When in the ping mode of operation, the sender of the echo request message requires the FEC of the last segment to the target S-PE/T-PE node. This information can either be configured manually or be obtained by inspecting the corresponding sub-TLV's of the pseudowire switching point TLV. However, the pseudowire switching point TLV is optional and there is no guarantee that all S-PE nodes populate it with their system address and the pseudowire ID of the last pseudowire segment traversed by the label mapping message. Thus, the router implementation always makes use of the user configuration for these parameters.
When in the trace mode operation, the T-PE automatically learns the target FEC by probing one by one the hops of the MS-pseudowire path. Each S-PE node includes the FEC to the downstream node in the echo reply message in a similar way that LSP trace causes the probed node to return the downstream interface and label stack in the echo reply message.