- A
The neighbor 10.1.1.2 is advertising three new prefixes: 192.168.1.0/24, 192.168.2.0/24, and 192.168.3.0/24.
Why wrong: The third update includes 'unreachable', meaning it is a withdrawal, not an advertisement.
- B
The neighbor 10.1.1.2 is withdrawing the prefix 192.168.3.0/24 while advertising 192.168.1.0/24 and 192.168.2.0/24.
The 'unreachable' keyword in the debug output indicates a route withdrawal for 192.168.3.0/24.
- C
The neighbor 10.1.1.2 is flapping all three prefixes due to a BGP session reset.
Why wrong: The output shows only one withdrawal; the other two are normal updates. No flap is indicated.
- D
The neighbor 10.1.1.2 is sending a malformed update for 192.168.3.0/24, causing it to be marked as unreachable.
Why wrong: The 'unreachable' is a standard BGP withdrawal mechanism, not an error condition.
Quick Answer
The correct interpretation is that neighbor 10.1.1.2 is withdrawing the prefix 192.168.3.0/24 while advertising 192.168.1.0/24 and 192.168.2.0/24. This is confirmed by the third update line, which includes the keyword “unreachable” after the attribute set, signaling that the route is being removed from the BGP table rather than added or updated. In the context of the Cisco CCNP ENARSI 300-410 exam, interpreting debug ip bgp updates output tests your ability to distinguish between route advertisements and withdrawals, a common troubleshooting skill for BGP convergence issues. A frequent trap is misreading the “unreachable” flag as an error or assuming all prefixes in the output are being installed; instead, remember that any prefix listed immediately after the “unreachable” keyword is being withdrawn. A useful memory tip is to think of “unreachable” as “undo” — the neighbor is telling you the route is no longer valid.
300-410 BGP Troubleshooting Practice Question
This 300-410 practice question tests your understanding of bgp troubleshooting. 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.
A network engineer runs the following command to troubleshoot a BGP Troubleshooting issue:
R1# debug ip bgp updates
BGP(0): 10.1.1.2 UPDATE rcvd w/ attr: nexthop 10.1.1.2, origin i, localpref 100, metric 0 BGP(0): 10.1.1.2 rcvd 192.168.1.0/24 BGP(0): 10.1.1.2 rcvd UPDATE w/ attr: nexthop 10.1.1.2, origin i, localpref 100, metric 0 BGP(0): 10.1.1.2 rcvd 192.168.2.0/24 BGP(0): 10.1.1.2 rcvd UPDATE w/ attr: nexthop 10.1.1.2, origin i, localpref 100, metric 0 -- unreachable BGP(0): 10.1.1.2 rcvd 192.168.3.0/24
What does this output indicate?
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 neighbor 10.1.1.2 is withdrawing the prefix 192.168.3.0/24 while advertising 192.168.1.0/24 and 192.168.2.0/24.
The debug ip bgp updates output shows received BGP updates from neighbor 10.1.1.2. The third update includes 'unreachable', indicating that the neighbor is withdrawing the route 192.168.3.0/24. The other prefixes are being advertised normally.
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 neighbor 10.1.1.2 is advertising three new prefixes: 192.168.1.0/24, 192.168.2.0/24, and 192.168.3.0/24.
Why it's wrong here
The third update includes 'unreachable', meaning it is a withdrawal, not an advertisement.
- ✓
The neighbor 10.1.1.2 is withdrawing the prefix 192.168.3.0/24 while advertising 192.168.1.0/24 and 192.168.2.0/24.
Why this is correct
The 'unreachable' keyword in the debug output indicates a route withdrawal for 192.168.3.0/24.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The neighbor 10.1.1.2 is flapping all three prefixes due to a BGP session reset.
Why it's wrong here
The output shows only one withdrawal; the other two are normal updates. No flap is indicated.
- ✗
The neighbor 10.1.1.2 is sending a malformed update for 192.168.3.0/24, causing it to be marked as unreachable.
Why it's wrong here
The 'unreachable' is a standard BGP withdrawal mechanism, not an error condition.
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 output shows only one withdrawal; the other two are normal updates. No flap is indicated.
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?
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 neighbor 10.1.1.2 is withdrawing the prefix 192.168.3.0/24 while advertising 192.168.1.0/24 and 192.168.2.0/24. — The debug ip bgp updates output shows received BGP updates from neighbor 10.1.1.2. The third update includes 'unreachable', indicating that the neighbor is withdrawing the route 192.168.3.0/24. The other prefixes are being advertised normally.
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. A network engineer runs the following command to troubleshoot a BGP Troubleshooting issue: R1# debug ip bgp 10.1.1.2 updates BGP: 10.1.1.2 sending UPDATE with 2 prefixes, 0 withdrawn BGP: 10.1.1.2 sending UPDATE with 0 prefixes, 1 withdrawn What does this output indicate?
medium- ✓ A.The neighbor 10.1.1.2 is receiving two new routes and one route withdrawal.
- B.The neighbor 10.1.1.2 is sending two new routes and one route withdrawal.
- C.The BGP session with 10.1.1.2 is flapping.
- D.The neighbor 10.1.1.2 is not reachable.
Why A: The debug ip bgp updates with neighbor IP shows updates sent to the neighbor. The first line indicates an update with two prefixes advertised. The second line indicates an update with one prefix withdrawn. This is normal BGP operation.
Last reviewed: Jun 18, 2026
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