iority2 attribute allows 256 additional priorities to be used before the BMCA goes to a tiebreaker. The tiebreaker is based on the clockIdentity field. The clockClass (from the Announce message) signals the traceability of clock quality. The value of 6 indicates that the clock has a traceable path back to a PRTC source of time, meaning the grand- master at the top of the figure. In Figure 7-4, they all have this traceability. clockClass = 6 Grandmaster stepsRemoved = 0 clockClass 6 stepsRemoved = 0 Slave port 0 Boundary clock Slave port 0 clockClass = 6 clockClass = 6 Boundary clock stepsRemoved = 1 stepsRemoved = 1 Master port 3 Boundary clock Master port 1 clockClass = 6 stepsRemoved = 1 Announce messages Slave port 0 clockClass 6 Ordinary slave clock stepsRemoved = 2 Figure 7-4 BMCA Running on a Multi-Port BC The stepsRemoved indicates how many different PTP boundary clocks have been crossed in the way from the grandmaster to the current clock. Therefore, the grandmaster sends out an Announce message with the value set to zero, while the left and center boundary clocks will send it out with the value of one and the right boundary clock with value of two. Given that the clock class in this network is the same (they are all traceable to a PRTC) for every Announce message, BMCA will select the port receiving the Announce message with the smallest value for the steps removed, and the master and slave ports will align themselves (master, slave, and passive) as shown in the figure. 7.3.1 PTP Datasets Both ordinary and boundary clocks maintain two types of datasets, known as clock datasets and port datasets. In IEEE 1588-2008 terminology, the fields in a dataset are known as members. The clock dataset names and the corresponding sections of 1588-2008 that cover them are as follows: Default dataset or defaultDS: Attributes describing the entire clock and common to every port (section 8.2.1). Current dataset or currentDS: Attributes related to the current state of the clock resulting from the mechanisms of synchronizat