Specifications

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Cisco MWR 2941 Mobile Wireless Edge Router Release 3.5 Software Configuration Guide, Cisco IOS Release 15.1(3)MR
OL-26895-01
Chapter 15 Configuring Ethernet OAM, CFM, and E-LMI
Understanding the Ethernet OAM Protocol
• 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, a power failure. 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.
From a user's perspective, an interface in loopback mode is in a link-up state.
Cisco Vendor-Specific Extensions
Ethernet OAM allows vendors to extend the protocol by allowing them to create their own
type-length-value (TLV) fields.