Quick Answer
IS-IS is the correct choice for an IGP in an MPLS core network requiring fast convergence, because as a link-state protocol it natively supports mechanisms like incremental SPF (iSPF) and prefix-independent convergence (PIC) that dramatically reduce reconvergence time after a topology change. Unlike OSPF, which relies on opaque LSAs for MPLS Traffic Engineering extensions, IS-IS uses built-in TLVs for the same purpose, making it more scalable and efficient in large service provider cores. On the Cisco SPCOR 350-501 exam, this question tests your understanding of why IS-IS is the de facto IGP for MPLS backbones—a common trap is choosing OSPF because of its familiarity, but remember that IS-IS is the protocol historically designed for IS-IS (Intermediate System-to-Intermediate System) environments and excels in fast convergence scenarios. For a memory tip: think "IS-IS for ISP"—the double "IS" stands for the protocol that ISPs trust for speed and scalability in MPLS cores.
This 350-501 practice question tests your understanding of networking. 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.
An ISP is designing an MPLS core network and needs to choose an IGP that supports fast convergence. Which IGP meets this requirement and is most commonly used in MPLS core networks?
IS-IS
IS-IS provides fast convergence and is the predominant IGP in service provider MPLS cores.
OSPFv3
Why wrong: OSPFv3 is used for IPv6, but IS-IS is more common in MPLS cores.
EIGRP
Why wrong: EIGRP is proprietary to Cisco and not suitable for multi-vendor MPLS cores.
RIPng
Why wrong: RIPng has slow convergence and is not suitable for MPLS core.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
IS-IS
IS-IS is the correct choice because it is a link-state IGP that inherently supports fast convergence through mechanisms like incremental SPF (iSPF) and prefix-independent convergence (PIC). It is widely deployed in MPLS core networks due to its scalability, extensibility via TLVs, and native support for MPLS Traffic Engineering (MPLS-TE) without requiring additional protocol extensions like OSPF's opaque LSA.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
IS-IS
Why this is correct
IS-IS provides fast convergence and is the predominant IGP in service provider MPLS cores.
Related concept
Read the scenario before looking for a memorised answer.
OSPFv3
Why it's wrong here
OSPFv3 is used for IPv6, but IS-IS is more common in MPLS cores.
EIGRP
Why it's wrong here
EIGRP is proprietary to Cisco and not suitable for multi-vendor MPLS cores.
RIPng
Why it's wrong here
RIPng has slow convergence and is not suitable for MPLS core.
Common exam traps
Cisco often tests the misconception that OSPF is the default IGP for all networks, but in MPLS core environments, IS-IS is the preferred choice due to its native TE support and hierarchical scalability, making OSPF a distractor despite its fast convergence capabilities.
Detailed technical explanation
IS-IS uses a two-level hierarchical design (Level 1 and Level 2) that naturally aligns with MPLS core/edge segmentation, and its TLV-based extensibility allows seamless integration of MPLS-TE extensions (RFC 5305) and Segment Routing (RFC 8667). Under the hood, IS-IS fast convergence is achieved through techniques like LSP flooding optimization, incremental SPF (iSPF) that recalculates only affected prefixes, and BGP PIC edge-node protection, which can achieve sub-50ms failover in MPLS networks.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
What to study next
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FAQ
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
The correct answer is: IS-IS — IS-IS is the correct choice because it is a link-state IGP that inherently supports fast convergence through mechanisms like incremental SPF (iSPF) and prefix-independent convergence (PIC). It is widely deployed in MPLS core networks due to its scalability, extensibility via TLVs, and native support for MPLS Traffic Engineering (MPLS-TE) without requiring additional protocol extensions like OSPF's opaque LSA.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 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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