its own. A MEP receiving ETH-AIS recognizes that the fault is at a lower level and then
suppresses alarms at current level.
ACX Series routers support ETH-AIS PDU generation for server MEPs based on the
following defect conditions:
•
Loss of connectivity (physical link loss detection)
•
Layer 2 circuit or Layer 2 VPN down
Alarm indication signaling is done through the transmission and propagation of ETH-AIS
PDUs. ETH-AIS should be enabled on MEPs. A MEP which is configured to issue packets
with ETH-AIS information is generally of server layer and continues to transmit periodic
packets with ETH-AIS information until the defect condition is cleared. CFM MEPs, upon
receiving ETH-AIS PDUs, suppresses loss of continuity alarms associated with its peer
MEPs. A MEP resumes loss of continuity alarm generation upon detecting loss of continuity
defect conditions in the absence of an ETH-AIS condition.
For point-to-point Ethernet connectivity, a MEP has only a single peer MEP. Therefore, a
MEP suppress alarms on its peer MEP when it receives the ETH-AIS information.
For multi-point Ethernet connectivity, a MEP which receives ETH-AIS information cannot
determine the exact MEP encountered a fault condition and therefore it will not be able
to isolate the exact peer MEP for alarm suppression. ITU-T Y.1731 recommends suppressing
alarms for all peer MEPs irrespective of the connectivity status.
AIS transmission—A MEP upon detecting a defect condition transmits ETH-AIS PDUs in
a direction opposite to its peer MEPs. The transmission of ETH-AIS PDUs is based on a
configured ETH-AIS transmission period. An ETH-AIS transmission period of 1 second is
recommended. The first ETH-AIS PDU must be transmitted immediately following the
detection of a defect condition.
AIS reception—A MEP upon receiving ETH-AIS PDUs examines it to ensure that its
maintenance domain (MD) level corresponds to the same MD level. Upon receiving an
ETH-AIS PDU, the MEP detects a defect condition. Following the detection of a defect
condition, if there are no ETH-AIS PDUs received within an interval of 3.5 times the
ETH-AIS transmission period indicated in the ETH-AIS PDUs received earlier, the MEP
clears the defect condition. After the fault condition is cleared, MEPs continue to report
alarms.
NOTE: ACX Series routers do not support ITU-T Y.1731 ETH-AIS for layer 2
services (bridging).
The following are the limitations for server MEP
•
Triggering of ETH-AIS messages over services (Layer 2 circuit and Layer 2 VPN) by the
link-loss server MEP is done on a best-effort manner. This is because the transmission
of ETH-AIS messages is independent of the service status and there is no guarantee
for delivering the ETH-AIS messages before service goes down.
1137Copyright © 2017, Juniper Networks, Inc.
Chapter 34: Configuring Operations, Administration, and Management (OAM)
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