Aggregate traffic management

[Stanislav Rost]

Dear NANOGers,

I have a very hands-on question:
Suppose I am a network operator for a decent-sized ISP, and I decide
that I want to "divide" aggregate traffic flowing through a router
toward some destination, in order to then send some of it through one
route and the remainder through another route.  Thus, I desire to
enforce some traffic engineering decision.

How would I be able to accomplish this "division"?  What technologies
(even if vendor-specific) would I use?  

I can think of some methods like prefix-matching classification and
ECMP, but I am still not sure exactly how the latter works in practice
(at the router level) and how one may set them up to achieve such
load-sharing.

Thank you for your expertise and lore,

-- 
Stanislav Rost <stanrost@lcs.mit.edu>
Laboratory for Computer Science, MIT