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Does anyone know how the F5 rebalances traffic when you configure "Action on Service Down" to None. i.e say there are active connections and your have updated the health check so that the server is down. The exsisting connection are active, but then you stop the apache service. Would the F5 reselect based on not seeing any responses to the previous connections or do you need to use the 2 iRule events ... LB_FAILED and LB_RESELECT ?


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If the pool config for 'action on service down' is set to the default of 'none' LTM will not do anything with existing connections when a pool member is marked down.

New connections would be load balanced/persisted to a new pool member.

For most application traffic processing you generally can't reselect a new pool member and create a new serverside connection mid-client connection. The client won't typically have an application session created on the newly selected server.

For HTTP, it generally makes sense to either use the default 'action on service down' setting of None or use Reject. Reject triggers LTM to send a TCP reset to the client and allow it to manually retry the request on a new TCP connection to a new pool member.


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Disclaimer: I haven't touched a f5 LTM in a year.

It will reselect as you are suggesting (load balancing method will determine how), but there is a caveat: I've found that if one has cookie persistence enabled in conjunction with an iRule, it's possible that the monitor state will be ignored and the traffic will be passed through to an inactive pool member regardless, even if all pool members are inactive. That scenario is most likely where the LB_ events you mentioned will come in handy.

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