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Overview of Ethernet OAM

Ethernet OAM provides the 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 local area network (VLAN) identifier for troubleshooting
Work independently of the actual Ethernet transport and function over physical Ethernet ports, or a virtual service such as pseudowire, and so on.
Isolate faults over a flat (or single operator) network architecture or a nested or hierarchical (or multi-provider) networks.

OAM can provide simple link-level information, provide performance statistics, or track end-to-end connectivity across the network. Simple link fault management (LFM) for Ethernet links is defined in IEEE 802.3ah. The most complete connectivity fault management (CFM) is defined in IEEE 802.1ag. This chapter emphasizes the use of CFM in a Metro Ethernet environment.

CFM can be used to monitor an Ethernet network at a per-service level, unlike LFM, which functions at the physical link level. The service monitored could be a virtual local area network (VLAN), concatenation of VLANs or a virtual private LAN service (VPLS) instance.

The major features of CFM are:

Fault monitoring using the continuity check protocol. This is a neighbor discovery and health check protocol which discovers and maintains adjacencies at the VLAN or link level.
Path discovery and fault verification using the linktrace protocol. Similar to IP traceroute, this protocol maps the path taken to a destination MAC address through one or more bridged networks between the source and destination.
Fault isolation using the loopback protocol. Similar to IP ping, this protocol works with the continuity check protocol during troubleshooting.

Ethernet OAM functions are implemented as:

Fault detection and notification (provided by continuity check messages)
Path discovery (provided by the linktrace protocol)
Fault isolation, verification, and recovery (isolation and verification are provided by a combination of protocols, while recovery is the function of protocols such as spanning tree)

CFM partitions the service network into various administrative domains. For example, operators, providers, and customers may be part of different administrative domains. Each administrative domain is mapped into one maintenance domain providing enough information to perform its own management, thus avoiding security breaches and making end-to-end monitoring possible. Each maintenance domain is associated with a maintenance domain level from 0 through 7. Level allocation is based on the network hierarchy, where outermost domains are assigned a higher level than the innermost domains. Customer end points have to highest maintenance domain level. In a CFM maintenance domain, each service instance is called a maintenance association. A maintenance association can be thought as a full mesh of maintenance endpoints (MEPs) having similar characteristics. MEPs are active CFM entities generating and responding to CFM protocol messages. There is also a maintenance intermediate point (MIP), which is a CFM entity similar to the MEP, but more passive (MIPs only respond to CFM messages).

MEPs can be up MEPs or down MEPs. A link can connect a MEP at level 5 to a MEP at level 7. The interface at level 5 is an up MEP (because the other end of the link is at MEP level 7) and the interface at level 7 is a down MEP (because the other end of the link is at MEP level 5).

The loopback protocol used in Ethernet OAM is modeled on the standard IP ping. After a fault is detected, the loopback protocol performs fault verification and isolation under the direction of a network operator. The loopback is performed using request and response message pairs. A unicast loopback message is generated by a MEP and a loopback reply is generated by the destination MIP or MEP. The target MAC address is learned by the continuity check protocol or linktrace protocol. The loopback message's packet is always forwarded to a unique port by the originating MEP, as determined by a MAC table lookup or the MEP interface MAC address. The target MIP or MEP generates a unicast loopback reply in response to the received loopback message. The loopback message follows the same path as a data packet, and intermediate bridges simply forward the packet to the destination MIP or MEP.

In all the examples in this chapter, CFM can be used at two levels:

By the service provider to check the connectivity among its provider edge (PE) routers
By the customer to check the connectivity among its customer edge (CE) routers

Note: The configured customer CFM level must be greater than service provider CFM level.

The examples in this chapter use CFM to monitor connectivity over a VPLS and bridge network.

Note: The configurations in this chapter are only partial examples of complete and functional router configurations. Do not copy these configurations and use them directly on an actual system.

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