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Understanding Ethernet OAM Link Fault Management for ACX Series Routers

The Juniper Networks Junos operating system (Junos OS) for Juniper Networks ACX Series routers allows the Ethernet interfaces on these routers to support the IEEE 802.3ah standard for the Operation, Administration, and Maintenance (OAM) of Ethernet in access networks. The standard defines OAM link fault management (LFM). You can configure IEEE 802.3ah OAM LFM on point-to-point Ethernet links that are connected either directly or through Ethernet repeaters. The IEEE 802.3ah standard meets the requirement for OAM capabilities even as Ethernet moves from being solely an enterprise technology to a WAN and access technology, and the standard remains backward compatible with the existing Ethernet technology.

Ethernet OAM provides tools that network management software and network managers can use to determine how a network of Ethernet links is functioning. Ethernet OAM should:

• Rely only on the media access control (MAC) address or virtual LAN identifier for troubleshooting.
• Work independently of the actual Ethernet transport and function over physical Ethernet ports or a virtual service such as a pseudowire.
• Isolate faults over a flat (or single-operator) network architecture or nested or hierarchical (or multiprovider) networks.

The following OAM LFM features are supported on ACX Series routers:

• Discovery and Link Monitoring

  The discovery process is triggered automatically when OAM is enabled on the interface. The discovery process permits Ethernet interfaces to discover and monitor the peer on the link if it also supports the IEEE 802.3ah standard. You can specify the discovery mode used for IEEE 802.3ah OAM support. In active mode, the interface discovers and monitors the peer on the link if the peer also supports IEEE 802.3ah OAM functionality. In passive mode, the peer initiates the discovery process. After the discovery process has been initiated, both sides participate in the process. The router performs link monitoring by sending periodic OAM protocol data units (PDUs) to advertise OAM mode, configuration, and capabilities.

  You can specify the number of OAM PDUs that an interface can skip before the link between peers is considered down.

• Remote Fault Detection

  Remote fault detection uses flags and events. Flags are used to convey the following:

  • Link Fault means a loss of signal
  • Dying Gasp means an unrecoverable condition such as a power failure
  • Critical Event means an unspecified vendor-specific critical event

  You can specify the interval at which OAM PDUs are sent for fault detection.

  Note: ACX Series routers support the receipt of dying-gasp packets, but cannot generate them.

• Remote Loopback Mode

  Remote loopback mode ensures link quality between the router and a remote peer during installation or troubleshooting. In this mode, when the interface receives a frame that is not an OAM PDU or a PAUSE frame, it sends it back on the same interface on which it was received. The link appears to be in the active state. You can use the returned loopback acknowledgement to test delay, jitter, and throughput.

  If a remote data terminal equipment (DTE) supports remote loopback mode, Junos OS can place the remote DTE into loopback mode. When you place a remote DTE into loopback mode, the interface receives the remote loopback request and puts the interface into remote loopback mode. When the interface is in remote loopback mode, all frames except OAM PDUs and PAUSE frames are looped back. No changes are made to the frames. OAM PDUs continue to be sent and processed.