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Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide
OL-23826-09
Chapter 10 Configuring Ethernet OAM
Discovery includes an optional phase in which the local station can accept or reject the configuration of
the peer OAM entity. For example, a node may require that its partner support loopback capability to be
accepted into the management network. These policy decisions may be implemented as vendor-specific
extensions.
Link Monitoring
Link monitoring in Ethernet OAM detects and indicates link faults under a variety of conditions. Link
monitoring uses the event notification OAM PDU and sends events to the remote OAM entity when there
are problems detected on the link. The error events include the following:
• Error Symbol Period (error symbols per second)—The number of symbol errors that occurred
during a specified period exceeded a threshold. These errors are coding symbol errors.
• Error Frame (error frames per second)—The number of frame errors detected during a specified
period exceeded a threshold.
• Error Frame Period (error frames per n frames)—The number of frame errors within the last n
frames has exceeded a threshold.
• Error Frame Seconds Summary (error seconds per m seconds)—The number of error seconds
(1-second intervals with at least one frame error) within the last m seconds has exceeded a threshold.
Since IEEE 802.3ah OAM does not provide a guaranteed delivery of any OAM PDU, the event
notification OAM PDU may be sent multiple times to reduce the probability of a lost notification. A
sequence number is used to recognize duplicate events.
Remote Failure Indication
Faults in Ethernet connectivity that are caused by slowly deteriorating quality are difficult to detect.
Ethernet OAM provides a mechanism for an OAM entity to convey these failure conditions to its peer
via specific flags in the OAM PDU. The following failure conditions can be communicated:
• Link Fault—Loss of signal is detected by the receiver; for instance, the peer's laser is
malfunctioning. A link fault is sent once per second in the information OAM PDU. Link fault applies
only when the physical sublayer is capable of independently transmitting and receiving signals.
• Dying Gasp—This notification is sent for power failure, link down, router reload and link
administratively down conditions. This type of condition is vendor specific. A notification about the
condition may be sent immediately and continuously.
• Critical Event—An unspecified critical event occurs. This type of event is vendor specific. A critical
event may be sent immediately and continuously.
Remote Loopback
An OAM entity can put its remote peer into loopback mode using the loopback control OAM PDU.
Loopback mode helps an administrator ensure the quality of links during installation or when
troubleshooting. In loopback mode, every frame received is transmitted back on the same port except for
OAM PDUs and pause frames. The periodic exchange of OAM PDUs must continue during the loopback
state to maintain the OAM session.
The loopback command is acknowledged by responding with an information OAM PDU with the
loopback state indicated in the state field. This acknowledgement allows an administrator, for example,
to estimate if a network segment can satisfy a service-level agreement. Acknowledgement makes it
possible to test delay, jitter, and throughput.