Hello David,
The load-balancing algorithm will be use at the time of forwarding packets based on the function you determine to use (Default is to generate a hash from the source and destination IP address) and as you already know which interface will be used is determined in a round-robin fashion starting with the lowest port ID.
Looking for some Networking Assistance?
Contact me directly at jcarvaja@laguiadelnetworking.com
I will fix your problem ASAP.
Cheers,
Julio Carvajal Segura
http://laguiadelnetworking.com
Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC