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Exporting Version 9 Flow Data Records to a Log Collector Overview Using an MX Series Router or NFX250

 

A flow record template defines a collection of fields with corresponding descriptions of the format and syntax for the elements or attributes that are contained in it. Network elements (such as routers and switches), which are called exports, accumulate the flow data and export the information to collectors, which are hosts or external devices that can save a large volume of such system log messages for events or system operations. The collected data provides granular, finer-level metering and statistical data for highly flexible and detailed resource usage accounting. Templates that are sent to the collector contain the structural information about the exported flow record fields; therefore, if the collector cannot interpret the formats of the new fields, it can still process the flow record.

The version 9 flow template has a predefined format. An export packet consists of a packet header followed by one or more FlowSet fields. The FlowSet fields can be any of the possible three types—Template, Data, or Options Template. The template flowset describes the fields that are in the data flowsets (or flow records). Each data flowset contains the values or statistics of one or more flows with the same template ID. An interleaved NetFlow version 9 export packet contains the packet header, Template FlowSet, and Data FlowSet fields. A Template FlowSet field signifies each event such as the creation of a NAT entry or the release of a NAT entry allocated, and the Data FlowSet field denotes the NAT sessions for which the Template FlowSet (or the event type) is associated. For example, if a NAT address entry creation, exhaustion of addresses in a NAT pool, and a NAT entry deletion or release occur, an interleaved version 9 export packet contains the packet header, one Template FlowSet field for NAT address creation, two Data FlowSet fields for the two sessions for which address creation is performed, another TemplateSet field for NAT address deletion, two Data FlowSet fields for the two sessions for which address deletion event occurs, and the other TemplateSet field for NAT pool consumption having exceeded the configured number of pools.

The following are the possible combinations that can occur in an export packet:

  • An export packet that consists of interleaved template and data FlowSets—A collector device should not assume that the template IDs defined in such a packet have any specific relationship to the data FlowSets within the same packet. The collector must always cache any received templates, and examine the template cache to determine the appropriate template ID to interpret a data record.

  • An export packet consisting entirely of data FlowSets—After the appropriate template IDs have been defined and transmitted to the collector device, most of the export packets consist solely of data FlowSets.

  • An export packet consisting entirely of template FlowSets—Although this case is the exception, it is possible to receive packets containing only template records. Ordinarily, templates are appended to data FlowSets. However, in some instances only templates are sent. When a router first boots up or reboots, it attempts to synchronize with the collector device as quickly as possible. The router can send template FlowSets at an accelerated rate so that the collector device has sufficient information to parse any subsequent data FlowSets. Also, template records have a limited lifetime, and they must be periodically refreshed. If the refresh interval for a template occurs and no appropriate data FlowSet that needs to be sent to the collector device is present, an export packet consisting only of template FlowSets is sent.