The answer is that the mutual redistribution routing loop is caused by the absence of route-map filters on the redistribute commands. When R1 redistributes EIGRP into OSPF and R2 redistributes OSPF back into EIGRP without any filtering, each router re-injects the other’s learned route, creating a feedback loop where the same prefix is continuously advertised back and forth. This occurs because OSPF’s default administrative distance (110) and EIGRP’s (90) allow both routers to prefer the redistributed route over the original, causing the loop to persist. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of redistribution loop prevention, a common trap where engineers forget that mutual redistribution inherently requires filtering or administrative distance tuning to break the cycle. A reliable memory tip is “filter or flip”—always apply a route-map to filter out routes learned from the other protocol, or flip the administrative distance to prefer the original source, preventing the loop from forming.
300-410 Route Redistribution Practice Question
This 300-410 practice question tests your understanding of route redistribution. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 engineer notices that after redistributing EIGRP into OSPF, a routing loop occurs between two routers. Router R1 config:
router ospf 1
redistribute eigrp 100 subnets
!
router eigrp 100
redistribute ospf 1 metric 10000 100 255 1 1500
R1# show ip route 192.168.1.0
Routing entry for 192.168.1.0/24
Known via "eigrp 100", distance 90, metric 128256
Redistributing via eigrp 100
Last update from 10.1.1.2 on GigabitEthernet0/0
R2# show ip route 192.168.1.0
Routing entry for 192.168.1.0/24
Known via "ospf 1", distance 110, metric 20
Redistributing via ospf 1
Last update from 10.1.1.1 on GigabitEthernet0/0
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The redistribute commands are missing route-map filters, causing mutual redistribution and a loop.
Mutual redistribution between EIGRP and OSPF without route filtering causes a routing loop. R1 learns the route via EIGRP and redistributes into OSPF; R2 learns via OSPF and redistributes back into EIGRP, creating a loop. The fix is to use route-maps to filter redistributed routes or set administrative distance to prefer one source.
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 redistribute commands are missing route-map filters, causing mutual redistribution and a loop.
Why this is correct
Without filtering, routes learned from one protocol are redistributed back, creating a loop.
The EIGRP metric is too low, causing OSPF to prefer the redistributed route.
Why it's wrong here
The metric is fine; the loop is due to mutual redistribution, not metric.
✗
The OSPF administrative distance is higher than EIGRP, causing suboptimal path selection.
Why it's wrong here
Distance difference is normal; the loop is due to redistribution without filters.
✗
The subnets keyword under OSPF redistribution is causing classful behavior.
Why it's wrong here
The subnets keyword allows classless routes; it does 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
The subnets keyword allows classless routes; it does 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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.
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: The redistribute commands are missing route-map filters, causing mutual redistribution and a loop. — Mutual redistribution between EIGRP and OSPF without route filtering causes a routing loop. R1 learns the route via EIGRP and redistributes into OSPF; R2 learns via OSPF and redistributes back into EIGRP, creating a loop. The fix is to use route-maps to filter redistributed routes or set administrative distance to prefer one source.
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.
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