350-501 MPLS and Segment Routing Practice Question
Exhibit
Refer to the exhibit. The output is from a Cisco router after configuring Segment Routing with TI-LFA. R1# show isis fast-reroute summary Number of prefixes with backup paths: 1 TI-LFA: Primary Protection: Enabled Node Protection: Enabled SRLG Protection: Disabled
What does the output indicate about the TI-LFA protection on R1?
⚠ Common exam trap
Cisco often tests the misconception that TI-LFA always provides a backup path for every prefix, but in reality, the backup is only installed when a loop-free post-convergence path exists; candidates may incorrectly assume all prefixes are protected.
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
✓
Only 1 prefix has a backup path.
The output shows that for the protected prefix 10.1.1.1/32, the backup path is listed as 'N/A', meaning no backup path was computed. However, for the other prefix (likely a different /32), a backup path is present. This indicates that only one prefix has a backup path, making option B correct. TI-LFA computes a backup path only when a viable post-convergence path exists without microloops; if the topology doesn't allow it, the backup is marked as not available.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
All prefixes have backup paths.
Why it's wrong here
Only 1 prefix is shown to have a backup; this does not indicate all.
- ✓
Only 1 prefix has a backup path.
Why this is correct
The line 'Number of prefixes with backup paths: 1' confirms this.
- ✗
0 prefixes have backup paths.
Why it's wrong here
The output explicitly states 1 prefix has a backup path.
- ✗
2 prefixes have backup paths.
Why it's wrong here
The output states 1, not 2.
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