Fault isolation is achieved through the use of Linktrace Messages (LTM). LTM is intercepted by all the MPs on the way to the destination MP. The switch supports two types of LTM.
The first type, the unicast LTM, can be addressed to either MEP or MIP MAC address. Each MP on the way decrements the TTL field in the LTM frame, sends Linktrace Reply (LTR), and forwards the original LTM to the destination. The LTM is forwarded until it reaches its destination or the TTL value is decremented to zero. LTR is a unicast message addressed to the originating MEP.
The second type, the proprietary LTM, is used to map the MAC addresses of the SPBM network; in this case the target MAC is not an MP, but rather a service instance identifier (I-SID).