Cisco ASR 920 series, Руководство по настройке Ethernet-маршрутизатора

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Normal link operation does not require Ethernet OAM. OAM frames, called OAM protocol data units (PDUs),
use the slow protocol destination MAC address 0180.c200.0002. They are intercepted by the MAC sublayer
and cannot propagate beyond a single hop within an Ethernet network.
Ethernet OAM is a relatively slow protocol with modest bandwidth requirements. The frame transmission
rate is limited to a maximum of 10 frames per second; therefore, the impact of OAM on normal operations is
negligible. However, when link monitoring is enabled, the CPU must poll error counters frequently. In this
case, the required CPU cycles will be proportional to the number of interfaces that have to be polled.
Two major components, the OAM client and the OAM sublayer, make up Ethernet OAM. The following two
sections describe these components.
OAM Client
The OAM client is responsible for establishing and managing Ethernet OAM on a link. The OAM client also
enables and configures the OAM sublayer. During the OAM discovery phase, the OAM client monitors OAM
PDUs received from the remote peer and enables OAM functionality on the link based on local and remote
state as well as configuration settings. Beyond the discovery phase (at steady state), the OAM client is
responsible for managing the rules of response to OAM PDUs and managing the OAM remote loopback
mode.
OAM Sublayer
The OAM sublayer presents two standard IEEE 802.3 MAC service interfaces: one facing toward the superior
sublayers, which include the MAC client (or link aggregation), and the other interface facing toward the
subordinate MAC control sublayer. The OAM sublayer provides a dedicated interface for passing OAM
control information and OAM PDUs to and from a client.
The OAM sublayer is made up of three components: control block, multiplexer, and packet parser (p-parser).
Each component is described in the following sections.
Control Block
The control block provides the interface between the OAM client and other blocks internal to the OAM
sublayer. The control block incorporates the discovery process, which detects the existence and capabilities
of remote OAM peers. It also includes the transmit process that governs the transmission of OAM PDUs to
the multiplexer and a set of rules that govern the receipt of OAM PDUs from the p-parser.
Multiplexer
The multiplexer manages frames generated (or relayed) from the MAC client, control block, and p-parser.
The multiplexer passes through frames generated by the MAC client untouched. It passes OAM PDUs generated
by the control block to the subordinate sublayer; for example, the MAC sublayer. Similarly, the multiplexer
passes loopback frames from the p-parser to the same subordinate sublayer when the interface is in OAM
remote loopback mode.
P-Parser
The p-parser classifies frames as OAM PDUs, MAC client frames, or loopback frames and then dispatches
each class to the appropriate entity. OAM PDUs are sent to the control block. MAC client frames are passed
to the superior sublayer. Loopback frames are dispatched to the multiplexer.
Carrier Ethernet Configuration Guide (Cisco ASR 920 Series)
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Using Ethernet Operations Administration and Maintenance
Ethernet OAM