The Link Trace procedure identifies all MIPs and MEPs belonging to a ME. It can be thought of as a "layer 2 trace route". The procedure is similar to the loopback procedure.
A MEP initiates the Link Trace by sending an LTM message using a multicast MAC DA and the appropriate MEG level for the ME. It is recommended that LTM makes use of the highest CoS ID available, which will yield the lowest possible loss for a particular ETH service (EVC or OVC). Each MIP that belongs to this ME and the remote MEP:
replies by generating a Link Trace Response LTR unicast message
forwards the LTM onward towards the remote MEP (except for the MEP)
If there are no faults in the path, Link Trace allows the originator to map the entire route to the remote MEP. If there is a fault in the path, the originator will receive LTRs from all MIPs before the fault thereby facilitating fault isolation.
On the Subscriber MEG, LTM PDUs should be sent with same CE-VLAN ID that maps to the monitored EVC, that way it is guaranteed that the LTM passes the same path as the monitored EVC's service frames.
In this example, an LTM procedure is issued from MEP1. MIP1 and MIP2 reply, but MIP3 does not reply.
The failure is between MIP2 and MIP3, or at the NE implementing MIP3.
Since none of the MIPs nor the MEP beyond MIP2 receive the LTM, they will not (attempt to) send an LTR.