan>
              </vlan-id-create-delete>
            </vlan>

          </__XML__MODE__exec_configure>
        </configure>
      </config>
 _edit_config /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py:102
2015-06-25 16:44:24.664 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.nexus_network_driver [-] NexusDriver created VLAN: 502 create_and_trunk_vlan /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py:331
2015-06-25 16:44:24.828 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.nexus_network_driver [-] GET call returned interface ethernet 1/22 config get_interface_switch_trunk_allowed /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py:190
2015-06-25 16:44:24.828 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.nexus_network_driver [-] NexusDriver edit config: 
      <config xmlns:xc="urn:ietf:params:xml:ns:netconf:base:1.0">
        <configure>
          <__XML__MODE__exec_configure>
          <interface>
            <ethernet>
              <interface>1/22</interface>
              <__XML__MODE_if-ethernet-switch>
                <switchport>
                  <trunk>
                    <allowed>
                      <vlan>
                        <add>
                          <vlan_id>502</vlan_id>
                        </add>
                      </vlan>
                    </allowed>
                  </trunk>
                </switchport>
              </__XML__MODE_if-ethernet-switch>
            </ethernet>
          </interface>

          </__XML__MODE__exec_configure>
        </configure>
      </config>
 _edit_config /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py:102
2015-06-25 16:44:24.942 13954 DEBUG neutron.plugins.ml2.drivers.cisco.nexus.nexus_network_driver [-] Successfully added switchport trunk vlan 502 on int ethernet 1/22. enable_vlan_on_trunk_int /usr/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/cisco/nexus/nexus_network_driver.py:317