A MAC trace operates like an LSP trace with variations. Operations in a MAC trace are triggered when the VC TTL is decremented to 0.
Like a MAC ping, a MAC trace can be sent either by the control plane or the data plane.
When a MAC trace request is sent by the control plane, the destination IP address is determined from the control plane mapping for the destination MAC. If the destination MAC is known to be at a specific remote site, then the far-end IP address of that SDP is used. If the destination MAC is not known, then the packet is sent as a unicast transmission to all SDPs in the service with the appropriate squelching.
A control plane MAC traceroute request is sent via UDP/IP. The destination UDP port is the LSP ping port. The source UDP port is assigned by the system, where the source UDP port is really the demultiplexer that identifies the particular instance that sent the request, when that demultiplexer correlates the reply. The source IP address is the system IP address of the sender.
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, then the data plane request is sent down the known SDP with the appropriate tunnel encapsulation and VC label. If it is not known, then 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 address of the sender.
The Reply Mode is either 3 (control plane reply) or 4 (data plane reply), 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.