The OAM features as defined by IEEE 802.3ah, Ethernet in the First Mile, are discovery, Link Monitoring, Remote Fault Detection, Remote Loopback, and Cisco Proprietary Extensions.
Discovery is the first phase of Ethernet OAM and it identifies the devices in the network and their OAM capabilities. Discovery
uses information OAM PDUs. During the discovery phase, the following information is advertised within periodic information
OAM mode—Conveyed to the remote OAM entity. The mode can be either active or passive and can be used to determine device
OAM configuration (capabilities)—Advertises the capabilities of the local OAM entity. With this information a peer can determine
what functions are supported and accessible; for example, loopback capability.
OAM PDU configuration—Includes the maximum OAM PDU size for receipt and delivery. This information along with the rate limiting
of 10 frames per second can be used to limit the bandwidth allocated to OAM traffic.
Platform identity—A combination of an organization unique identifier (OUI) and 32-bits of vendor-specific information. OUI
allocation, controlled by the IEEE, is typically the first three bytes of a MAC address.
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 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
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.
For more information on Dying Gasp, see the Dying Gasp Support for Loss of Power Supply Through SNMP, Syslog and Ethernet
OAM chapter in the Cisco NCS 520 Series Router Configuration Guide.
Critical Event—An unspecified critical event has occurred. This type of event is vendor specific. A critical event may be
sent immediately and continuously.
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.