tive to understand the active topology under consideration than it is with mechanisms such as RSTP. Figure 14.8 illustrates the architecture for 1+1 bidirectional protection switching. Traffic is duplicated on both the working and protection paths, and the appropriate path is selected by the sink. As is common with other transport technologies, the switching architectures provide a number of features that give network providers operational flexibility, including Nonrevertive switching: The traffic remains on the protection path and does not revert to the working path after the defect has cleared. This is generally employed in 1+1 Chapter 14 Next Generation Ethernet Working Transport Entity Protection Transport Entity Protection- Protection- Switching Switching Process Process Working Transport Entity Protection Transport Entity Fig. 14.8 Ethernet 1+1 protection-switching architecture. schemes and has the advantage that it minimizes the number of times that traffic is impacted by switching events. Revertive switching: The traffic is returned to the working path from the protection path after the defect has cleared. Generally speaking, the protection path will often have lower performance (e.g., larger delay) than the working path, and for this reason reversion is desirable. If there is an intermittent fault, it is desirable to avoid frequent switching events. This can be achieved by implementing a wait-to-restore timer that waits a period of time before switching can occur. The time can be configured in steps of 1 minute between 5 and 12 minutes. Hold-off timers: In multilayer networks there are often protection schemes operating at different levels, and it is not desirable to have multiple protection events in different layers occurring simultaneously in response to a single event. It is preferable to give protection schemes in lower layers sufficient time to restore service and only to respond at a higher layer if it is clear that the lower layer has had sufficient time to clear the defect and has failed to do S