Basic Concepts

OAMPDUs

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

Figure 1 OAMPDU format

**Table 1** Fields and descriptions in an OAMPDU
Field	Description
Dest addr	Destination MAC address, which is a slow-protocol multicast address. 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, which has a fixed value of 0x8809.
Subtype	Subtype of a slow protocol. The value is 0x03, which means that the slow sub-protocol is EFM.
Flags	Status of an EFM entity: Remote Stable Remote Evaluating Local Stable Local Evaluating Critical Event Link Fault
Code	OAMPDU type: 0X00: Information OAMPDU 0X01: Event Notification OAMPDU 0X04: Loopback Control OAMPDU Table 2 describes common types of OAMPDUs.

**Table 2** OAMPDU types
OAMPDU Type	Description
Information OAMPDU	Used for peer discovery. Two OAM entities in the handshake phase send OAM PUDs at a specified interval to monitor link connectivity. Used for fault notification. Upon receipt of an OAM PDU carrying Critical Event in the Flags field, the local end sends an alarm to notify the NMS of the remote device fault.
Event Notification OAMPDU	Used to monitor links. If an errored frame event, errored symbol period event, or errored frame second summary 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. Table 3 describes capabilities of processing OAMPDUs in the two modes.

**Table 3** Capabilities of 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

An EFM connection can be initiated only 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. Two OAM entities that both work in passive mode cannot establish an EFM connection between them.
An OAM entity that is to initiate a loopback request must work in active mode.