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Configuration Guide - Reliability

CloudEngine 12800 and 12800E V200R002C50

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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CFM Fundamentals

CFM Fundamentals

CFM supports Continuity Check (CC), Loopback (LB), and Linktrace (LT) functions.


CC monitors connectivity of links between MEPs. A MEP periodically sends multicast Continuity Check Messages (CCMs) to an RMEP in the same MA. If an RMEP does not receive a CCM within a period 3.5 times the timeout interval at which CCMs are sent, the RMEP considers the path between itself and the MEP faulty.

Figure 7-10 CC
The CC procedure is as follows:
  1. CCM generation

    A MEP generates and sends CCMs. In Figure 7-10, MEP1, MEP2, and MEP3 belong to the same MA and are enabled to send CCMs to one another at the same interval.

    Each CCM carries a level that is equal to the MEP level.

  2. MEP database establishment

    Every CFM-enabled device has a MEP database. The MEP database records information about the local MEP and RMEPs in the same MA. The local MEP and RMEPs are manually configured, and their information is automatically recorded in the MEP database.

  3. Fault identification

    If a MEP does not receive CCMs from its RMEP within a period 3.5 times the timeout interval at which CCMs are sent, the MEP considers the path between itself and the RMEP faulty. A log is generated to provide information for fault diagnosis. LB or LT can be used to locate the fault. MEPs in an MA exchange CCMs to monitor links, implementing multi-point to multi-point (MP2MP) detection.

  4. CCM termination

    CCMs are generated and terminated by MEPs. A MEP forwards received CCMs of a higher level but drops CCMs of a lower level or of the same level. In this manner, CCMs from a low-level MD are not transmitted to the MD of a higher level.


LB is also called 802.1ag MAC ping. Similar to IP ping, LB monitors connectivity of a path between a MEP and an RMEP.

A MEP initiates an 802.1ag MAC ping test to monitor reachability of a MEP or MIP of the same level in the same or different MAs. The MEP sends loopback messages (LBMs) to the MEP or MIP. After receiving the messages, the MEP or MIP replies with Loopback Reply (LBR) messages. LB helps locate a faulty node because a faulty node cannot send an LBR message in response to an LBM. LBMs and LBR messages are unicast packets.

The following example illustrates LB implementation on the network shown in Figure 7-11.

Figure 7-11 LB networking

CFM is configured to monitor a path between PE1 (MEP1) and PE4 (MEP2). The MD level of these MEPs is 6. A MIP with level 6 is configured on PE2 and PE3. If a fault in a link between PE1 and PE4 is detected, LB can be used to locate the fault. Figure 7-12 illustrates the LB procedure.

Figure 7-12 LB procedure

MEP1 can measure the network delay based on 802.1ag MAC ping results or the frame loss ratio based on the difference between the number of LBMs and the number of LBR messages.


LT is also called 802.1ag MAC trace. Similar to IP traceroute, it identifies a path between two MEPs.

A MEP initiates an 802.1ag MAC trace test to monitor reachability of a MEP or MIP of the same level in the same or different MAs. A source MEP constructs and sends a Linktrace Message (LTM) to a destination MEP. After receiving the LTM, each MIP forwards the LTM and replies with a Linktrace Reply (LTR) message. Upon receiving the LTM, the destination MEP replies with an LTR message and does not forward the LTM. The source MEP obtains topology information about each hop on the path based on the LTR messages. LTMs are multicast packets and LTR messages are unicast packets.

Figure 7-13 LT networking

The following example illustrates LT implementation on the network shown in Figure 7-13.

  1. MEP1 sends an LTM to MEP2. The LTM carries a time to live (TTL) value and the MAC address of destination MEP2.

  2. After the LTM arrives at MIP1, MIP1 reduces the TTL in the LTM by 1 and forwards the LTM if the TTL is not zero. MIP1 then replies with an LTR message to MEP1. The LTR message carries forwarding information and the TTL value carried by the LTM.

  3. After the LTM reaches MIP2 and MEP2, the process described on MIP1 is repeated for MIP2 and MEP2. In addition, MEP2 finds that its MAC address is the destination address carried in the LTM and therefore does not forward the LTM.

  4. The LTR messages from MIP1, MIP2, and MEP2 provide MEP1 with information about the forwarding path between MEP1 and MEP2.

    If a fault occurs on the path between MEP1 and MEP2, MEP2 or a MIP cannot receive the LTM or reply with an LTR message. MEP1 can locate the faulty node. For example, if the link between MEP1 and MIP2 works properly but the link between MIP2 and MEP2 is faulty, MEP1 can receive LTR messages from MIP1 and MIP2 but fails to receive a response from MEP2. MEP1 then considers the path between MIP2 and MEP2 faulty.

Updated: 2019-03-21

Document ID: EDOC1000166606

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