300-510 · Question #9
Refer to the exhibit. Which router does R1 install as an alternate next hop when trying to reach R3 if LFA is enabled?
The correct answer is D. R2. Loop-Free Alternate (LFA) fast reroute pre-computes a backup next hop that can be used immediately upon primary link failure without risk of creating a forwarding loop. A neighbor qualifies as an LFA if its shortest path cost to the destination is strictly less than the…
Question
Refer to the exhibit. Which router does R1 install as an alternate next hop when trying to reach R3 if LFA is enabled?
Exhibit
Options
- AR5
- BR3
- CR4
- DR2
How the community answered
(21 responses)- A14% (3)
- B5% (1)
- C10% (2)
- D71% (15)
Explanation
Loop-Free Alternate (LFA) fast reroute pre-computes a backup next hop that can be used immediately upon primary link failure without risk of creating a forwarding loop. A neighbor qualifies as an LFA if its shortest path cost to the destination is strictly less than the neighbor's cost back through the computing router plus that router's cost to the destination (the RFC 5286 inequality). In this topology, R2 satisfies the LFA condition for R1 to reach R3, making R2 the pre-installed backup next hop.
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