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 migrating from OSPF to IS-IS in the core. During the migration, both protocols are running. Some prefixes are learned via both OSPF and IS-IS. The network uses BGP for external routes and MPLS for VPNs. The engineer notices that BGP routes are pointing to a next-hop that is reachable via OSPF but not via IS-IS, causing traffic to be dropped. What is the best course of action to ensure that during the migration, BGP uses only one IGP protocol?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Increase the administrative distance of IS-IS to be higher than OSPF
Option C is correct because increasing the administrative distance of IS-IS to be higher than OSPF (e.g., from 115 to 120) makes OSPF routes preferred over IS-IS routes in the routing table. Since BGP uses the IGP route to resolve its next-hop, this ensures BGP always selects the OSPF path, preventing traffic drops when the next-hop is only reachable via OSPF. This is a simple, protocol-agnostic way to control IGP preference during a migration without altering route redistribution or tunneling.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Use BGP next-hop tracking to prefer IS-IS routes
Why it's wrong here
Next-hop tracking does not force preference of one IGP over another.
✗
Configure a route-map to selectively distribute routes
Why it's wrong here
Route-maps filter route advertisement, not next-hop resolution.
✓
Increase the administrative distance of IS-IS to be higher than OSPF
Why this is correct
Higher admin distance makes IS-IS less preferred, ensuring OSPF routes are used for next-hop resolution.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Implement MPLS TE tunnels for BGP next-hops
Why it's wrong here
TE tunnels add complexity and do not address the IGP preference issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that BGP next-hop tracking (Option A) or route-maps (Option B) can influence IGP preference, when in fact administrative distance is the direct and simplest mechanism to control route selection between two IGPs.
Detailed technical explanation
How to think about this question
Administrative distance (AD) is a Cisco-specific metric used to rank routes from different sources; OSPF has a default AD of 110, while IS-IS has 115. By raising IS-IS AD above 110 (e.g., to 120), OSPF routes are always preferred in the RIB, and BGP, which relies on the RIB for next-hop resolution, will consistently use the OSPF path. In a migration scenario, this ensures traffic stability while both protocols are active, and the AD can be reverted once IS-IS is fully deployed.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Increase the administrative distance of IS-IS to be higher than OSPF — Option C is correct because increasing the administrative distance of IS-IS to be higher than OSPF (e.g., from 115 to 120) makes OSPF routes preferred over IS-IS routes in the routing table. Since BGP uses the IGP route to resolve its next-hop, this ensures BGP always selects the OSPF path, preventing traffic drops when the next-hop is only reachable via OSPF. This is a simple, protocol-agnostic way to control IGP preference during a migration without altering route redistribution or tunneling.
What should I do if I get this 350-501 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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