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Configuration Guide - Reliability

CloudEngine 8800, 7800, 6800, and 5800 V200R003C00

This document describes the configurations of Reliability, including BFD Configuration, VRRP Configuration, DLDP Configuration, Smart Link and Monitor Link Configuration, EFM Configuration, and CFM Configuration.
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Basic Concepts of CFM

Basic Concepts of CFM

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.

The device provides IEEE Std 802.1ag-2007.


Maintenance Domains (MDs) are discrete areas within which connectivity fault detection is enabled. The boundary of an MD is determined by Maintenance Association End Points (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, and 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 7-2. 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 these 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 7-2, 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 7-2 MD networking

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

Default MD

Each device can be configured with a single default MD priority according to IEEE Std 802.1ag-2007.

Figure 7-3 Networking for the default MD

In Figure 7-3, if the default MD with the same level as the high-level MD is configured on the device in a low-level MD, Maintenance Association Intermediate Points (MIPs) are generated based on the default MD to send loopback reply (LBR) or linktrace reply (LTR) messages to devices in the high-level MD. CFM detects topology changes and monitors connectivity of both high-level and low-level MDs.

The level of the default MD must be higher than that of all MDs to which Maintenance Association End Points (MEPs) configured on the local device belong. In addition, the default MD must be of the same level as the high-level MD. The default MD transmits high-level continuity check messages (CCMs) and creates MIPs to send LTR messages.

IEEE Std 802.1ag-2007 defines that one default MD can be configured on each device and associated with multiple virtual local area networks (VLANs). On VLAN interfaces, MIPs are automatically generated based on the default MD and generation rules.


Each Maintenance Association (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 7-4 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 7-4 Inward- and outward-facing MEPs


MIPs are located on a link between two MEPs within an MD, facilitating management. More MIPs make network management and control easier. Customers set up more MIPs for important services than the MIPs for ordinary services.

MIP creation modes

MIPs can be automatically generated based on rules or manually created on interfaces. Table 7-1 lists MIP creation modes.

Table 7-1 MIP creation modes

Creation Mode



The MIP level must be set. Manually configured MIPs take precedence over automatically generated MIPs. Although it is easy to configure MIPs manually, it is difficult to manage many manually configured MIPs and errors may occur.


A device automatically generates MIPs based on configured creation rules. Configuring creation rules is complex, but properly configured rules ensure correct MIP settings.

MIP automatic creation rules

A device automatically creates MIPs based on a specific rule. The creation rules can be configured and are classified into explicit, default, or none, as listed in Table 7-2.

Table 7-2 MIP creation rules


Manual MIPs Exist on an Interface

Creation Rule

MEPs Are Configured on Low-Level MDs

MIPs Are Created

IEEE Std 802.1ag-2007











The procedure for identifying a lower-level MD is as follows:
  1. Identify a service instance associated with the MA in the MD.

  2. Query all interfaces in the service instance and check whether MEPs are configured on these interfaces.

  3. Query levels of all MEPs and locate the MEP with the highest level.

MIPs are separately calculated in each service instance such as a VLAN. In a single service instance, MAs in MDs with different levels have the same VLAN ID but different levels.

For each service instance on each interface, the device attempts to calculate a MIP from the lowest-level MEP based on the rules listed in Table 7-2 and the following conditions:
  • Each MD on a single interface has a specific level and is associated with multiple creation rules. The creation rule with the highest priority takes effect. An explicit rule has a higher priority than a default rule.

  • The level of a MIP must be higher than any MEP on the same interface.

  • An explicit rule applies only to the interface with MEPs configured.

  • A single MIP can be generated on a single interface. If multiple rules for generating MIPs with different levels are used, a MIP with the lowest level is generated.

MIP creation rules help detect and locate faults by level because a MEP in an MD with a lower level can be mapped to a MIP in an MD with a higher level.

For example, CCMs are sent to detect a fault in a level-7 MD on the network shown in Figure 7-5. Loopback or linktrace is used to locate the fault along the link between MIPs that may be in level-5 MD. This process is repeated until the faulty link or device is located.

Figure 7-5 Hierarchical MIPs in MDs

The following example illustrates how to create a MIP based on the default rule defined in IEEE Std 802.1ag-2007.

In Figure 7-6, MD1 through MD5 are nested into MD7, and MD2 through MD5 are nested into MD1. MD7 has a higher level than MD1 through MD5, and MD1 has a higher level than MD2 through MD5. Multiple MEPs are configured on SwitchA in MD1 and the MEPs belong to MDs with different levels.

Figure 7-6 MIP creation based on IEEE Std 802.1ag-2007

A default rule is configured on SwitchA to create a MIP in MD1. The procedure for creating the MIP is as follows:

  1. SwitchA compares MEP levels and finds the MEP at level 5 (highest level). The MEP level is determined by the level of the MD to which the MEP belongs.

  2. SwitchA selects the MD at level 6, which is higher than the MEP of level 5.

  3. SwitchA generates a MIP at level 6.

If MDs at level 6 or higher do not exist, no MIP is generated.

If MIPs at level 1 already exist on SwitchA, MIPs at level 6 cannot be generated.

Hierarchical MPs

MEPs and MIPs are Maintenance Points (MPs). MPs are configured on interfaces and belong to specific MAs shown in Figure 7-7.

Figure 7-7 MPs

Figure 7-8 shows locations of MEPs and MIPs.

Figure 7-8 Hierarchical MPs

Operator 1, Operator 2, Service provider, and Customer use MDs with levels 2, 3, 5, and 6, respectively. A higher MD level indicates a larger-scale MD.

CFM Protocol Packets

CFM sends tagged protocol packets to detect link faults. Figure 7-9 shows the format of CFM protocol packets.

Figure 7-9 Format of CFM protocol packets

Table 7-3 describes the main fields in CFM protocol packets.

Table 7-3 Main 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 packet. Table 7-4 lists the types of CFM protocol packets.

Table 7-4 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: 2019-05-08

Document ID: EDOC1100004359

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