The MIB describes this simply as a count of CFM frames received tagged with a given MD level, but the object's name, RxInvalidMAID, identifies its real purpose as counting CCMs whose Maintenance Association ID does not match this MEP's own MAID, the classic cross-connect symptom where a CCM from an unrelated service lands on the wrong MEP. A nonzero count here is one of the strongest signals of a VLAN or service misconfiguration somewhere upstream, since two different maintenance associations should never see each other's heartbeats at all. If this counter starts incrementing right after a new service was provisioned on a shared trunk, that new service's VLAN mapping is the first place to check.