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350-501 · Question #361

Which condition must be met for TI-LFA to protect LDP traffic?

The correct answer is D. For double-segment protection, the P and Q nodes must be SR-capable. For TI-LFA to protect LDP traffic, the nodes involved in constructing and forwarding the Segment Routing repair path, specifically the P and Q nodes in double-segment protection, must be Segment Routing-capable.

MPLS and Segment Routing

Question

Which condition must be met for TI-LFA to protect LDP traffic?

Options

  • AFor single-segment protection, the PQ node must be LDP and SR-capable.
  • BThe protected destination must have an associated LDP label and prefix-SID.
  • CThe point of local repair must be LDP-capable.
  • DFor double-segment protection, the P and Q nodes must be SR-capable.

How the community answered

(29 responses)
  • A
    10% (3)
  • B
    3% (1)
  • C
    7% (2)
  • D
    79% (23)

Why each option

For TI-LFA to protect LDP traffic, the nodes involved in constructing and forwarding the Segment Routing repair path, specifically the P and Q nodes in double-segment protection, must be Segment Routing-capable.

AFor single-segment protection, the PQ node must be LDP and SR-capable.

While the PQ node is part of TI-LFA calculation, its LDP capability is not strictly required for SR-based TI-LFA protection; it primarily needs to be SR-capable.

BThe protected destination must have an associated LDP label and prefix-SID.

The protected destination having an LDP label is true for LDP traffic, and a prefix-SID is required for SR repair paths, but the presence of both on the *destination* isn't the fundamental condition for TI-LFA *protection* itself; the PLR and forwarding nodes need SR capability.

CThe point of local repair must be LDP-capable.

The point of local repair (PLR) must be SR-capable to perform TI-LFA calculation and encapsulation, as TI-LFA is an SR-specific technology; simply being LDP-capable is insufficient for TI-LFA's functionality.

DFor double-segment protection, the P and Q nodes must be SR-capable.Correct

TI-LFA is a Segment Routing (SR) based fast reroute mechanism. For TI-LFA to provide double-segment protection, both the P (repair path first-hop) and Q (repair path next-hop) nodes must be SR-capable to understand and forward traffic based on the Segment Identifiers (SIDs) used to construct the repair path, even when protecting primary LDP data plane paths.

Concept tested: TI-LFA protection for LDP traffic

Source: https://www.cisco.com/c/en/us/td/docs/iosxr/asr9000/segment-routing/69x/b-segment-routing-cg-69x-asr9k/b-segment-routing-cg-69x-asr9k_chapter_011.html

Topics

#TI-LFA#Segment Routing#LDP Traffic Protection#Fast Reroute

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