ign the timing flow for normal and reasonable failure conditions. Do not just turn it on every link and expect it to work reliably. Note that this lesson ap- plies especially to SyncE, PTP profiles such as G.8275.1 are more forgiving, just do not go over- board with large numbers of input ports. Be careful of the correct configuration of TDM and SyncE frequency inputs and their quality levels. For the North American markets, the SyncE for any network equipment should be configured as an option 2 EEC clock, and for other markets, it is usually option 1 (ITU-T/ETSI). This difference is a mirror of the distinction between the E1 and T1 systems. Remember that option 1 (the E1 option) might be the default; it certainly is on Cisco devices. The SyncE value for the configured option may take a default value that aligns with any configura- tion for TDM, SO setting option 2 for TDM might make option 2 the default for ESMC. Either way, especially if you are working on systems in North America, double-check that the configuration of SSM and ESMC is set to be option 2. Another trap is the so-called revertive mode, which can be specified as an option for the frequency selection process on some devices. Normally, when the primary source of frequency (say, SyncE from Ethernet interface numbered as port 1) goes down and port 2 is valid (and configured) as a secondary source, the frequency system will select port 2. When port 1 signals that it has regained traceability to a QL-PRC/QL-PRS source of frequency, most operators expect the system to select the higher-priority port 1 again as the source of fre- quency for the system. This is the case only when revertive mode is set (and, of course, the QL re- ceived on port 1 is the same as on port 2), because otherwise it will continue to use port 2 until there is another rearrangement in the sources of frequency. This catches out a lot of engineers. A similar problem arises with a timer associated with the frequency source. If revertive mode is configured, then port 1 will become the 