6-4Figure 6-4 Levels of MPsBasic Functions of CFDCFD works effectively only in properly-configured networks. Its functions, which are implementedthrough the MPs, include:z Continuity check (CC);z Loopback (LB)z Linktrace (LT)Continuity checkContinuity check is responsible for checking the connectivity between MEPs. Connectivity faults areusually caused by device faults or configuration errors. This function is implemented through periodicsending of continuity check messages (CCMs) by the MEPs. As a multicast message, a CCM sent byone MEP is intended to be received by all the other MEPs in the same MA. If a MEP fails to receive theCCMs within 3.5 sending periods, the link is regarded as faulty and a corresponding log is generated.When multiple MEPs send CCMs at the same time, the multipoint-to-multipoint link check is achieved.LoopbackSimilar to ping at the IP layer, loopback is responsible for verifying the connectivity between a localdevice and a remote device. To implement this function, the local MEP sends loopback messages(LBMs) to the remote MEP. Depending on whether the local MEP can receive a loopback replymessage (LBR) from the remote MEP, the link state between the two can be verified. LBMs and LBRsare unicast messages. They are used to verify the connectivity between two points.LinktraceLinktrace is responsible for identifying the path between the source MEP and the destination MEP. Thisfunction is implemented in the following way: the source MEP multicasts linktrace messages (LTMs) tothe destination MEP. After receiving the messages, the destination MEP and the MIPs that the LTMspass send back linktrace reply messages (LTRs) to the source MEP. Based on the reply messages,
