350-501 MPLS and Segment Routing Practice Question
A service provider is using Segment Routing with TI-LFA for fast convergence. During a link failure, the router performing the local repair must compute a backup path that avoids the failed link. Which type of Adjacency Segment Identifier (Adj-SID) is required for the backup path to be loop-free?
⚠ Common exam trap
Cisco often tests the misconception that any Adj-SID can be used for TI-LFA backup, but only the Protected Adj-SID (B-flag set) guarantees loop-free behavior, while Unprotected Adj-SID leads to potential loops or blackholing.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Protected Adj-SID
In Segment Routing with TI-LFA, the router performing local repair must compute a backup path that is guaranteed loop-free. A Protected Adj-SID (Adj-SID with the B-flag set) is required because it indicates that the adjacency segment is protected by a fast-reroute mechanism, allowing the router to use a precomputed backup path that avoids the failed link while ensuring no forwarding loops occur.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Anycast Adj-SID
Why it's wrong here
Anycast Adj-SID is used for load balancing, not local protection.
- ✗
Unprotected Adj-SID
Why it's wrong here
Unprotected Adj-SIDs are not used for TI-LFA backup.
- ✓
Protected Adj-SID
Why this is correct
Protected Adj-SID enables fast reroute protection in SR networks.
- ✗
Backup Adj-SID
Why it's wrong here
There is no standard 'Backup Adj-SID'; TI-LFA uses the protected flag.
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Senior Network & Security Engineer · founder of Courseiva
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