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 explicitly specify the SAII and TAII to use, or the user can specify only the spoke-sdp-fec-id of the MS-PW in the vccv-ping command.
If the SAII:TAII is entered by the user in the vccv-ping command, those values are used for the VCCV-ping echo request. However, 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.
If SAII:TAII is entered in addition to the spoke-sdp-fec-id, the system will verify the entered values against the values stored in the context for that spoke-sdp-fec-id.
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 received in the initial label mapping message for the reverse direction of the MS-PW (with respect to the sending PE), the SAII:TAII to use in the target FEC stack of the VCCV-ping echo request message is derived by parsing that 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, 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, since it is not configured for a specific MS-PW at S-PE nodes.