350-501 Architecture Practice Question
A large service provider operates a national MPLS backbone with over 200 P routers and 500 PE routers. They use IS-IS as the IGP with segment routing and have deployed TI-LFA for link and node protection. Recently, a core router (P1) suffered a complete failure, and during the failure, traffic for some prefixes was dropped for over 200ms. After the failure, the network recovered within seconds. The engineer suspects that TI-LFA did not provide the expected sub-50ms protection for some destinations. Further analysis reveals that the affected prefixes have their BGP next-hop on a router that is multiple hops away, and the P1 failure impacted both the primary path and the backup path computed by TI-LFA. The engineer reviews the TI-LFA configuration and finds that 'fast-reroute per-prefix ti-lfa' is enabled under router isis. The engineer also notes that P1 was not a protecting node for those prefixes. Which action should the engineer take to improve convergence time for these prefixes?
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
✓
Add the 'sr-protect' option under the fast-reroute configuration to enable node protection
For TI-LFA to provide node protection, the 'sr-protect' option should be configured, which forces the node to attempt to provide protection even for traffic that transits through the node. Option A is wrong because interface-level TI-LFA would not help; the issue is node-level. Option C is wrong because increasing IGP timers may cause slower convergence. Option D is wrong because there is no need for an additional LDP backup.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Configure TI-LFA on all interfaces using 'fast-reroute per-prefix ti-lfa interface'
Why it's wrong here
Interface-level TI-LFA only protects against link failures, not node failures; the issue is node protection.
- ✓
Add the 'sr-protect' option under the fast-reroute configuration to enable node protection
Why this is correct
The 'sr-protect' option ensures that the node acts as a protecting node for transit traffic, providing node protection even if the node itself is the failure point.
- ✗
Reduce the IGP timers and enable incremental SPF to accelerate convergence
Why it's wrong here
Reducing timers may help but does not provide the sub-50ms protection that TI-LFA is designed for; the issue is that TI-LFA did not compute a backup path.
- ✗
Deploy LDP as a fallback label distribution protocol to provide additional backup paths
Why it's wrong here
LDP does not provide fast reroute guarantees and would not achieve sub-50ms convergence.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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