CFD works effectively only in properly-configured networks. Its functions, which are implemented
through the MPs, include:
Continuity check (CC)
Continuity check is responsible for checking the connectivity between MEPs. Connectivity faults are
usually caused by device faults or configuration errors. This function is implemented through periodic
sending of continuity check messages (CCMs) by the MEPs. As a multicast message, a CCM sent by
one MEP is intended to be received by all the other MEPs in the same MA. If a MEP fails to receive the
CCMs within 3.5 sending periods, the link is considered 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.
Similar to ping at the IP layer, loopback is responsible for verifying the connectivity between a local
device 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 reply
message (LBR) from the remote MEP, the link state between the two can be verified. LBMs and LBRs
are unicast messages.
Linktrace is responsible for identifying the path between the source MEP and the destination MEP. This
function is implemented in the following way: the source MEP multicasts linktrace messages (LTMs) to
the destination MEP. After receiving the messages, the destination MEP and the MIPs that the LTMs
pass send back linktrace reply messages (LTRs) to the source MEP. Based on the reply messages, the
source MEP can identify the path to the destination MEP. Note that LTMs are multicast frames while
LTRs are unicast frames.
Protocols and Standards
The CFD function is implemented in accordance with IEEE 802.1ag: Virtual Bridged Local Area
Networks Amendment 5: Connectivity Fault Management.
CFD Configuration Task List
For CFD to work effectively, you should first design the network by performing the following tasks:
Grade the MDs in the entire network, and define the boundary of each MD.
Assign a name for each MD. Make sure that the same MD has the same name on different devices.
Define the MA in each MD according to the VLAN you want to monitor.
Assign a name for each MA. Make sure that the same MA in the same MD has the same name on
At the edges of MD and MA, MPs should be designed at the device port. MEPs can be designed on
devices or ports that are not at the edges.
Complete the following tasks to configure CFD: