FRR is configured on a network. What occurs when the headend router on the path is alerted to a link failure over IGP?

Question

Options

• ALSP attempts fast switching on the backup path until the primary path returns to the active state.
• BThe headend router uses a presignaled LSP to bypass the failure point.
• CA new backup tunnel is established past the PLR to pass through the protected nodes.
• DBackup tunnel is established and intersects with the primary tunnel at the headend.

• A
  73% (19)
• B
  15% (4)
• C
  8% (2)
• D
  4% (1)

Explanation

Option A is correct because when the headend router learns of a link failure through IGP updates, it recognizes the primary LSP path is compromised and the traffic continues flowing on the pre-established FRR backup path - the headend works to restore or re-signal the primary LSP while the backup carries traffic in the interim. Option B is wrong because using a presignaled bypass tunnel is the action taken by the PLR (Point of Local Repair), not the headend - the PLR activates the backup tunnel locally within milliseconds of detecting the failure, before the headend even knows. Option C is wrong because FRR backup tunnels are pre-signaled, not newly established after a failure is detected; creating tunnels on-the-fly would defeat the purpose of fast reroute. Option D is wrong because FRR backup tunnels merge back into the primary path at the Merge Point (MP), which is a downstream router - not the headend, which is the tunnel source.

Memory tip: Think of FRR as a two-act play - Act 1 is the PLR acting fast (local repair, sub-50ms, uses presignaled bypass); Act 2 is the headend reacting via IGP (switches to backup path and works to restore primary). The exam question specifies "via IGP" to signal you're in Act 2, which points to the headend's role of maintaining the backup path during recovery.

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