Unidirectional Link Fault Management

With EFM OAM, certain physical layers can support a limited unidirectional capability.

The ability to operate a link in a unidirectional mode for diagnostic purposes supports the maintenance objective of failure detection and notification. Unidirectional OAM operation is not supported on some legacy links but is supported on newer links such as 100BASE-X PCS, 1000BASE-X PCS, and 10GbE RS. On technologies that support the feature, OAM PDUs can be transmitted across unidirectional links to indicate fault information. To the higher layers, the link is still failed in both directions, but to the OAM layer, some communication capabilities exist. The distinction between a unidirectional link and a normal link is shown in the following figure.

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Normal Link and Unidirectional Operation
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You can enable unidirectional link fault detection and notification on individual ports with CLI commands. This allows appropriate register settings to transmit OAM PDUs even on a link that has a slowly deteriorating quality receive path or no receive path at all. Then, when a link is not receiving a signal from its peer at the physical layer (for example, if the peer‘s laser is malfunctioning), the local entity can set a flag in an OAM PDU to let the peer know that its transmit path is inoperable.

The operation of OAM on an Ethernet interface does not adversely affect data traffic because OAM is a slow protocol with very limited bandwidth potential, and it is not required for normal link operation. By utilizing the slow protocol MAC address, OAM frames are intercepted by the MAC sub layer and cannot propagate across multiple hops in an Ethernet network. This implementation assures that OAM PDUs affect only the operation of the OAM protocol itself and not user data traffic.

The IEEE 802.3ah standard defines fault notifications based on one-second timers. But by sending triggered OAM PDUs on detecting link down/local fault rather that waiting to send on periodic PDUs, failure detection is less than one second can be achieved, thereby accelerating fault recovery and network restoration.

EFM OAM uses standard length Ethernet frames within the normal frame length of 64 to 1518 bytes as PDUs for their operation. The following table describes the fields of OAM PDUs.

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OAM PDU Fields

Field Octets Description Value
Destination Address 6 Slow protocol multicast address 01:80:C2:00:00:02
Source Address 6 Port‘s individual MAC address Switch MAC
Length/Type 2 Slow protocol type 0x8809
Subtype 1 Identifies specific slow protocol 0x03
Flags 2 Contains status bits see the following figure
Code 1 Identifies OAM PDU type 0x00 (Information TLV)
Data/Pad 42-1496 OAM PDU data 0x00 (END of TLV)
FCS 4 Frame check sequence