When the flow maps back to itself, the outbound flow appears to originate from the VM to the front end, and the corresponding inbound flow appears to originate from the VM to itself. From the guest operating system's point of view, the inbound and outbound parts of the same flow don't match inside the virtual machine. The TCP stack won't recognize these halves of the same flow as being part of the same flow. The source and destination don't match. When the flow maps to any other VM in the backend pool, the halves of the flow do match and the VM can respond to the flow.
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