Overview

Ethernet in the First Mile (EFM) is the Ethernet physical layer specification which is mainly used between the link of the access layer and the aggregation layer for Ethernet management and maintenance. EFM is the link-level OAM (Operation and Management). For the link between two directly connected devices, it provides link connectivity detection function, link failure monitoring function and remote loopback function.

Figure 1.     Networking diagram for Ethernet OAM

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Types of EFM OAMPDU

EFM works at the data link layer, and its protocol messages are called OAMPDUs (OAM Protocol Data Units). EFM detects and reports link status through periodic OAMPDUs between devices, enabling network administrators to manage the network effectively. Table 1 shows the OAMPDUs types supported by PICOS.

Table 1. Types of EFM OAMPDU Supported by PICOS

EFM OAM Mode

There are two EFM OAM modes: active mode and passive mode. EFM connections can only be initiated by active mode OAM entities, while passive mode OAM entities can only wait for connection requests from peer OAM entities. Different connection modes have different processing capabilities for OAMPDU which is shown in Table 2.

Table 2. Comparison of OAMPDU processing capability between active and passive modes

EFM OAM Operation Mechanism

EFM OAM operation supported by PICOS includes Discovery and Remote Loopback.

EFM OAM Discovery

The implementation of EFM OAM function is based on EFM OAM connectivity. The process of establishing EFM OAM connectivity is also known as the Discovery phase, i.e., the process by which the local OAM entity discovers the remote OAM entity and establishes a stable connectivity with it.

When the EFM OAM function is enabled on an interface, and the EFM mode of the interface is active mode, then an EFM OAM connection is initiated from the interface to the remote end. During the process of establishing an EFM OAM connection, the connected OAM entities exchange the EFM OAM configuration information through the Information OAMPDUs to decide whether to establish an EFM OAM connection.

Figure 2.     EFM OAM Discovery Diagram

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As shown in Figure 2, interface Ge-1/1/1 on Switch A operates in EFM OAM active mode:

(1) Switch A sends an Information OAMPDU message to Switch B, which contains the EFM OAM configuration information of Switch A.

(2) After receiving the OAMPDU, Switch B compares the EFM OAM configuration of its own with Switch A, then replies to Switch A with an Information OAMPDU, which includes not only the EFM OAM configuration information for both Switch A and Switch B, but also the flag information indicating whether EFM OAM configurations of Switch B match Switch A.

After receiving the response OAMPDU from Switch B, Switch A will then determine whether the EFM OAM configuration of Switch B matches its own configuration.

Through the above process, if the EFM OAM configurations of both sides match, the EFM OAM connection is established, after which the OAM entities at both ends will send Information OAMPDUs periodically to detect whether the connection is normal.

Remote Loopback

The remote loopback function means that when an OAM entity in active mode sends a message to the far end, the far end does not forward the message (except for OAMPDUs) based on its destination address after receiving it, but returns the message to the sender along the original path. It can be used to locate link faults and detect link quality: the network administrator can judge the link performance (including packet loss rate, delay, jitter, etc.) by observing the return of non-OAMPDU messages.

Figure 3.     Schematic diagram of remote loopback

As shown in Figure 3, the EFM OAM connection between Switch A and Switch B is established, the interface Ge-1/1/1 of Switch A is working in EFM OAM active mode.

(1) When implementing the operations of start EFM OAM remote loopback tests on Switch A, Switch A sends a Loopback Control OAMPDU carried with remote loopback enabling information to Switch B, then waits for a reply.

(2) After receiving the OAMPDU, Switch B enters to the loopback state and replies to Switch A with an Information OAMPDU of the state change.

(3) & (4) In loopback state, Switch B returns all non-OAMPDU messages received along the original path to Switch A.

(5) When implementing the operations of stop EFM OAM remote loopback tests on Switch A, Switch A sends a Loopback Control OAMPDU carried with remote loopback disabling information to Switch B.

(6) When Switch B receives this OAMPDU, it exits the loopback state and replies to Switch A with an Information OAMPDU of the state change.

When remote loopback function is implemented, 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). This is because when two connected ports are in a loopback session, no packets other than the OAM PDUs are sent to the CPU for software processing, services will be disrupted.