The appropriate port set size is determined from the combination of the port set size received via the PORT_SET option and the locally configured policy which may limit the port set size.
If the requested port set is initially not available at the Suggested External Port, an attempt is made to find a new set of ports of the appropriate size. The appropriate PORT_SET size in this context means a combination of the requested PORT_SET size and the local limits set by the operator in the SR OS node.
If the available number of consecutive ports in a set for the specified external IP address is fewer that the requested amount or stated in the policy, the maximum number of available consecutive ports are allocated to the client.
In summary, the SR OS tries to find the biggest available port set (as dictated by the combination of the requested size and local policy), instead of allocating random port sets available at the suggested external port.