An administrator finds that traffic from a large file download over an HTTP session is not load-balanced across multiple links in a LAG. What is the possible cause?

In a Link Aggregation Group (LAG), the behavior described—where traffic from a large file download over an HTTP session is not load-balanced across multiple links—is typically what you would expect, particularly with TCP connections. This is due to the way that TCP sessions work. A TCP session, once established, will generally send all packets for that session over the same path, leading to the phenomenon where it appears that not all links are being utilized for that particular download.

Load balancing in this context often occurs at a session or flow level, rather than at an individual packet level. Therefore, because file downloads are typically managed by a single TCP connection (resulting in a single flow), the load balancing mechanism does not split the session traffic across the links in the LAG, and thus it uses one link exclusively for that session.

This behavior does not indicate a configuration issue or an error in the system; rather, it reflects a fundamental characteristic of TCP sessions and the hash algorithms that determine traffic distribution in a LAG. In scenarios with multiple flows or sessions, the traffic would be load-balanced effectively across the available links.