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.
A service provider is deploying segment routing in their MPLS core. They want to use an IGP as the control plane for label distribution without running LDP or RSVP-TE. Which IGP is best suited for this purpose?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
BGP
Why wrong: BGP is an EGP, not an IGP, and requires additional components.
EIGRP
Why wrong: EIGRP does not support segment routing.
IS-IS
IS-IS supports segment routing extensions and is widely used in SP cores.
RIP
Why wrong: RIP is outdated and does not support segment routing.
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 best-suited IGP for segment routing in an MPLS core because it natively supports the Segment Routing (SR) extensions defined in RFC 8667. These extensions allow IS-IS to advertise Prefix-SIDs and Adjacency-SIDs directly within the link-state database, enabling label distribution without requiring LDP or RSVP-TE. This makes IS-IS a natural fit for service providers deploying SR-MPLS.
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.
BGP
Why it's wrong here
BGP is an EGP, not an IGP, and requires additional components.
EIGRP
Why it's wrong here
EIGRP does not support segment routing.
IS-IS
Why this is correct
IS-IS supports segment routing extensions and is widely used in SP cores.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
RIP
Why it's wrong here
RIP is outdated and does not support segment routing.
Common exam traps
The trap here is that candidates may think OSPF is also a valid choice, but the question specifically asks for the IGP best suited for segment routing without LDP or RSVP-TE, and while OSPF does support SR (RFC 8665), IS-IS is historically more common in service provider cores due to its native support for CLNS and easier migration from LDP to SR.
Detailed technical explanation
IS-IS uses sub-TLVs within the IS-IS Link State PDUs (LSPs) to carry Segment Routing information, such as the SR Capabilities sub-TLV (TLV 242) and the SID/Label Binding sub-TLV (TLV 149). This allows each router to compute the SR-MPLS label stack for any destination using the IGP's SPF algorithm, eliminating the need for a separate label distribution protocol. In real-world deployments, IS-IS SR also supports TI-LFA (Topology-Independent Loop-Free Alternate) for fast reroute, which is critical for service provider networks requiring sub-50ms convergence.
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.
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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 best-suited IGP for segment routing in an MPLS core because it natively supports the Segment Routing (SR) extensions defined in RFC 8667. These extensions allow IS-IS to advertise Prefix-SIDs and Adjacency-SIDs directly within the link-state database, enabling label distribution without requiring LDP or RSVP-TE. This makes IS-IS a natural fit for service providers deploying SR-MPLS.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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