VCCV-ping supports the use of FEC129 AII type 2 in the target FEC stack of the ping echo request message. The FEC to use in the echo request message is derived in one of two ways: Either the user can specify only the spoke-sdp-fec-id of the MS-PW in the vccv-ping command, or the user can explicitly specify the SAII and TAII to use.
If the SAII:TAII is entered by the user in the vccv-ping command, then those values are be used for the VCCV ping echo request, but their order is be reversed before being sent so that they match the order for the downstream FEC element for an S-PE, or the locally configured SAII:TAII for a remote T-PE of that MS-PW. If SAII:TAII is entered in addition to the spoke-sdp-fec-id, the system verifies the entered values against the values stored in the context for that spoke-sdp-fec-id.
Otherwise, if the SAII:TAII to use in the target FEC stack of the VCCV ping message is not entered by the user, and if a switching point TLV was previously received in the initial label mapping message for the reverse direction of the MS-PW (with respect to the sending PE), then the SAII:TAII to use in the target FEC stack of the VCCV ping echo request message is derived by parsing that switching point TLV based on the user-specified TTL (or a TTL of 255 if none is specified). In this case, the order of the SAII:TAII in the switching point TLV is maintained for the VCCV ping echo request message.
If no pseudowire switching point TLV was received, then the SAII:TAII values to use for the VCCV ping echo request are derived from the MS-PW context, but their order is reversed before being sent so that they match the order for the downstream FEC element for an S-PE, or the locally configured SAII:TAII for a remote T-PE of that MS-PW. Note that the use of spoke-sdp-fec-id in vccv-ping is only applicable at T-PE nodes, because it is not configured for a specific MS-PW at S-PE nodes.