All downstream traffic on an MLPPPoX bundle with multiple links is always MLPPPoX encapsulated. Some traffic is fragmented and served in a octet oriented round robin fashion over multiple member links. However, fragments are never delayed when the bundle contains multiple links.
In a per fragment/packet load sharing algorithm, there is always the possibility that there is uneven load utilization between the member links. A single link overload can go unnoticed in the network all the way to the Access Node. The access node is the only node in the network that actually has multiple physical links connected to it. All other session-aware nodes (LAC and LNS) only see MLPPPoX as a bundle with multiple sessions without any mechanism to shape traffic per physical link. Other nodes in this case being 7750 SRs. Other vendors may have the ability to condition (shape) traffic per session.
If one of the member sessions is perpetually overloaded by the LNS, traffic is dropped in the last mile because the corresponding physical link cannot absorb traffic beyond its physical capabilities. This would have detrimental effects on the whole operation of the MLPPPoX bundle. To prevent this perpetual overloading of the member links that can be caused by per packet/fragment load balancing scheme, the load balancing scheme that considers the number of octets transmitted over each member link. The octet counter of a new link is initialized to the lowest value of any existing link counter. Otherwise the load balancing mechanism would show significant bias toward the new link until the byte counter catches up with the rest of the links.