- A
In SR-MPLS, the Node-SID is allocated from the dynamic label range and is unique only within the IGP area.
Why wrong: Incorrect. Node-SID is allocated from SRGB and must be unique globally (domain-wide).
- B
LDP and SR-MPLS cannot coexist on the same router; the router must be configured exclusively for one or the other.
Why wrong: Incorrect. They can coexist with mapping if needed.
- C
The Segment Routing Global Block (SRGB) is a reserved label range that is used to allocate Prefix-SIDs, ensuring end-to-end label consistency.
Correct. SRGB is a global label range for Prefix-SIDs.
- D
Adjacency-SID in SR-MPLS is a local label representing a specific link and is used for traffic steering over that link.
Correct. Adjacency-SID is locally significant for a specific adjacency.
- E
In SR-MPLS, the label stack is built by the ingress router and nodes swap the top label without pushing additional labels.
Why wrong: Incorrect. In SR-MPLS, the ingress pushes a label stack; intermediate nodes may swap (or pop) labels based on the SID type.
350-501 MPLS and Segment Routing Practice Question
This 350-501 practice question tests your understanding of mpls and segment routing. 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.
A service provider is transitioning from LDP-based MPLS to Segment Routing (SR-MPLS). Which TWO statements correctly describe differences or interoperability considerations? (Choose two.)
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
The Segment Routing Global Block (SRGB) is a reserved label range that is used to allocate Prefix-SIDs, ensuring end-to-end label consistency.
SR-MPLS uses IGP (IS-IS/OSPF) to distribute Prefix-SIDs and Adjacency-SIDs, eliminating the need for LDP. SRGB is a global block of labels reserved for SR. LDP and SR can coexist with mapping. Options B and D are correct.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
In SR-MPLS, the Node-SID is allocated from the dynamic label range and is unique only within the IGP area.
Why it's wrong here
Incorrect. Node-SID is allocated from SRGB and must be unique globally (domain-wide).
- ✗
LDP and SR-MPLS cannot coexist on the same router; the router must be configured exclusively for one or the other.
Why it's wrong here
Incorrect. They can coexist with mapping if needed.
- ✓
The Segment Routing Global Block (SRGB) is a reserved label range that is used to allocate Prefix-SIDs, ensuring end-to-end label consistency.
Why this is correct
Correct. SRGB is a global label range for Prefix-SIDs.
Related concept
OSPF neighbours must agree on key parameters.
- ✓
Adjacency-SID in SR-MPLS is a local label representing a specific link and is used for traffic steering over that link.
Why this is correct
Correct. Adjacency-SID is locally significant for a specific adjacency.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
In SR-MPLS, the label stack is built by the ingress router and nodes swap the top label without pushing additional labels.
Why it's wrong here
Incorrect. In SR-MPLS, the ingress pushes a label stack; intermediate nodes may swap (or pop) labels based on the SID type.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-501 OSPF questions on adjacency and route selection.
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MPLS and Segment Routing — study guide chapter
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FAQ
Questions learners often ask
What does this 350-501 question test?
MPLS and Segment Routing — This question tests MPLS and Segment Routing — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The Segment Routing Global Block (SRGB) is a reserved label range that is used to allocate Prefix-SIDs, ensuring end-to-end label consistency. — SR-MPLS uses IGP (IS-IS/OSPF) to distribute Prefix-SIDs and Adjacency-SIDs, eliminating the need for LDP. SRGB is a global block of labels reserved for SR. LDP and SR can coexist with mapping. Options B and D are correct.
What should I do if I get this 350-501 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-501 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jul 4, 2026
This 350-501 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-501 exam.
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