When multiples equal-cost paths satisfy the constraints of a specific RSVP LSP path, CSPF in the router head-end node selects a path so that LSP bandwidth is balanced across the network links. In releases before R7.0, CSPF used a random number generator to select the path and returned it to MPLS. In the course of time, this method actually balances the number of LSP paths over the links in the network; it does not necessarily balance the bandwidth across those links.
The least-fill path selection algorithm identifies the single link in each of the equal cost paths which has the least available bandwidth in proportion to its maximum reserved bandwidth. It then selects the path which has the largest value of this figure. The net effect of this algorithm is that LSP paths are spread over the network links over time such that percentage link utilization is balanced. When the least-fill option is enabled on an LSP, during a manual reset CSPF applies this method to all path calculations of the LSP, also at the time of the initial configuration.