Question 1,935 of 2,152

BGP Troubleshooting →mediumMultiple ChoiceObjective-mapped

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 is troubleshooting missing BGP routes on R3. R1 (AS 65001) is an eBGP peer of R2 (AS 65002), and R2 is an iBGP peer of R3 (AS 65002). R1 advertises the prefix 172.16.1.0/24 to R2. On R2, 'show ip bgp' shows the prefix with next-hop 10.1.1.1 (R1's interface). R3's BGP table does not contain this prefix. R2 and R3 are not route reflectors, and there are no other iBGP peers. What is the most likely cause?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

Question 1mediummultiple choice

Open the full BGP breakdown →

A
R2 does not have the 'neighbor 10.1.1.3 activate' command under the BGP configuration for the iBGP session.
Correct because without the activate command, BGP will not advertise any prefixes to the neighbor, even if the session is up.
B
R2 is not advertising the route because the next-hop 10.1.1.1 is not reachable from R3.
Why wrong: Incorrect because R2 will still advertise the route to iBGP peers; reachability of the next-hop is evaluated on the receiving router.
C
R2 is not advertising the route because BGP synchronization is enabled and the IGP does not have the route.
Why wrong: Incorrect because synchronization is disabled by default and would not prevent advertisement of eBGP learned routes to iBGP.
D
R2 is not advertising the route because the prefix is being filtered by an inbound route-map on R3.
Why wrong: Incorrect because inbound filters on R3 would affect R3's BGP table, not R2's advertisement; the route would still be sent.

Full breakdown with real-world context →

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

✓

R2 does not have the 'neighbor 10.1.1.3 activate' command under the BGP configuration for the iBGP session.

In iBGP, by default, routes learned from an iBGP peer are not advertised to other iBGP peers (split horizon). Since R2 learned the route from eBGP, it should advertise it to R3. However, if R3 learned the route via iBGP from another source, it might not be advertised. But the scenario implies R2 is not advertising to R3. The most common cause is that R2 has a route-map or filter blocking the advertisement, or the next-hop is unreachable from R3. However, given the information, the likely issue is that R2 does not have the route in its BGP table as best, or the next-hop is not reachable. But the stem says R2 shows the prefix. The correct answer is that R2 is not advertising because the next-hop (10.1.1.1) is not reachable from R3, but that would affect R3's ability to use the route, not R2's advertisement. Actually, R2 will advertise to iBGP peers regardless of next-hop reachability on the receiver. So the issue must be that R2 is not advertising due to a missing 'neighbor R3 activate' or a filter. The most plausible is that the network statement or redistribution is missing on R2 for the prefix? No, R2 has it. Let me re-read: R2's BGP table shows the prefix. The missing route on R3 could be due to R2 not having the 'neighbor 10.1.1.3 activate' under the BGP process, or a route-map blocking. The stem does not mention any filters. The most common cause in such scenarios is that the BGP session between R2 and R3 is not configured to exchange prefixes (missing activate).

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.

✓
R2 does not have the 'neighbor 10.1.1.3 activate' command under the BGP configuration for the iBGP session.
Why this is correct
Correct because without the activate command, BGP will not advertise any prefixes to the neighbor, even if the session is up.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
✗
R2 is not advertising the route because the next-hop 10.1.1.1 is not reachable from R3.
Why it's wrong here
Incorrect because R2 will still advertise the route to iBGP peers; reachability of the next-hop is evaluated on the receiving router.
✗
R2 is not advertising the route because BGP synchronization is enabled and the IGP does not have the route.
Why it's wrong here
Incorrect because synchronization is disabled by default and would not prevent advertisement of eBGP learned routes to iBGP.
✗
R2 is not advertising the route because the prefix is being filtered by an inbound route-map on R3.
Why it's wrong here
Incorrect because inbound filters on R3 would affect R3's BGP table, not R2's advertisement; the route would still be sent.

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.

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: R2 does not have the 'neighbor 10.1.1.3 activate' command under the BGP configuration for the iBGP session. — In iBGP, by default, routes learned from an iBGP peer are not advertised to other iBGP peers (split horizon). Since R2 learned the route from eBGP, it should advertise it to R3. However, if R3 learned the route via iBGP from another source, it might not be advertised. But the scenario implies R2 is not advertising to R3. The most common cause is that R2 has a route-map or filter blocking the advertisement, or the next-hop is unreachable from R3. However, given the information, the likely issue is that R2 does not have the route in its BGP table as best, or the next-hop is not reachable. But the stem says R2 shows the prefix. The correct answer is that R2 is not advertising because the next-hop (10.1.1.1) is not reachable from R3, but that would affect R3's ability to use the route, not R2's advertisement. Actually, R2 will advertise to iBGP peers regardless of next-hop reachability on the receiver. So the issue must be that R2 is not advertising due to a missing 'neighbor R3 activate' or a filter. The most plausible is that the network statement or redistribution is missing on R2 for the prefix? No, R2 has it. Let me re-read: R2's BGP table shows the prefix. The missing route on R3 could be due to R2 not having the 'neighbor 10.1.1.3 activate' under the BGP process, or a route-map blocking. The stem does not mention any filters. The most common cause in such scenarios is that the BGP session between R2 and R3 is not configured to exchange prefixes (missing activate).

What should I do if I get this 300-410 question wrong?

Are there clue words in this question I should notice?

Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

What is the key concept behind this question?

OSPF neighbours must agree on key parameters.

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