The answer is that the route 10.2.2.0/24 is being incorrectly advertised back to the neighbor from which it was learned. This output shows R1 advertising a route with a next hop of 10.1.12.2 to the very same neighbor at 10.1.12.2, which violates the BGP split-horizon principle—a fundamental rule that prevents a router from readvertising routes learned from an eBGP neighbor back to that neighbor, as doing so can create routing loops. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of BGP route advertisement rules and common misconfigurations like missing AS-path filtering or outbound prefix-lists. A common trap is assuming the advertised-routes output always shows correct behavior; instead, always check whether the next hop matches the neighbor’s IP. Memory tip: “Don’t send it back where it came from—split-horizon keeps the path from going slack.”
300-410 BGP Troubleshooting Practice Question
This 300-410 practice question tests your understanding of bgp troubleshooting. 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 runs the following command on Router R1:
R1# show bgp neighbors 10.1.12.2 advertised-routes
BGP table version is 15, local router ID is 10.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 10.1.1.0/24 0.0.0.0 0 32768 i
*> 10.2.2.0/24 10.1.12.2 0 0 65002 i
Total number of prefixes 2
Based on this output, what can be concluded about the route 10.2.2.0/24?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
The route 10.2.2.0/24 is being advertised back to the neighbor from which it was learned, which is incorrect.
The next hop is 10.1.12.2, the same as the neighbor, indicating the route is being sent back to the source, which violates BGP loop prevention unless the AS path is manipulated.
B
The route 10.2.2.0/24 is locally originated.
Why wrong: The path shows AS 65002, so it is learned from AS 65002, not locally originated.
C
The route 10.2.2.0/24 has a weight of 0.
Why wrong: Weight is 0, which is default for learned routes, but this is not a problem.
D
The route 10.2.2.0/24 is not valid.
Why wrong: It is marked with * and >, meaning valid and best.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The route 10.2.2.0/24 is being advertised back to the neighbor from which it was learned, which is incorrect.
The route 10.2.2.0/24 is being advertised to the neighbor 10.1.12.2 with next hop 10.1.12.2 itself. This indicates that R1 is advertising a route it learned from the same neighbor back to it, which is a classic case of BGP split-horizon or route advertisement causing a potential loop. However, since it is advertised, it may be due to missing AS-path filtering or a misconfiguration.
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 route 10.2.2.0/24 is being advertised back to the neighbor from which it was learned, which is incorrect.
Why this is correct
The next hop is 10.1.12.2, the same as the neighbor, indicating the route is being sent back to the source, which violates BGP loop prevention unless the AS path is manipulated.
Clue confirmation
The clue word "best" in the question point toward this answer.
The path shows AS 65002, so it is learned from AS 65002, not locally originated.
✗
The route 10.2.2.0/24 has a weight of 0.
Why it's wrong here
Weight is 0, which is default for learned routes, but this is not a problem.
✗
The route 10.2.2.0/24 is not valid.
Why it's wrong here
It is marked with * and >, meaning valid and best.
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
The path shows AS 65002, so it is learned from AS 65002, not locally originated.
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.
BGP Troubleshooting — This question tests BGP Troubleshooting — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The route 10.2.2.0/24 is being advertised back to the neighbor from which it was learned, which is incorrect. — The route 10.2.2.0/24 is being advertised to the neighbor 10.1.12.2 with next hop 10.1.12.2 itself. This indicates that R1 is advertising a route it learned from the same neighbor back to it, which is a classic case of BGP split-horizon or route advertisement causing a potential loop. However, since it is advertised, it may be due to missing AS-path filtering or a misconfiguration.
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.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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