Hi Sarah,
Indeed, the router will load-balance every time it uses the default route. However, with the CEF turned on (by default), there are two methods of load-balancing: the per-destination and the per-packet approach.
The per-destination load balancing method is the default. It means that packets towards X.X.X.X matching the default route will always be sent through one next hop, while packets towards another destination Y.Y.Y.Y also matching the default route will be sent through another next hop. This is done to prevent packet reordering due to different delays over different paths, and also, to prevent problems with stateful firewalls that would drop subsequent packets if they didn't receive the first packet.
The per-packet load balacing does exactly what you described earlier: each time the default route entry is used, the packet will be forwarded to a different next hop. This makes the link utilization more equal but unfortunately, there are negative side effects to possible packet reordering, and it is totally unusable if stateful firewalls, NAT boxes, proxies or similar devices are put into path.
Best regards,
Peter
