- A
Reduces the number of labels in the control plane.
Why wrong: SR with label stacking can actually increase the number of labels in the data plane.
- B
Label allocation is per-prefix, not per-interface.
SR assigns a single label per prefix independent of the outgoing interface, simplifying label management.
- C
Eliminates the need for IGP convergence for label distribution.
Why wrong: SR relies on IGP extensions for label distribution, so IGP convergence still affects label availability.
- D
Simplifies MPLS TE tunnel configuration.
Why wrong: SR does not inherently simplify TE; it offers a different approach but LDP also supports TE with RSVP.
- E
Supports TI-LFA for fast reroute with full topology protection.
SR natively enables TI-LFA, providing link and node protection without additional protocols.
Quick Answer
The answer is support for TI-LFA with full topology protection and native per-prefix label allocation. Segment Routing enables Topology Independent Loop-Free Alternate fast reroute because it uses the IGP to precompute backup paths that protect against any single link or node failure, unlike LDP which relies on per-link LFA and often fails to provide full coverage in complex topologies. Additionally, SR allocates labels natively per prefix using the IGP’s shortest-path tree, eliminating the need for a separate label distribution protocol and reducing control-plane complexity. On the Cisco SPCOR 350-501 exam, this question tests your understanding of why SR is preferred in modern service provider cores, with a common trap being that SR still requires IGP convergence for label distribution—this is false because SR labels are derived directly from the IGP database. Another trap is confusing SR-TE with LDP-TE, but remember that LDP can also support TE, so that is not a unique benefit. Memory tip: think “TI-LFA for full protection, native labels for no extra protocol.”
350-501 Architecture Practice Question
This 350-501 practice question tests your understanding of architecture. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Which TWO are benefits of using Segment Routing over LDP in a service provider core?
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
Label allocation is per-prefix, not per-interface.
Options A and C are correct. SR supports TI-LFA for fast reroute, and native per-prefix label allocation. Option B is wrong because SR still requires IGP convergence for label distribution. Option D is wrong because SR can be used with TE, but LDP also supports TE. Option E is wrong because SR may actually increase the number of labels due to SID stack.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Reduces the number of labels in the control plane.
Why it's wrong here
SR with label stacking can actually increase the number of labels in the data plane.
- ✓
Label allocation is per-prefix, not per-interface.
Why this is correct
SR assigns a single label per prefix independent of the outgoing interface, simplifying label management.
Related concept
Static NAT maps one inside address to one outside address.
- ✗
Eliminates the need for IGP convergence for label distribution.
Why it's wrong here
SR relies on IGP extensions for label distribution, so IGP convergence still affects label availability.
- ✗
Simplifies MPLS TE tunnel configuration.
Why it's wrong here
SR does not inherently simplify TE; it offers a different approach but LDP also supports TE with RSVP.
- ✓
Supports TI-LFA for fast reroute with full topology protection.
Why this is correct
SR natively enables TI-LFA, providing link and node protection without additional protocols.
Related concept
Static NAT maps one inside address to one outside address.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
- Static NAT maps one inside address to one outside address.
- PAT allows many inside hosts to share one public address using ports.
- Inside local and inside global describe the private and translated addresses.
- NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
- Identify inside and outside interfaces first.
- Check whether the scenario needs static NAT, dynamic NAT or PAT.
- Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 350-501 NAT questions on configuration and troubleshooting.
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FAQ
Questions learners often ask
What does this 350-501 question test?
Architecture — This question tests Architecture — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Label allocation is per-prefix, not per-interface. — Options A and C are correct. SR supports TI-LFA for fast reroute, and native per-prefix label allocation. Option B is wrong because SR still requires IGP convergence for label distribution. Option D is wrong because SR can be used with TE, but LDP also supports TE. Option E is wrong because SR may actually increase the number of labels due to SID stack.
What should I do if I get this 350-501 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 350-501 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 24, 2026
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