The answer is mutual redistribution without route filtering, which directly causes a routing loop. When two OSPF processes redistribute into each other as shown, routes from OSPF 1 are advertised into OSPF 2, then re-injected back into OSPF 1 as external routes, creating a feedback loop. This explains why R2’s traceroutes show traffic traversing R1 twice—packets bounce between domains instead of taking a direct path. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of redistribution loop prevention, often appearing as a troubleshooting question where suboptimal paths or routing instability are the symptoms. A common trap is assuming administrative distance alone prevents loops, but OSPF external routes (E2) have a fixed distance of 110, so no distance-based tiebreaker stops the re-redistribution. The fix requires route tagging and filtering to block re-advertised routes. Memory tip: “Mutual redistribution without filters is a round-trip ticket for packets—tag and filter to break the loop.”
300-410 Network Logging and Syslog Practice Question
This 300-410 practice question tests your understanding of network logging and syslog. 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.
Two OSPF domains are redistributed into each other on router R1. R1 has:
router ospf 1
redistribute ospf 2 subnets
router ospf 2
redistribute ospf 1 subnets
Router R2 (in OSPF 1) shows:
R2# show ip route ospf
O E2 10.1.1.0/24 [110/20] via 10.2.1.1, 00:00:05, Serial0/0/0
O E2 10.2.1.0/24 [110/20] via 10.2.1.1, 00:00:05, Serial0/0/0
R2# traceroute 10.1.1.1 source 10.2.1.2
Type escape sequence to abort.
Tracing the route to 10.1.1.1
1 10.2.1.1 4 msec 4 msec 4 msec
2 10.1.1.1 8 msec 8 msec 8 msec
R2# traceroute 10.2.1.1 source 10.1.1.2
Type escape sequence to abort.
Tracing the route to 10.2.1.1
1 10.1.1.1 4 msec 4 msec 4 msec
2 10.2.1.1 8 msec 8 msec 8 msec
Traffic between the two domains is taking suboptimal paths. What is the root cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The redistribution is mutual without any route filtering, causing routing loops.
Mutual redistribution without route filtering creates a routing loop. Routes from OSPF 1 are redistributed into OSPF 2, and then back into OSPF 1, causing suboptimal paths. The traceroute shows traffic going through R1 twice. The fix is to use route tagging and filtering to prevent re-redistribution.
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 redistribution is mutual without any route filtering, causing routing loops.
Why this is correct
Routes from OSPF 1 are redistributed into OSPF 2 and then back into OSPF 1, creating a loop.
The 'subnets' keyword is missing from one of the redistribute commands.
Why it's wrong here
Both redistribute commands include 'subnets'.
✗
OSPF administrative distance is set incorrectly, preferring external routes.
Why it's wrong here
The default AD for OSPF external routes is 110, which is not causing the loop.
✗
R1 has a routing table entry for 10.1.1.0/24 pointing to R2.
Why it's wrong here
The traceroute shows R1 as the next hop, not R2.
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
Command / output trap
Both redistribute commands include 'subnets'.
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.
Network Logging and Syslog — This question tests Network Logging and Syslog — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The redistribution is mutual without any route filtering, causing routing loops. — Mutual redistribution without route filtering creates a routing loop. Routes from OSPF 1 are redistributed into OSPF 2, and then back into OSPF 1, causing suboptimal paths. The traceroute shows traffic going through R1 twice. The fix is to use route tagging and filtering to prevent re-redistribution.
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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