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[Thierry Carrez <thierry.carrez+lp@xxxxxxxxx>]

** Changed in: neutron
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1348737

Title:
  Once a Gateway is set L-3 agent attempts to update external Gateway on
  every router update

Status in OpenStack Neutron (virtual network service):
  Fix Released

Bug description:
  once an external gateway is set, due to a logic error, for every
  subsequent router update the L-3 agent incorrectly concludes there is
  a change in external gateway.  This causes the codepath to set
  external gateway getting  invoked un-necessarily.

  In process_router(...)

    ....
          ex_gw_port = self._get_ex_gw_port(ri)                               returns ri.router.get('gw_port')
    ...
            if ex_gw_port and ex_gw_port != ri.ex_gw_port:
              self._set_subnet_info(ex_gw_port)                                 <--------
   ...
           ri.ex_gw_port = ex_gw_port

  The _set_subnet_info adds an element to the ex_gw_port thus making it
  different from the gw_port obtained out of router dict. Any subsequent
  ex_gw_port != ri.ex_gw_port would result True, incorrectly.

  One way to fix it would be to change

  From:

           if ex_gw_port and ex_gw_port != ri.ex_gw_port:
  To:

          if (ex_gw_port and (not ri.ex_gw_port
                              or ex_gw_port['id'] != ri.ex_gw_port['id'])):

  
  To reproduce:  
  a)	Create a router 
  b)	Add external gateway to it (router-gateway-set  <router_name>  <external_nw>)
  c)	Do something that would cause the Plugin to send down an update to the router. This can be an action un-related to the external gateway e.g. add an interface to the router ( router-interface-add <router_name> <subnet>)
  d)	Besides adding the interface, the agent code would treat this as if router gateway has been added or changed. It  re-attempts to plug the router-gateway port and activities associated with it. This can be observed by reading the code and I believe observing the logs. I think you will see gratuitous arps getting sent out. I instrumented the code for my debugging so don’t remember if you will see clear logs without instrumentation or not.
      This definitely contributes to degradation of service due to slowing down things but don’t know yet if there will be a communication interruption or not. It does affect DVR too because DVR has additional actions such as setting up SNAT redirection as part of setting up the external gateway and can cause duplicate entries.

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