Direct Server Return (SwitchBack) configures the ServerIron ADX to instruct real servers to send client responses directly to the clients instead of sending the responses back through the ServerIron ADX. As a result, the clients receive faster response time and the ServerIron ADX is free to support even more sessions to serve more clients. (A connection is two sessions, one in each direction.)
When configured for this feature, the ServerIron ADX sends client requests addressed to a VIP to a load balanced real server, as in standard Server Load Balancing (SLB) configurations. However, to enhance server-to-client response time and to increase the overall connection capacity of the ServerIron ADX, the software in a SwitchBack configuration formats the client request packets in such a away that the real servers respond directly to the clients, instead of sending the responses back through the ServerIron ADX.
Figure 2.4 shows an example of two ServerIron ADXs deployed in a SLB SwitchBack configuration.
You configure SwitchBack on an individual TCP/UDP port basis when you configure your virtual servers. The feature requires that you configure a loopback interface on each real server and give the loopback interface the IP address of the VIP. The ServerIron ADX sends the client traffic to the real server without translating the destination address from the VIP into the real server's IP address. Thus, the real server receives the client traffic addressed to its loopback address and responds directly to the client.
The SwitchBack feature can be used on a single ServerIron ADX supporting a single server farm, but is also is quite powerful when used on multiple ServerIron ADXs in combination with the Symmetric SLB feature (see
“Symmetric SLB”).
