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CLI-based Configuration Guide - Reliability

AR100, AR120, AR150, AR160, AR200, AR1200, AR2200, AR3200, and AR3600 V200R010

This document provides guidance for configuring reliability services, including interface backup, BFD, VRRP, and EFM.
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Basic Concepts

Basic Concepts

CFM Versions

Ethernet CFM supports E2E connectivity fault detection, fault advertisement, fault acknowledgement, and fault location. CFM monitors network connectivity and pinpoints connectivity faults. It is used with protection switching techniques to improve network reliability.

802.1ag has many versions during development. Huawei devices conform to only IEEE Std 802.1ag-2007. Table 9-1 describes the version support.

Table 9-1  Support of IEEE Std 802.1ag-2007


IEEE Std 802.1ag-2007

Maintenance Domain (MD)


Default MD


Maintenance Association (MA)


Maintenance Association End Point (MEP)


Maintenance Association Intermediate Point (MIP)

Not supported


MDs are discrete areas within which connectivity fault detection is enabled. The boundary of an MD is determined by MEPs that are configured on interfaces. An MD is identified by an MD name.

To help locate faults, MDs are separated into levels 0 through 7. A larger value indicates a higher level and a larger area that the MD covers. One MD can be tangential to or nested into another MD, but MDs cannot overlap. A lower-level MD can be nested into a higher-level MD, but a higher-level MD cannot be nested into a lower-level MD.

Classifying MDs based on levels facilitates fault diagnosis. MD2 is nested into MD1 on the network shown in Figure 9-2. If a fault occurs in MD1, PE2 through PE7 and all the links between the PEs are checked. If no fault is detected in MD2, PE2, PE3, and PE4 are working properly. That is, the fault occurs on PE5, PE6, or PE7 or on a link between PEs.

In practice, a nested MD can monitor connectivity of the higher-level MD into which it is nested. In this case, 802.1ag packets can traverse the nested MD. In Figure 9-2, MD2 with level 3 is nested into MD1 with level 6, so 802.1ag packets can traverse MD2 to monitor connectivity of MD1 and 802.1ag packets of MD2 are not transmitted in MD1. Proper MD planning helps locate faults.

Figure 9-2  MD networking

802.1ag packets are exchanged and CFM functions are implemented based on MDs. Proper MD planning helps a network administrator locate faults.


Each MA consists of MEPs. Multiple MAs can be configured in an MD as needed. An MA is identified by an MD name and an MA name.

An MA serves a specific service such as VLAN. A MEP in an MA sends packets carrying tags of the specific service and receives packets sent by other MEPs in the MA.


MEPs are located at the edge of an MD and an MA. The service type and level of packets sent by a MEP are determined by the MD and MA to which the MEP belongs. A MEP processes packets at specific levels based on its own level, and sends packets carrying its own level. If a MEP receives a packet carrying a level higher than its own, the MEP does not process the packet and forwards the packet based on the original path. If a MEP receives a packet carrying a level lower than or equal to its own, it processes the packet.

A MEP is manually configured on an interface. The MEP level is equal to the MD level.

The MEP that is configured on a CFM-enabled device is called a local MEP. That MEPs that are configured on other devices in the same MA are called Remote Maintenance Association End Points (RMEPs).

Figure 9-3 shows the types of MEPs:
  • Inward-facing MEP: sends packets to other interfaces on the same device.

  • Outward-facing MEP: sends packets to the interface on which the MEP is configured.

Figure 9-3  Inward- and outward-facing MEPs

CFM Protocol Packets

CFM sends tagged protocol packets to detect link faults. Figure 9-4 shows the format of CFM protocol packets.

Figure 9-4  Format of CFM protocol packets

Table 9-2 describes the fields in CFM protocol packets.

Table 9-2  Fields in CFM protocol packets



MD Level

Level of an MD. The value ranges from 0 to 7. A larger value indicates a higher level.


Number of the CFM version. The current version is 0.


Message code value, specifying a specific type of CFM protocol packets. Table 9-3 lists the types of CFM protocol packets.

Table 9-3  Types of CFM protocol packets

OpCode Value

Packet Type



Continuity Check Message (CCM)

Monitors E2E link connectivity.


Loopback Reply (LBR) message

Replies to an LBM.


Loopback Message (LBM)

Is sent by an interface that initiates loopback detection.


Linktrace Reply (LTR) message

Replies to an LTM.


Linktrace Message (LTM)

Is sent by an interface to initiate a linktrace test.

Updated: 2019-08-12

Document ID: EDOC1100034076

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