ServerIron ADX Server Load Balancing Guide
Release 12.0.00
June 15, 2009

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Health Checks > Reassign Threshold

Reassign Threshold
The reassign threshold specifies the number of contiguous inbound TCP-SYN packets a real server can fail to respond to before the ServerIron ADX changes the application state to FAILED and the server state to TEST. The default reassign threshold is 21. The server and application states are described in “Server and Application Port States”.
If the field reaches the reassign threshold, the ServerIron ADX marks the application failed. The value of an application’s Reas field is reset to 0 when the ServerIron ADX receives a TCP SYN ACK from the application. No other type of traffic can clear this field.
If a real server seems to be triggering the reassign threshold too frequently, you can increase the reassign threshold. The default is 21 and the range of values is 6 – 254. This is a global parameter. See “Reassign Threshold”.
In addition to the circumstances described in “Server and Application Port States”, a real server’s state also can be affected by the reassign threshold. The reassign threshold specifies how many consecutive TCP SYN requests a real server can fail to respond to before the ServerIron ADX changes the application state to FAILED and the server state to test. The default reassign threshold is 20.
The value of an application’s Reas field is reset to 0 when the ServerIron ADX receives a TCP SYN ACK from the application. No other type of traffic can clear this field.
NOTE: It is possible to take a service down without triggering the reassign threshold. For example, in a lab environment where the server is not receiving TCP SYNs, the service might be down but since the ServerIron ADX is not sending new requests to the server, the server does not fail to respond to enough consecutive TCP SYNs to meet the reassign threshold. As a result, the ServerIron ADX indicates the server and the service are ACTIVE when in fact they are offline.
NOTE: The reassign threshold counter is not incremented in SwitchBack (Direct Server Return) configurations.
NOTE: The reassign threshold does not apply to servers in SwitchBack (Direct Server Return) configurations. In a SwitchBack configuration, traffic from the real server does not pass back through the ServerIron ADX. As a result, the ServerIron ADX cannot monitor the TCP SYN ACKs from the server. See “DSR”.
NOTE: The ServerIron ADX does not try to reassign the client’s request to another server if you configure the application port to be sticky. The sticky option configures the ServerIron ADX to override load-balancing and send all client requests for the application to the same server during a given session.
NOTE: If a real server seems to be triggering the reassign threshold too frequently, you can increase the reassign threshold. This is a global parameter.
To modify the SYN-ACK threshold to 215, enter a command such as the following:
ServerIron(config)# server reassign-threshold 215
Syntax: server reassign-threshold <threshold value>
The range of values for the <threshold value> variable is 6 – 4000. The default value is 20.
NOTE: The SYN-ACK threshold is OFF by default.
Preventing State Flapping
You can prevent state flapping caused by the reassignment counter.
By default, the ServerIron ADX brings an application port down if the port's reassignment count exceeds the reassign threshold. The default reassign threshold is 21. If a port fails to respond with a SYN ACK to 21 client SYNs in a row, the ServerIron ADX marks the port failed. Once the port is marked failed, the port can be re-activated as a result of a successful health check on the port.
In some networks, the reassignment counter can cause needless state flapping of application ports. This occurs if the network conditions cause the counter to frequently reach the threshold and cause the ServerIron ADX to bring ports down even though the applications are healthy. The applications will remain unavailable for the amount of time it takes the ServerIron ADX to send health checks, interpret the results, and activate the ports in response to successful results.
NOTE: The reassignment count applies to the total number of contiguous (back-to-back) unanswered SYNs from all clients who have sent SYNs to the server.
To revent state flapping caused by the reassignment counter, enter the following command:
ServerIron(config)# server no-reassign-count
When you enter this command, the ServerIron ADX will stop incrementing the reassignment counters for real server applications.
Syntax: [no] server no-reassign-count
NOTE: THe command server no-reassign-count is enabled by default.

Health Checks > Reassign Threshold

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