1-4
Figure 1-4 Levels of MPs
Device A Device B Device C Device D Device E Device F
5 5
3
5 5
3 3
3
2 2 2 2
0 0 0 0 0 0
MD level 5
MD level 3
MD Level 2 MD Level 2
MD level 0
Port
5
Outward-facing MEP and MD level
5
MIP and MD level
Maintenance association
Logical path of CFD messages
Port 1 Port 2
MD level 0 MD level 0
3
Inward-facing MEP and MD level
2 2 2 2
MEP list
A MEP list is a collection of local MEPs allowed to be configured in an MA and the remote MEPs to be
monitored. It lists all the MEPs configured on different devices in the same MA. The MEPs all have
unique MEP IDs. When a MEP receives from a remote device a continuity check message (CCM) that
carries a MEP ID not included in the MEP list of the MA, it drops the message.
CFD Functions
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 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.
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.
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