Understanding backup path calculation
The TI-LFA backup path calculation method remains unchanged, but in this scenario, there is no overlapping PQ node between the extended P-Space of P2 and the Q-Space of PE5. The following explains how TI-LFA uses a combination of a Node-SID and Adjacency-SID to route traffic over a link that is not the primary path on its own.
The following topology diagram in Figure 8.2 is relevant to the following TI-LFA calculation section:
Figure 8.2 – Double-segment FRR path calculation
The following explanation details the computation and selection of the primary path and backup path.
Primary path
The primary path from P2 to PE5 goes via P3, with the end-to-end path, cost of 30 (P2 - P3 - P4 - PE5), making it the shortest path.
Backup path
The backup path is classified as TI-LFA (link) because the classic LFA for link protection inequality condition is not met here.
Classic LFA
As per RFC 5286, Inequality...
