History
NetFlow was originally a Cisco packet switching technology for Cisco routers, implemented in IOS 11.x around 1990. It was originally a software implementation for the Cisco 7000, 7200 and 7500, where it was thought as an improvement over the then current Cisco Fast Switching.
The idea was that the first packet of a flow would create a NetFlow switching record. This record would then be used for all later packets of the same flow, until the expiration of the flow. Only the first packet of a flow would require an investigation of the route table to find the most specific matching route. This is an expensive operation in software implementations, especially the old ones without Forwarding information base. The NetFlow switching record was actually some kind of route cache record, and old versions of IOS still refer to the NetFlow cache as ip route-cache.
This technology was advantageous for local networks. This was especially true if some of the traffic had to be filtered by an ACL as only the first packet of a flow had to be evaluated by the ACL.
NetFlow switching soon turned out to be unsuitable for big routers, especially Internet backbone routers, where the number of simultaneous flows was much more important than those on local networks, and where some traffic causes lots of short-lived flows, like Domain Name System requests (whose source port is random for security reasons).
As a switching technology, NetFlow was replaced around 1995 by Cisco Express Forwarding. This first appeared on Cisco 12000 routers, and later replaced NetFlow switching on advanced IOS for the Cisco 7200 and Cisco 7500.
As of 2012, technologies similar to NetFlow switching are still in use in most firewalls and software-based IP routers. For instance the conntrack feature of the Netfilter framework used by Linux.
Read more about this topic: NetFlow
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