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What Is NetFlow?

NetFlow technology efficiently provides the metering base for a key set of applications including network traffic accounting, usage-based network billing, user and application monitoring, profiling, network planning and analysis, outbound marketing, and data warehousing for both service provider and enterprise customers.

NetFlow enables you to collect traffic flow statistics on your routing devices. NetFlow is based on identifying packet flows for ingress IP packets. It does not involve any connection-setup protocol either between routers or to any other networking device or end station and does not require any change externally - either to the traffic or packets themselves or to any other networking device. NetFlow is completely transparent to your existing network, including end stations and application software and network devices like LAN switches. Also, NetFlow is performed independently on each internetworking device, it need not be operational on each router in the network. Using NetFlow Data Export (NDE), you can export data to a remote workstation for data collection and further processing. Network planners can selectively invoke NDE on a router or on a per-subinterface basis to gain traffic performance, control, or accounting benefits in specific network locations.

View: Netflow formats, configuration, RFC, applications etc.

A network flow is a sequence of packets between a given source and destination in one direction only. Cisco routers store and export information about the network flows they handle for network management purposes; high-end routers and switches use network flows to accelerate security processing. In order to distinguish flows from one another, the source and destination addresses and application (TCP/UDP) port numbers are used. The IP Type of Service byte, protocol type and the ifIndex of the input interface are also used to uniquely identify the flow to which a packet belongs. A flow is determined to have ended when it has been idle for a specified length of time, when it has become older than a specified age (30 minutes by default) or when the flow is a TCP connection a FIN or RST has been sent. The router may expire flows more aggressively if it is running out of cache space.

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