350-501 Networking Practice Question
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?
⚠ Common exam trap
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.
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
✓
Increase the administrative distance of IS-IS to be higher than OSPF
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.
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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.