Yeah...we never had success with the OSS solutions here. We used a lot of Tiara Networks (later bought by Nortel) boxes for this sort of thing. Worked great, but expensive, closed source, yada-yada. If there were only a couple of parallel paths, routers worked reliably if not always optimally or cheaply; sticking to one vendors multilink PPP was generally reliable, and worst case there is OSPF as one mysteriously downvoted commenter upstream noted.