JN0-106 Routing Fundamentals Practice Question
A network is experiencing intermittent routing loops. The engineer discovers that routes are being redistributed from OSPF into BGP and then from BGP back into OSPF on different routers. What is the most effective way to prevent this?
⚠ Common exam trap
Watch out — candidates often think adjusting administrative distance or metrics can break redistribution loops, but only explicit route filtering (or tagging) prevents the cycle of mutual redistribution.
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
✓
Configure route filters to prevent mutual redistribution.
Mutual redistribution between OSPF and BGP creates routing loops when routes are exported from OSPF into BGP on one router and then re-injected back into OSPF on another router. The most effective way to prevent this is to use route filters (e.g., prefix lists, route-maps, or policy-statement in Junos) to control which routes are redistributed, ensuring that routes learned from one protocol are not re-advertised back into the same protocol. This breaks the redistribution cycle without altering protocol preference or metrics.
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 a higher preference for BGP routes.
Why it's wrong here
BGP preference (administrative distance) determines which route is installed in the routing table when the same prefix is learned from multiple protocols, but it has no effect on redistribution policy. A route with a higher BGP preference can still be advertised into OSPF and then re-advertised back into BGP, creating a loop. Preference only changes which route wins route selection, not which routes are exported or re-injected between protocols.
- ✓
Configure route filters to prevent mutual redistribution.
Why this is correct
Configuring route filters is the correct fix because mutual redistribution loops occur when a route from protocol A is exported to protocol B, then re-imported into protocol A, creating a feedback cycle. Route filters can block this by matching on route tags, protocol origin, or prefix lists at the redistribution boundary, preventing a route from being re-advertised back into its source protocol. For example, tagging OSPF routes when redistributing into BGP and then filtering out that tag on the reverse redistribution breaks the loop.
- ✗
Set a lower metric on OSPF routes.
Why it's wrong here
Setting a lower OSPF metric makes OSPF routes more preferred inside the OSPF domain by affecting the SPF calculation, but it does not control redistribution behavior. The loop persists because the route is still exported to BGP and then re-imported into OSPF, regardless of its metric value. Metric manipulation only changes the cost associated with a route, not the policy that allows mutual redistribution, so it cannot prevent the loop.
- ✗
Increase the OSPF cost on interfaces.
Why it's wrong here
Increasing the OSPF cost on an interface changes the link metric used in the shortest-path calculation, making routes through that interface less favorable. However, interface cost does not govern whether a route is redistributed into OSPF; a redistributed route from BGP will still be injected into OSPF with a cost set by the redistribution policy. Since the routing loop is caused by re-advertisement between protocols, interface cost changes do not interrupt the feedback cycle and therefore cannot solve the issue.
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This JN0-106 practice question is part of Courseiva's free Juniper Networks 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 JN0-106 exam.