MPLS-TP OAM Alarm Suppression Packets
Short answer
BC AIS and FDI packets are used in MPLS and MPLS-TP OAM to suppress downstream alarms.
Question
Key Takeaways
This question tests the specific MPLS-TP OAM packets responsible for alarm suppression, trapping learners who might confuse backward notification (BDI) or defect detection (CC) with forward defect suppression.
In MPLS and MPLS-TP OAM, AIS and FDI packets are used to suppress downstream alarms caused by an upstream fault, while BDI and CC serve other purposes.
Common mistake: Selecting BDI because it is a defect indication packet, failing to recognize that BDI notifies the upstream end of a fault rather than suppressing downstream alarms.
Expert Analysis
Alarm Suppression Principle: In MPLS and MPLS-TP OAM, alarm suppression is a critical mechanism to prevent a single upstream fault from flooding the network with cascading alarms from downstream nodes. When a fault is detected, specific OAM packets are generated and sent downstream to inform subsequent nodes that the defect is already known, thereby suppressing secondary alarms at downstream Maintenance Entity Group (MEG) endpoints.
AIS and FDI Roles: The Alarm Indication Signal (AIS) and Forward Defect Indication (FDI) packets both serve this forward-direction suppression purpose. Option B (AIS) is the traditional and most common packet used to suppress downstream alarms following a signal fail or degrade condition. Option C (FDI) operates similarly, carrying defect type and location information forward to suppress alarms. In many ITU-T standards, AIS is considered a specific type of FDI, making both correct choices for alarm suppression.
BDI and CC Functions: Option D (BDI) is the Backward Defect Indication packet, which is sent in the opposite direction—upstream—to notify the source of the failure, allowing for fault localization or protection switching at the remote end, but it does not suppress alarms. Option A (CC) represents Continuity Check packets, which are used proactively to detect loss of continuity (LOC) defects; they are the source of defect detection, not the mechanism for alarm suppression.
Community Corroboration: Community input correctly identifies AIS and FDI as the alarm suppression packets, explicitly noting that BDI is used to send fault indicators upstream to notify the remote end rather than to suppress alarms.
Official Reference
https://tools.ietf.org/html/rfc6374
This link opens the official vendor documentation in a new tab.
Exam Strategy
When asked about OAM alarm suppression in transport networks, immediately associate it with forward-direction defect indicators (AIS, FDI). Backward indicators (BDI) are for upstream notification, and proactive checks (CC) are for defect detection. Grouping OAM packets by their directional purpose (forward vs. backward, detection vs. suppression) prevents mixing their functions on the exam.
Community Votes
50% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 1 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 1
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ethassan 👍 1AIS and FDI. BDI is not used to suppress the alarm, instead it is used to send fault indicators upstream to notify the remote end of a local failure.
Reader comments & corrections
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