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A packet that crosses multiple source interfaces is copied twice to the monitoring application, causing additional load on the destination interface and the collection device. The duplication can skew metric calculations. To avoid duplication in this scenario, use only the front tier as the source of a session.
The preceding figure identifies the port to use as the source when a monitoring application talks between two servers. When the front-end server is the source, the monitoring application sees transmitted and received data at the client and at the back-end server. When more than two tiers are talking, use the interface of every alternate tier as a source.
The preceding figure shows how to avoid packet duplication by sourcing the port mirror from every other tier of a multi-tiered application. Notice that each flow of data is only circled (sourced) once, and each flow crosses only one source port.
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