y: Operators can allocate priority values to the assigned fre- quency sources to design a preferred network synchronization flow. Prioritization of the sources helps in the cases where the same QL value is received from two (or more) different frequency sources but the operator has a preference. Of course, setting equal priorities indicates that neither is favored over the other. External configuration commands: ITU-T G.781 recommends multiple configuration com- mands that operators can use to influence the clock selection process. The primary use of these commands is for maintenance purposes (such as maintenance of a physical port). These direct- ives can be categorized as commands to do the following: Detach one or more physical ports from the clock selection process. These are referred to as lockout commands. If a port is configured for lockout, the clock selection process does not consider the port for any frequency selection decisions. G.781 recommends both a set and clear operation to enable and disable the lockout for physical ports. Switch to a different frequency source from the one currently chosen by the clock selection process. G.781 recommends that vendors implement a forced switch command for an inter- face that can override the currently selected synchronization source and force selection of another (assuming the port is question is not in locked-out state). It should support both a set and clear operation. They also suggest a manual switch command for a port that would influence the clock se- lection process to switch to the port in question. This command is accepted only if the con- figured port is enabled, not locked out, not in signal fail condition, and has a better QL than DNU (in QL-enabled mode); otherwise it is rejected. It should also support both a set and clear operation. In summary, the clock selection process selects the frequency source with the highest QL, provided it is not experiencing a signal fail condition. If multiple sources have the same QL, the source with the highest prior