This 300-410 practice question tests your understanding of bgp troubleshooting. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
R1 and R2 are iBGP peers in AS 65001. R1 has: neighbor 10.1.1.2 route-reflector-client. R2 advertises a prefix 10.0.0.0/8 with a community of no-export. R1 reflects this prefix to its other client R3. R3 is in a different AS (65002) via eBGP. R3 receives the prefix but does not advertise it to its eBGP neighbor R4 in AS 65003. 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 no-export community on the prefix prevents R3 from advertising it to eBGP neighbor R4.
The community no-export attached to the prefix prevents it from being advertised to any eBGP peers. When R1 reflects the prefix to R3 (iBGP), the community is preserved. R3, upon advertising to R4 (eBGP), checks the community and withholds the advertisement because of the no-export community. The root cause is that the no-export community is present on the prefix, which blocks eBGP advertisement.
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 no-export community on the prefix prevents R3 from advertising it to eBGP neighbor R4.
Why this is correct
no-export community means the route should not be advertised to any eBGP peers.
R1 should have stripped the community before reflecting to R3.
Why it's wrong here
Route reflectors do not modify communities by default.
✗
R3 is missing the send-community command to its eBGP neighbor.
Why it's wrong here
The community is already present; the issue is that it blocks advertisement.
✗
The route-reflector-client command on R1 should be on R2 instead.
Why it's wrong here
That would not affect the community behavior.
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.
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 no-export community on the prefix prevents R3 from advertising it to eBGP neighbor R4. — The community no-export attached to the prefix prevents it from being advertised to any eBGP peers. When R1 reflects the prefix to R3 (iBGP), the community is preserved. R3, upon advertising to R4 (eBGP), checks the community and withholds the advertisement because of the no-export community. The root cause is that the no-export community is present on the prefix, which blocks eBGP advertisement.
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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