Basic Concepts of EFM

OAM PDUs

EFM works at the data link layer and uses protocol packets called OAM Protocol Data Units (PDUs). EFM devices periodically exchange OAMPDUs to report the link status, helping network administrators effectively manage networks. Figure 1 shows the format of an OAMPDU as well as the common types of OAMPDUs. Table 1 describes fields in an OAMPDU.

Figure 1 OAMPDU format

**Table 1** Fields in an OAMPDU
Field	Description
Destination addr	Destination MAC address, which is a slow protocol multicast address 0x0180-C200-0002. Network bridges cannot forward slow protocol packets. EFM OAMPDUs cannot be forwarded over multiple devices, even if OAM is supported or enabled on the devices.
Source addr	Source address, which is a unicast MAC address of a port on the transmit end. If no port MAC address is specified on the transmit end, the bridge MAC address of the transmit end is used.
Type	Slow protocol type that has the fixed value of 0x8809.
Subtype	Subtype of a slow protocol. The value is 0x03, indicating that the slow subprotocol is EFM.
Flags	EFM entity status: Remote Stable Remote Evaluating Local Stable Local Evaluating Critical Event Dying Gasp Link Fault
Code	OAMPDU type: 0X00: Information OAMPDU 0X01: Event Notification OAMPDU 0X04: Loopback Control OAMPDU Table 2 describes OAMPDU types.

**Table 2** OAMPDU types
OAMPDU Type	Description
Information OAMPDU	Used to discover a remote EFM entity, initiate a handshake process, and establish an EFM connection. After the EFM connection is established, both EFM entities periodically exchange Information OAMPDUs to monitor link connectivity. Used to advertise fault information. Upon receiving a critical link event carried in the Flags field of an Information OAMPDU sent by the remote EFM entity, the local EFM entity sends a trap to the NMS.
Event Notification OAMPDU	Used to monitor links. If an errored frame event, errored code period event, or errored frame second event occurs on an interface, the interface sends an Event Notification OAMPDU to notify the remote interface of the event.
Loopback Control OAMPDU	Used to enable or disable the remote loopback function.

Connection Modes

EFM supports two connection modes: active and passive. An EFM connection can only be initiated by an OAM entity working in active mode. An OAM entity working in passive mode waits to receive a connection request from its peer entity. Table 3 lists capabilities for processing OAMPDUs in the two connection modes.

**Table 3** Capabilities for processing OAMPDUs in active and passive modes
Capability	Active Mode	Passive Mode
Initiate a connection request by sending an Information OAMPDU during the discovery process	Supported	Not supported
Respond to a connection request during the discovery process	Supported	Supported
Send Information OAMPDUs	Supported	Supported
Send Event Notification OAMPDUs	Supported	Supported
Send Loopback Control OAMPDUs	Supported	Not supported
Respond to Loopback Control OAMPDUs	Supported (The remote EFM entity must work in active mode.)	Supported