- A
Use a higher preference for BGP routes.
Why wrong: Preference changes do not stop redistribution; they only affect route selection.
- B
Configure route filters to prevent mutual redistribution.
Route filters can block routes from being redistributed back into the originating protocol, breaking the loop.
- C
Set a lower metric on OSPF routes.
Why wrong: Metric changes do not prevent redistribution loops.
- D
Increase the OSPF cost on interfaces.
Why wrong: Increasing cost does not prevent redistribution loops.
Question 142 of 514
How to Prevent Routing Loops from Mutual Distribution Using Route Filters
This JNCIA-JUNOS practice question tests your understanding of routing fundamentals. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 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?
Quick Answer
The answer is to configure route filters to prevent mutual redistribution. This is the most effective solution because mutual redistribution creates a feedback loop where routes learned from OSPF are injected into BGP, then re-advertised back into OSPF on a different router, causing the same prefix to be learned with conflicting metrics and next hops, which triggers continuous route flapping and intermittent loops. On the JNCIA-Junos exam, this scenario tests your understanding of redistribution dangers and the Junos routing policy framework, often appearing as a multiple-choice trap where options like increasing administrative distance or enabling loop prevention protocols seem plausible but fail to address the root cause. A common memory tip is to think of route filters as a one-way valve: they ensure that routes redistributed from OSPF into BGP are tagged and then blocked from being re-injected back into OSPF, breaking the cycle at its source. Remember the mnemonic “Filter the Feed” to stop loops before they start.
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.
Option B is correct because 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.
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 a higher preference for BGP routes.
Why it's wrong here
Preference changes do not stop redistribution; they only affect route selection.
- ✓
Configure route filters to prevent mutual redistribution.
Why this is correct
Route filters can block routes from being redistributed back into the originating protocol, breaking the loop.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Set a lower metric on OSPF routes.
Why it's wrong here
Metric changes do not prevent redistribution loops.
- ✗
Increase the OSPF cost on interfaces.
Why it's wrong here
Increasing cost does not prevent redistribution loops.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that 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.
Detailed technical explanation
How to think about this question
In Junos, redistribution is controlled via policy-statement with 'from protocol' and 'to protocol' conditions. When redistributing OSPF into BGP, you should tag routes with a community or set a specific metric to identify them, then use a filter on the BGP-to-OSPF redistribution to reject routes carrying that tag. This is analogous to using route-tagging in Cisco IOS to prevent routing loops, as described in RFC 1403 for OSPF-BGP interaction. A real-world scenario is an MPLS L3VPN where CE routers redistribute BGP routes into OSPF; without proper filtering, the PE router could re-learn its own routes, causing instability.
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.
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.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this JNCIA-JUNOS question test?
Routing Fundamentals — This question tests Routing Fundamentals — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Configure route filters to prevent mutual redistribution. — Option B is correct because 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.
What should I do if I get this JNCIA-JUNOS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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