The root cause is that the `summary-only` keyword on the `aggregate-address` command suppresses all more specific routes, so the 172.16.1.0/24 route is never advertised to R2, forcing R2 to use the less preferred aggregate. This occurs because BGP’s `aggregate-address summary-only` configuration instructs the router to advertise only the summarized prefix and to suppress all component routes, regardless of their individual path attributes or local preference on the receiving router. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how BGP route summarization interacts with path selection—a common trap is assuming that a higher local preference on a more specific route will override the aggregate, but the `summary-only` keyword prevents that route from being sent at all. Remember the memory tip: “Summary-only silences the specifics,” meaning if you want to keep granular control, avoid `summary-only` or use a `suppress-map` to selectively hide routes.
300-410 Route Summarization Practice Question
This 300-410 practice question tests your understanding of route summarization. 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.
BGP route summarization is causing unexpected path selection for prefix 172.16.0.0/16. Router R1 (AS 65001) has:
Router R2 (AS 65002) receives the aggregate and shows:
R2# show ip bgp 172.16.0.0/16
BGP routing table entry for 172.16.0.0/16, version 2
Paths: (1 available, best #1)
65001, (aggregated by 65001 10.0.0.1)
10.0.0.1 from 10.0.0.1 (10.0.0.1)
Origin IGP, localpref 100, valid, external, best
However, R2 has a more specific route for 172.16.1.0/24 via another path with higher local preference. What is the root cause?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The summary-only keyword suppresses all more specific routes, so the /24 route is not advertised to R2, forcing R2 to use the aggregate.
The aggregate-address with summary-only suppresses all more specific routes, so R2 does not see the /24 route. Even if R2 had a better path for the /24, it is not advertised due to summary-only. This can cause suboptimal routing or blackholing if the aggregate points to a less preferred path. The fix is to remove summary-only or use suppress-map to selectively suppress.
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 summary-only keyword suppresses all more specific routes, so the /24 route is not advertised to R2, forcing R2 to use the aggregate.
Why this is correct
summary-only prevents any more specific routes from being advertised.
Clue confirmation
The clue word "best" in the question point toward this answer.
The network command for 172.16.0.0/16 is missing, so the aggregate is not generated.
Why it's wrong here
The network command is present.
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 network command is present.
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.
Route Summarization — This question tests Route Summarization — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The summary-only keyword suppresses all more specific routes, so the /24 route is not advertised to R2, forcing R2 to use the aggregate. — The aggregate-address with summary-only suppresses all more specific routes, so R2 does not see the /24 route. Even if R2 had a better path for the /24, it is not advertised due to summary-only. This can cause suboptimal routing or blackholing if the aggregate points to a less preferred path. The fix is to remove summary-only or use suppress-map to selectively suppress.
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.
About these practice questions
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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 is troubleshooting BGP route summarization on a border router that advertises a summary route 172.16.0.0/16 to an ISP neighbor. The engineer notices that the ISP is receiving the summary route but also receiving the more specific routes (172.16.1.0/24, 172.16.2.0/24), causing suboptimal routing. What should the engineer do to ensure only the summary route is advertised?
medium
A.Configure the 'network' command for the summary route and remove the network statements for the specific subnets.
✓ B.Use the 'aggregate-address 172.16.0.0 255.255.0.0 summary-only' command under the BGP process.
C.Apply a route-map to the neighbor to filter out the specific routes using an ACL.
D.Configure the 'summary-address' command under the BGP process.
Why B: In BGP, to suppress more specific routes when advertising a summary, the engineer must use the 'aggregate-address' command with the 'summary-only' keyword.
Variation 2. A network engineer runs the following command on Router R1:
R1# show ip bgp neighbors 10.1.1.1 advertised-routes
BGP table version is 10, local router ID is 1.1.1.1
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
*> 10.0.0.0/8 0.0.0.0 0 32768 i
*> 10.1.0.0/16 0.0.0.0 0 32768 i
*> 10.1.1.0/24 0.0.0.0 0 32768 i
*> 10.1.2.0/24 0.0.0.0 0 32768 i
Based on this output, what is a problem with the BGP advertisements?
hard
A.The router is not advertising any routes.
✓ B.The router is advertising overlapping prefixes, including both summary and specific routes.
C.The router is only advertising the summary route.
D.The router is using incorrect next-hop.
Why B: The router is advertising both a summary route (10.0.0.0/8) and more specific routes (10.1.0.0/16, 10.1.1.0/24, etc.), which defeats the purpose of summarization and can cause routing loops or suboptimal routing.
Last reviewed: Jun 18, 2026
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