Engineered time synchronisation spanning tree (sync tree) for a given gPTP domain refers to the usage of external port configuration instead of BMCA to construct a desired sync tree with the Grandmaster PTP Instance as the root (see IEEE Std 802.1AS2020, 10.3.1).
One of the advantages of engineered sync trees is to enable a planned, deterministic, and stable configuration of the IEEE Std 802.1AS2020 sync tree for a given gPTP domain, e.g., prevent sync tree changes in case of UAFX Station addition or removal from the network. Working Clock is a use case of the engineered sync tree.
An external sync tree management entity should use the remote management interface described in clause 126.96.36.199.4 to set up an engineered sync tree.
- The externalPortConfigurationEnabled parameter is set to TRUE;
- The ptpPortEnabled parameter is set to TRUE.
For validation that the computed sync tree configuration can be applied to all PTP Ports intended to participate in the given gPTP domain, the management entity configuring the sync tree will, for example, verify that these ports are up, IEEE Std 802.1AS2020capable, and satisfy topology constraints by checking the following parameters:
- The status of oper-status parameter is up (see IETF RFC 8343) for all participating Ethernet links;
- The status of isMeasuringDelay (see IEEE Std 802.1AS2020, 14.16.4) parameter is TRUE;
- The status of asCapable (see IEEE Std 802.1AS2020, 14.8.7) is TRUE;
- The status of asCapableAcrossDomains (see IEEE Std 802.1AS2020, 14.16.5);
- The status of gmCapable (see IEEE Std 802.1AS2020, 14.2.7) is TRUE, only applicable to the Grandmaster PTP Instance;
- Verify that the number of PTP Relay Instances (hops) between the Grandmaster PTP Instance and any given PTP End Instance is within the limit prescribed by an external sync tree management entity;
- Verify per PTP link that the value of meanLinkDelay (see IEEE Std 802.1AS2020, 14.16.6) is less than or equal to meanLinkDelayThresh (see IEEE Std 802.1AS2020, 14.16.7 and IEEE Std 802.1AS2020 Table 11-1) value to detect, e.g., an anomaly in propagation delay;
NOTE Even if neighbouring PTP Instances do report asCapable, it can be that the link between asCapable neighbouring PTP Instances is not asCapable due to, for example, the wrong setting of meanLinkDelayThresh value. The meanLinkDelayThresh value reflects the estimated propagation delay of the installed link.
The sync tree needs to have the following properties to ensure consistent protocol behaviour and time synchronisation:
- The desiredState of all PTP Ports of the Grandmaster PTP Instance is set to MasterPort;
- The desiredState of exactly one PTP Port of all the other PTP Instances is set to SlavePort;
- The desiredState of remaining PTP Ports that are part of sync tree in non-Grandmaster PTP Relay Instances is set to MasterPort.
- The desiredState of all other PTP Ports is set to PassivePort.
After synthesis, the configuration of the gPTP domain and its engineered sync tree may then be applied and validated, for example: