Least Connections

If there are multiple pool members with the same lowest number of connections, then least-connections will round-robin between those members.

Note that there are two least-connections methods - node (which considers the number of connections on the node from all pools), and member (which considers only the number of connections to the node from that pool)

Also be aware that if you use least-connections and you have one server (pool member) that is responding quickly, but giving an error page, and all the other pool members are working, but take time to process the requests, then the BigIP will keep selecting the bad one, since it will always have less connections (as it responds to them more quickly)

You may wish to read this article, which explains how the various load balancing methods work