In Figure: VCCV Ping Over a Multi-Segment Pseudowire, a trace can be performed on the MS-PW originating from T-PE1 by a single operational command. The following process occurs:
T-PE 1 sends a VCCV echo request with TTL set to 1 and a FEC 128 containing the pseudowire information of the first segment (pseudowire1 between T-PE 1 and S-PE) to S-PE for validation.
S-PE validates the echo request with the FEC 128. Since it is a switching point between the first and second segment, it builds an echo reply with a return code of 8 and includes the FEC 128 of the second segment (pseudowire2 between S-PE and T-PE 2) and sends the echo reply back to T-PE 1.
T-PE 1 builds a second VCCV echo request based on the FEC 128 in the echo reply from the S-PE. It increments the TTL and sends the next echo request out to T-PE 2. The VCCV echo request packet is switched at the S-PE datapath and forwarded to the next downstream segment without any involvement from the control plane.
T-PE 2 receives and validates the echo request with the FEC 128 of the pseudowire2 from TPE 1. Since T-PE 2 is the destination node or the egress node of the MS-PW, it replies to T-PE1 with an echo reply with a return code of 3 (egress router) and no FEC 128 is included.
T-PE 1 receives the echo reply from T-PE 2. T-PE 1 is made aware that T-PE 2 is the destination of the MS-PW because the echo reply does not contain the FEC 128 and because its return code is 3. The trace process is completed.