Administrator Guide

1674 Audio Video Bridging
clock. If the best master clock is grandmaster-capable, then the clock
becomes the grandmaster clock for the 802.1AS domain, generating time
synchronization information periodically. The ANNOUNCE message also
includes a path trace TLV that tracks the path to best master clock. Each
time-aware system updates the received ANNOUNCE message by appending
its clock identity to the path trace TLV. The path trace TLV is mandatory as
long as the link MTU is not exceeded, in which case the ANNOUNCE
message is transmitted without path trace TLV information.
Time Synchronization
A grandmaster clock periodically synchronizes the attached slave clock
devices through the use of SYNC and FOLLOW_UP messages. The master
timestamps the transmission of each SYNC message and the slave
timestamps its reception. The transmit timestamp is conveyed to the slave via
the FOLLOW_UP message. This method is illustrated below:
Figure 47-2. SYNC and FOLLOW_UP Messages
The receipt of FOLLOW_UP message causes the local clock to be
synchronized. SYNC and FOLLOW_UP messages must be sent by the
current master clock, otherwise the slave port discards the messages. To
match a FOLLOW_UP message to SYNC message, there is a sequence ID