AFAICT, the only part of 802.1AS-2011 that mentions when to transmit an
announce message is Figure 10-15 on page 84. On the right hand side
there is the state TRANSMIT_ANNOUNCE which calls the txAnnounce method.

That state is only reachable if (selectedRole == MasterPort), and so
it looks like we should not transmit when is slave only mode, just
like in 1588.

Thanks,
Richard