Although tracing of the MS-PW path is possible using the methods described in the VCCV Ping section, these require multiple manual iterations and that require the FEC of the last pseudowire segment to the target T-PE/S-PE already be known at the node originating the echo request message for each iteration. This mode of operation is referred to as a "ping" mode.
The automated VCCV trace can trace the entire path of a pseudowire with a single command issued at the T-PE or at an S-PE. This is equivalent to LSP trace and is an iterative process by which the ingress T-PE or T-PE sends successive VCCV ping messages with incrementing TTL values, starting from TTL=1.
The method is described in draft-hart-pwe3-segmented-pw-vccv, VCCV Extensions for Segmented Pseudo-Wire, and is pending acceptance by the PWE3 working group. In each iteration, the source T-PE or S-PE builds the MPLS echo request message in a way similar to VCCV Ping. The first message with TTL=1 will have the next-hop S-PE T-LDP session source address in the Remote PE Address field in the pseudowire FEC TLV. Each S-PE that terminates and processes the message will include the FEC 128 TLV corresponding to the pseudowire segment to its downstream node, in the MPLS echo reply message. The inclusion of the FEC TLV in the echo reply message is allowed according to RFC 4379, Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures.
The source T-PE or S-PE then sends the next echo reply message with TTL=2 to test the next-next hop for the MS-PW. It copies the FEC TLV it received in the echo reply message into the new echo request message. The process is terminated when the reply is sent from the egress T-PE node or when a timeout occurs. If specified, the max-ttl parameter in the vccv-trace command will stop on SPE before reaching T-PE.
The results of VCCV trace can be displayed for fewer pseudowire segments of the end-to-end MS-PW path. In this case, the min-ttl and max-ttl parameters are configured accordingly. However, the T-PE/S-PE node will still probe all hops up to the min-ttl value in order to correctly build the FEC of the required subset of segments.
This method does not require the use of the downstream mapping TLV in the echo request and echo reply messages.