There are a few vendors who now provide traffic export from high-speed
interfaces by sampling those interfaces at a particular rate, and
using the sampled packets to populate the per-flow counters, rather
than looking at every packet.
Folks might be interested in the Internet Draft on "A framework for
passive packet measurement" that focuses on packet sampling. See
http://www.ietf.org/internet-drafts/draft-duffield-framework-papame-00.txt
There's a public mailing list at
listname : psamp@ops.ietf.org
subscribe: psamp-request@ops.ietf.org (with "subscribe" as the message)
archive : https://ops.ietf.org/lists/psamp/
The goal is to define a minimal set of requirements that need to be
supported in routers (or line cards) to meet the needs of folks that
operate networks. Feedback more than welcome!
-- Jen