A MAC trace functions like an LSP trace with some variations. Operations in a MAC trace are triggered when the VC TTL is decremented to 0.
Like a MAC ping, a MAC trace is sent using the data plane.
When a traceroute request is sent via the data plane, the data plane format is used. The reply can be via the data plane or the control plane.
A data plane MAC traceroute request includes the tunnel encapsulation, the VC label, and the OAM, followed by an Ethernet DLC, a UDP, and IP header. If the mapping for the MAC address is known at the sender, the data plane request is sent down the known SDP with the appropriate tunnel encapsulation and VC label. If the mapping is not known, it is sent down every SDP (with the appropriate tunnel encapsulation per SDP and appropriate egress VC label per SDP binding).
The tunnel encapsulation TTL is set to 255. The VC label TTL is initially set to the min-ttl (default is 1). The OAM label TTL is set to 2. The destination IP address is the all-routers multicast address. The source IP address is the system IP of the sender.
The destination UDP port is the LSP ping port. The source UDP port is whatever the system provides (this source UDP port is the demultiplexer that identifies the particular instance that sent the request, when correlating the reply).
The Reply Mode is either 3 (that is, reply using the control plane) or 4 (that is, reply through the data plane), depending on the reply-control option. By default, the data plane request is sent with Reply Mode 3 (control plane reply).
The Ethernet DLC header source MAC address is set to either the system MAC address (if no source MAC is specified) or to the specified source MAC. The destination MAC address is set to the specified destination MAC. The EtherType is set to IP.