How Ethernet OAM works
This section describes the working procedures of Ethernet OAM.
Ethernet OAM connection establishment
OAM connection establishment is also known as the Discovery phase, where an Ethernet OAM entity discovers the remote OAM entity to establish a session.
In this phase, two connected OAM entities exchange Information OAMPDUs to advertise their OAM configuration and capabilities to each other for a comparison. If their Loopback, link detection, and link event settings match, the OAM entities establish an OAM connection.
An OAM entity operates in active mode or passive mode. OAM entities in active mode initiate OAM connections, and OAM entities in passive mode wait and respond to the OAM connection requests. To set up an OAM connection between two OAM entities, you must set at least one entity to operate in active mode.
Table 2 shows the actions that a device can perform in different modes.
Table 2: Active Ethernet OAM mode and passive Ethernet OAM mode
Item | Active Ethernet OAM mode | Passive Ethernet OAM mode |
|---|---|---|
Initiating OAM Discovery | Available | Unavailable |
Responding to OAM Discovery | Available | Available |
Transmitting Information OAMPDUs | Available | Available |
Transmitting Event Notification OAMPDUs | Available | Available |
Transmitting Information OAMPDUs without any TLV | Available | Available |
Transmitting Loopback Control OAMPDUs | Available | Unavailable |
Responding to Loopback Control OAMPDUs | Available when both sides are operating in active OAM mode | Available |
After an Ethernet OAM connection is established, the Ethernet OAM entities exchange Information OAMPDUs at the handshake packet transmission interval to detect the availability of the Ethernet OAM connection. If an Ethernet OAM entity receives no Information OAMPDU within the Ethernet OAM connection timeout time, the Ethernet OAM connection is considered disconnected.
Link monitoring
Error detection in an Ethernet is difficult, especially when the physical connection in the network is not disconnected, but network performance is degrading gradually.
Link monitoring detects link faults in various environments. Ethernet OAM entities monitor link status by exchanging Event Notification OAMPDUs. When detecting one of the link error events listed in Table 3, an OAM entity sends an Event Notification OAMPDU to its peer OAM entity. The network administrator can keep track of network status changes by retrieving the log.
Table 3: Ethernet OAM link error events
Ethernet OAM link events | Description |
|---|---|
Errored symbol event | An errored symbol event occurs when the number of detected symbol errors in the detection window (specified number of received symbols) exceeds the predefined threshold. |
Errored frame event | An errored frame event occurs when the number of detected error frames in the detection window (specified detection interval) exceeds the predefined threshold. |
Errored frame period event | An errored frame period event occurs when the number of frame errors in the detection window (specified number of received frames) exceeds the predefined threshold. |
Errored frame seconds event | An errored frame seconds event occurs when the number of errored frame seconds (the second in which an errored frame appears is called an errored frame second) detected on a port in the detection window (specified detection interval) reaches the predefined threshold. |
Remote fault detection
Information OAMPDUs are exchanged periodically among Ethernet OAM entities across established OAM connections. When traffic is interrupted due to device failure or unavailability, the Ethernet OAM entity at the faulty end sends error information to its peer. The Ethernet OAM entity uses the flag field in Information OAMPDUs to indicate the error information (any critical link event type as shown in Table 4). You can use the log information to track ongoing link status and troubleshoot problems promptly.
Table 4: Critical link events
Type | Description | OAMPDU transmission frequencies |
|---|---|---|
Link Fault | Peer link signal is lost. | Once per second. |
Dying Gasp | An unexpected fault, such as power failure, occurred. | Non-stop. |
Critical Event | An undetermined critical event happened. | Non-stop. |
Remote loopback
Remote loopback is available only after the Ethernet OAM connection is established. With remote loopback enabled, the Ethernet OAM entity in active mode sends non-OAMPDUs to its peer. After receiving these frames, the peer does not forward them according to their destination addresses. Instead, it returns them to the sender along the original path.
Remote loopback enables you to check the link status and locate link failures. Performing remote loopback periodically helps to detect network faults promptly. Furthermore, performing remote loopback by network segments helps to locate network faults.