Question 948 of 2,152

Route Summarization →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the administrative distance of the redistributed routes is not modified, causing the redistributed route to be preferred over the original. This occurs because, by default, OSPF external routes have an administrative distance of 110, while EIGRP internal routes are 90 and external routes are 170; when mutual redistribution is configured without route tagging or filtering, a route learned via OSPF can be redistributed into EIGRP, then redistributed back into OSPF with a lower administrative distance than the original OSPF internal route, creating a redistribution routing loop. On the Cisco CCNP ENARSI 300-410 exam, this classic edge case tests your understanding of how administrative distance interacts with mutual redistribution between OSPF and EIGRP, often appearing as a trap where candidates overlook the need for route tags or distribute-lists. A reliable memory tip is “AD first, then metric”—always check administrative distance before metrics when troubleshooting loops, and remember that EIGRP’s default AD of 90 for internal routes can easily override OSPF’s 110.

300-410 Route Summarization Practice Question

This 300-410 practice question tests your understanding of route summarization. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 configures mutual redistribution between OSPF and EIGRP on a router. After configuration, routing loops occur. Which is the most likely explanation?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

Question 1hardmultiple choice

Review the full OSPF breakdown →

A
The administrative distance of the redistributed routes is not modified, causing the redistributed route to be preferred over the original.
Without adjusting the administrative distance (e.g., using 'distance' command on redistributed routes), the redistributed route may have a lower AD than the original, leading to a routing loop.
B
The seed metric is not configured for EIGRP redistribution.
Why wrong: Missing seed metric would prevent EIGRP from accepting the redistributed routes, not cause loops.
C
The OSPF process ID is mismatched.
Why wrong: Mismatched process ID would prevent OSPF from working altogether, not cause loops.
D
The redistribute command is missing the subnets keyword.
Why wrong: Missing the subnets keyword would only redistribute classful networks, not cause loops.

Full breakdown with real-world context →

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

✓

The administrative distance of the redistributed routes is not modified, causing the redistributed route to be preferred over the original.

Mutual redistribution without route tagging or filtering can cause routing loops. When a route redistributed from OSPF into EIGRP is then redistributed back into OSPF, it can be preferred over the original route if the administrative distance is lower, creating a loop. This is a classic edge case that requires careful use of route tags and distribute-lists.

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.

✓
The administrative distance of the redistributed routes is not modified, causing the redistributed route to be preferred over the original.
Why this is correct
Without adjusting the administrative distance (e.g., using 'distance' command on redistributed routes), the redistributed route may have a lower AD than the original, leading to a routing loop.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
✗
The seed metric is not configured for EIGRP redistribution.
Why it's wrong here
Missing seed metric would prevent EIGRP from accepting the redistributed routes, not cause loops.
✗
The OSPF process ID is mismatched.
Why it's wrong here
Mismatched process ID would prevent OSPF from working altogether, not cause loops.
✗
The redistribute command is missing the subnets keyword.
Why it's wrong here
Missing the subnets keyword would only redistribute classful networks, not cause loops.

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.

Trap categories for this question

Keyword trap
Missing the subnets keyword would only redistribute classful networks, not cause loops.

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 Summarization — This question tests Route Summarization — OSPF neighbours must agree on key parameters..

What is the correct answer to this question?

The correct answer is: The administrative distance of the redistributed routes is not modified, causing the redistributed route to be preferred over the original. — Mutual redistribution without route tagging or filtering can cause routing loops. When a route redistributed from OSPF into EIGRP is then redistributed back into OSPF, it can be preferred over the original route if the administrative distance is lower, creating a loop. This is a classic edge case that requires careful use of route tags and distribute-lists.

What should I do if I get this 300-410 question wrong?

Are there clue words in this question I should notice?

Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

What is the key concept behind this question?

OSPF neighbours must agree on key parameters.

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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. A redistribution setup between OSPF and EIGRP is causing a routing loop for subnet 10.1.1.0/24. Router R1 runs OSPF and EIGRP with redistribution. R1's configuration: router ospf 1 redistribute eigrp 100 subnets ! router eigrp 100 redistribute ospf 1 metric 10000 100 255 1 1500 ! interface GigabitEthernet0/0 ip address 10.1.1.1 255.255.255.0 ip summary-address eigrp 100 10.1.0.0 255.255.255.0 ! Router R2 (EIGRP neighbor) shows: R2# show ip route 10.1.1.0 Routing entry for 10.1.0.0/24, supernet Known via "eigrp 100", distance 90, metric 30720, type internal Last update from 10.1.1.1 on GigabitEthernet0/0, 00:00:05 ago What is the root cause?

hard

✓ A.The summary route 10.1.0.0/24 is less specific and can cause routing loops when combined with redistribution because R2 may send traffic for 10.1.1.0/24 back to R1.
B.The redistribution metric is too high, causing EIGRP to prefer the OSPF route via another path.
C.OSPF does not support subnets keyword, so the route is not redistributed correctly.
D.EIGRP is not enabled on the interface, so the summary is not advertised.

Why A: The summary route 10.1.0.0/24 is being advertised via EIGRP, but it is a less specific prefix than the actual /24. When R1 redistributes OSPF into EIGRP, the summary may cause R2 to prefer the summary over a more specific route, and if R2 sends traffic back to R1 for 10.1.1.0/24, R1 might forward it to R2 again if the OSPF route is not present, creating a loop. The summary should match the exact subnet or be more specific to avoid loops.

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