opagates around the ring. However, for redundancy, each node can accept synchronization from either of its neighbors (assuming the port is capable of it). The operators can influence the direction of flow by configuring the preferred source as having the highest priority (see the preceding section on the clock selection process). So, Switch C can extract timing and synchronize itself to either Switch B or Switch D. In this case, the operator configures it in a clockwise direction-with Switch B synchronized to Switch A, Switch C to Switch B, and Switch D to Switch C. This capability of being able to synchronize to either input (changing the dir- ection of flow) does increase the reliability of the network but at the same time also raises the pos- sibility of timing loops. These points expand on the details in Figure 6-11: Each link connecting the switches is shown as two unidirectional arrows; this is to show the timing transfer from one switch to the other in both directions. The direction of the arrows shows the transfer of timing information from one switch to another. All links carry timing information in the example. The solid arrows indicate the direction of a link that is selected as the input timing reference to a switch. The dashed arrows indicate the direction of a link that is not selected as the input timing reference but will be available for se- lection during a clock rearrangement. For example, Switch A is transferring the frequency toward Switch B and Switch D; the solid arrow from Switch A to Switch B shows that Switch B selected this link as its input timing ref- erence; whereas the dashed arrow from Switch A to Switch D shows that Switch D has not se- lected this link as input timing reference. This example shows Switch D selects Switch C as its preferred source of frequency even though Switch A might be "closer" to the PRC/PRS. This is because the network engineer con- figures the preferred priority in the order of possible frequency sources. Frequency traceability schemes such as SSM do not ha