The exhibit highlights in blue the primary path of a segment going from router S to router D. The exhibit also shows a backup path. The protected link fails and fast re-route is triggered on router S. If the backup path has been calculated using standard LFA, how many SIDs are included in the label stack of the data packet forwarded to router R1?
If the backup path has been calculated using standard Loop-Free Alternate (LFA), the primary requirement is to take one alternate next-hop when the primary path fails to quickly reroute traffic. In this case, since router R1 is the immediate next-hop on the backup path from router S and considering the metric calculation (20+10+10+10=50), only one Segment ID (SID) is needed for R1. Hence, the data packet forwarded to router R1 will include 1 SID in the label stack.
A is the correct answer
Standard LFA is 1 label forwarded to R1, shortest path from R1 to D is via r2, r3, r4 metric 40 vs back through S metric 50.