CFM MEP declares a connectivity fault when its defect flag is equal to or higher than its configured lowest defect priority. The defect can be any of the following depending on configuration:
DefRDICCM
DefMACstatus
DefRemoteCCM
DefErrorCCM
DefXconCCM
The following additional fault condition applies to Y.1731 MEPs:
reception of AIS for the local MEP level
Setting the lowest defect priority to allDef may cause problems when fault propagation is enabled in the MEP. In this scenario, when MEP A sends CCM to MEP B with interface status down, MEP B will respond with a CCM with RDI set. If MEP A is configured to accept RDI as a fault, then it gets into a dead lock state, where both MEPs will declare fault and never be able to recover.
The default lowest defect priority is DefMACstatus, which will not be a problem when interface status TLV is used. It is also very important that different Ethernet OAM strategies should not overlap the span of each other. In some cases, independent functions attempting to perform their normal fault handling can negatively impact the other. This interaction can lead to fault propagation in the direction toward the original fault, a false positive, or worse, a deadlock condition that may require the operator to modify the configuration to escape the condition. For example, overlapping Link Loss Forwarding (LLF) and ETH-CFM fault propagation could cause these issues.
For the DefRemoteCCM fault, it is raised when any remote MEP is down. So whenever a remote MEP fails and fault propagation is enabled, a fault is propagated to SMGR.