- A
Configure a higher administrative distance for routes learned via redistribution (e.g., distance 200 for OSPF external routes) to prefer the original protocol's routes.
Correct. By increasing the administrative distance for redistributed routes, you ensure that the original protocol's routes (with lower AD) are preferred, reducing the chance of a loop where a redistributed route is preferred over the original.
- B
Use the 'default-metric' command to set a consistent seed metric for redistributed routes, ensuring all routes have the same metric.
Why wrong: Incorrect. Setting a consistent metric helps with route selection but does not prevent loops. Loops occur when routes are re-redistributed, not due to metric inconsistency.
- C
Apply route tags to redistributed routes and use distribute lists or route maps to prevent re-redistribution of tagged routes back into the original protocol.
Correct. Tagging routes allows you to identify routes that were originally from one protocol and filter them when redistributing back, preventing loops.
- D
Enable 'auto-summary' on both routing protocols to summarize routes at classful boundaries, reducing the number of routes and loop potential.
Why wrong: Incorrect. Auto-summary can actually cause suboptimal routing and does not prevent loops. It is not a loop prevention mechanism.
- E
Configure 'passive-interface' on all interfaces where redistribution is performed to prevent routing updates from being sent.
Why wrong: Incorrect. Passive-interface prevents routing updates from being sent out an interface, but it does not prevent redistribution loops. It is used for other purposes like suppressing unnecessary updates.
Quick Answer
The correct answer is to apply route tags to redistributed routes and use distribute lists or route maps to prevent re-redistribution of tagged routes back into the original protocol. This works because mutual redistribution creates a feedback loop where a route learned from OSPF, for example, is redistributed into EIGRP, then re-injected back into OSPF, causing instability or a routing loop. Route tagging marks these routes with a unique value, and distribute lists or route maps filter out any tagged route from being re-redistributed into its source protocol. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of redistribution loop prevention mechanisms, often appearing as a multiple-select question where traps include “increase the metric” or “disable auto-summary”—neither of which stops re-injection. A common memory tip is “tag and block”: tag the route at the border, then block it from coming back home.
300-410 Route Redistribution Practice Question
This 300-410 practice question tests your understanding of route redistribution. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
An engineer is troubleshooting a route redistribution issue between EIGRP and OSPF. Which TWO actions will prevent routing loops in a mutual redistribution scenario? (Choose TWO.)
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 a higher administrative distance for routes learned via redistribution (e.g., distance 200 for OSPF external routes) to prefer the original protocol's routes.
To prevent routing loops during mutual redistribution, you must ensure that routes redistributed from one protocol are not re-injected back into the source protocol. Common methods include setting administrative distance values (e.g., using 'distance' command) or using route tagging with distribute lists or route maps to filter. Simply increasing the metric for redistributed routes does not prevent loops, and disabling auto-summary is unrelated to loop prevention.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Configure a higher administrative distance for routes learned via redistribution (e.g., distance 200 for OSPF external routes) to prefer the original protocol's routes.
Why this is correct
Correct. By increasing the administrative distance for redistributed routes, you ensure that the original protocol's routes (with lower AD) are preferred, reducing the chance of a loop where a redistributed route is preferred over the original.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
Use the 'default-metric' command to set a consistent seed metric for redistributed routes, ensuring all routes have the same metric.
Why it's wrong here
Incorrect. Setting a consistent metric helps with route selection but does not prevent loops. Loops occur when routes are re-redistributed, not due to metric inconsistency.
- ✓
Apply route tags to redistributed routes and use distribute lists or route maps to prevent re-redistribution of tagged routes back into the original protocol.
Why this is correct
Correct. Tagging routes allows you to identify routes that were originally from one protocol and filter them when redistributing back, preventing loops.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
Enable 'auto-summary' on both routing protocols to summarize routes at classful boundaries, reducing the number of routes and loop potential.
Why it's wrong here
Incorrect. Auto-summary can actually cause suboptimal routing and does not prevent loops. It is not a loop prevention mechanism.
- ✗
Configure 'passive-interface' on all interfaces where redistribution is performed to prevent routing updates from being sent.
Why it's wrong here
Incorrect. Passive-interface prevents routing updates from being sent out an interface, but it does not prevent redistribution loops. It is used for other purposes like suppressing unnecessary updates.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 300-410 question test?
Route Redistribution — This question tests Route Redistribution — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: Configure a higher administrative distance for routes learned via redistribution (e.g., distance 200 for OSPF external routes) to prefer the original protocol's routes. — To prevent routing loops during mutual redistribution, you must ensure that routes redistributed from one protocol are not re-injected back into the source protocol. Common methods include setting administrative distance values (e.g., using 'distance' command) or using route tagging with distribute lists or route maps to filter. Simply increasing the metric for redistributed routes does not prevent loops, and disabling auto-summary is unrelated to loop prevention.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
About these practice questions
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Same concept, more angles
1 more ways this is tested on 300-410
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. When redistributing routes between OSPF and EIGRP, which of the following is a recommended best practice to prevent routing loops?
medium- A.Use the same administrative distance for both protocols.
- ✓ B.Apply route filtering using route-maps or prefix-lists to control which routes are redistributed.
- C.Increase the default metric to a high value to discourage redistribution.
- D.Disable route summarization on both protocols.
Why B: To prevent routing loops during mutual redistribution, it is recommended to use route-maps with distribute-lists or prefix-lists to filter routes, and to adjust administrative distances to prefer one protocol over the other. This prevents routes from being redistributed back and forth.
Last reviewed: Jun 18, 2026
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