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--An unrecoverable condition has occurred; for example, when an interface is shut down.
This type of condition is vendor specific. A notification about the condition may be sent immediately
and continuously.
•
Critical Event--An unspecified critical event has occurred. 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.
When an interface is set to the remote loopback mode the interface no longer participates in any other Layer
2 or Layer 3 protocols; for example Spanning Tree Protocol (STP) or Open Shortest Path First (OSPF). The
reason is that when two connected ports are in a loopback session, no frames other than the OAM PDUs are
sent to the CPU for software processing. The non-OAM PDU frames are either looped back at the MAC level
or discarded at the MAC level.
Carrier Ethernet Configuration Guide (Cisco ASR 920 Series)
302
Using Ethernet Operations Administration and Maintenance
OAM Features
