Huawei MPLS OSN3500 equipment APS external inversion supports six types of clearing, forced inversion, manual inversion to work, manual inversion to protection, practice inversion, and protection locking, which are basically consistent with SDH protection methods such as MSP and SNCP. During the test of MPLS APS inversion priority, the customer first forcefully inverts the service to the protection channel, and then breaks the protection channel fiber, as a result, the APS protection group is inverted, and the priority Auto (SF) > Force Inversion (FS), which is different from SDH protection Force Inversion (FS) > Auto (SF). The customer is in doubt and asks for the reason.

Device version information:

OSN3500V1R9C03SPC100;

t2000v200r007c03spc004;

[Cause Analysis]: MPLS APS follows the standard ITU-T Y.1720. the Y.1720 protocol describes the external inversion command priority as follows:

Table1/Y.1720

Priority of local requests

NOTE 1 - A forced switch for workingLSPshould not be overridden by a Signal Fail on the protectionLSP. Since unidirectional protection switching is being performed and no APS protocol is supported over the protectionLSP, Signal Fail on the protectionLSPdoes not interfere with the ability to perform a protection switch. performed and no APS protocol is supported over the protectionLSP, Signal Fail on the protectionLSPdoes not interfere with the ability to perform a Signal Fail on the protectionLSPdoes not interfere with the ability to perform a forced switch for workingLSP.

NOTE 1-Forced switching for the workingLSP should not be performed without regard to Signal Fail on the protectionLSP. Since a unidirectional protection flip-flop will be performed and the APS protocol is not supported on the protectionLSP, Signal Fail on the protectionLSPdoes not interfere with the ability to perform a forced switch for workingLSP.

NOTE 2 - A forced switch for protectionLSPis not defined because this function may be achieved via a lockout of the protection command.

NOTE 2 - A forced switch for protectionLSPis not defined because this function may be achieved via a lockout of protection command.

The above table also specifies the priority ForcedSwitch>Signal Fail, but it should be noted that a special case is given in Note 1, in other words, the meaning can be interpreted as the SF priority of the protection channel is to be higher than the forced inversion of the working channel. In fact, it is not difficult to understand, because the APS protocol message is fixed to be sent on the protection channel, to ensure that the protection channel signal failure (SF) can not be involved in the corresponding protocol message transmission, otherwise the protection state of the two ends of the source and host will produce anomalies. The main point here is that there is no further refinement of the SF scenario. In this regard, we can further refer to the description of the corresponding APS inversion priority in the ITU-T G.8031/Y.1342 protocol Ethernet protection inversion:

Table 11-1/G.8031/Y.1342 - Code Points and Field Values for APS Specific Information

The above table further refines the SF scenarios into two types, SF-P and SF, where the signal failure SF-P priority of the protected channel is higher than the forced inversion.

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