3-4
Basic Functions of CFD
CFD works effectively only in properly-configured networks. Its functions, which are implemented
through the MPs, include:
z
Continuity check (CC);
z
Loopback (LB)
z
Linktrace (LT)
Continuity check
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 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.
Loopback
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. They are used to verify the connectivity between two points.
Linktrace
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 P802.1ag.
CFD Configuration Task List
For CFD to work effectively, you should first design the network by performing the following tasks:
z
Grade the MDs in the entire network, and define the boundary of each MD.
z
Assign a name for each MD. Make sure that the same MD has the same name on different
devices.
z
Define the MA in each MD according to the VLAN you want to monitor.
z
Assign a name for each MA. Make sure that the same MA in the same MD has the same name on
different devices.
z
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:
