The datapath procedures for the service origination router role, as described in Service Origination and Termination Roles, are modified as follows when the LFA backup is a repair tunnel.
The top SID of the primary path is copied into the LFA SRH as the only SID in the SID list. The Segments-Left field of the LFA SRH is set to 1.
The End SID of the PQ node or the End.X SID of the P-Q adjacency is copied in the DA field of the outer IPv6 header.
If the PQ node is the same as the node that owns the top SID of the primary path, the LFA SRH insertion must be skipped. This is not a datapath procedure per se, but IGP compresses the SID list of the backup path to look the same as that of the primary path.
The datapath procedures for the transit router role, as described in Transit Router Role with or without Segment Termination, are modified as follows when the LFA backup is a repair tunnel.
The next SID, read from the original SRH after decrementing Segments-Left field, or from the DA field if no SRH, is copied into the LFA SRH as the only SID in the SID list. The Segments-Left field of the LFA SRH is set to 1.
The SID of the PQ node is copied in the DA field of the outer IPv6 header.
If the PQ node is the same as the node that owns the next SID, the LFA SRH insertion must be skipped. This is not a datapath procedure per se, but IGP compresses the SID list of the backup path to look the same as that of the primary path.