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Interface Sampling

For any given IP interface, enabling J-Flow causes packets from the input stream to be sampled at a globally configured rate. For each packet sampled, the main flow cache is examined to see if there is an existing entry. If no entry exists, J-Flow creates a new entry and records attributes of the flow. If the packet matches an existing entry, J-Flow updates the existing flow.

In general, the system samples packets that it can forward. In other words, the system does not sample packets that it discards. As sampling occurs, the system records flow characteristics as they would appear for a packet that the virtual router transmits. This means, for example, that if a packet uses the address of an output interface or next-hop value altered by a policy setting, the system records the altered value in the flow record.

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