Question 167 of 2,152

Route Summarization →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is to add the 'summary-only' keyword to the BGP aggregate-address command. This keyword suppresses the advertisement of all more specific routes (like 192.168.0.0/24 and 192.168.1.0/24) to the ISP neighbor, leaving only the aggregate 192.168.0.0/22 in the BGP table. Without this keyword, BGP defaults to advertising both the aggregate and the specifics, and since BGP inherently prefers the most specific prefix, the ISP will always choose the /24 routes over the /22, defeating the purpose of summarization. On the Cisco CCNP ENARSI 300-410 exam, this is a classic trap: candidates often forget that 'aggregate-address' alone does not suppress specifics, and the 'summary-only' keyword is the explicit fix to enforce aggregate preference. A reliable memory tip is to think of "summary-only" as "only the summary survives"—it kills the details so the big picture wins.

300-410 Route Summarization Practice Question

This 300-410 practice question tests your understanding of route summarization. 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 a BGP route summarization issue. Router R1 is configured with the 'aggregate-address 192.168.0.0 255.255.252.0' command without any keywords. The engineer notices that the ISP neighbor is receiving both the aggregate route and the more specific routes (192.168.0.0/24, 192.168.1.0/24, etc.), causing the ISP to prefer the specific routes. What should the engineer do to ensure the aggregate route is preferred?

Question 1mediummultiple choice

Open the full BGP breakdown →

A
Add the 'summary-only' keyword to the aggregate-address command.
Correct. The summary-only keyword suppresses the advertisement of more specific routes, leaving only the aggregate route.
B
Remove the network statements for the specific subnets from the BGP process.
Why wrong: Incorrect. Removing network statements may cause the specific routes to not be injected, but the aggregate may still not be preferred if the specific routes are learned via other means.
C
Configure a route-map to set a higher local preference on the aggregate route.
Why wrong: Incorrect. Local preference is used for outbound traffic; it does not affect what the ISP prefers.
D
Use the 'aggregate-address' command with the 'as-set' keyword.
Why wrong: Incorrect. The as-set keyword preserves AS_PATH information but does not suppress specific routes.

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

✓

Add the 'summary-only' keyword to the aggregate-address command.

By default, BGP prefers more specific routes over less specific ones. To make the aggregate route preferred, the engineer can use the 'summary-only' keyword to suppress specific routes, or use attributes like AS_PATH to make the aggregate more attractive.

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.

✓
Add the 'summary-only' keyword to the aggregate-address command.
Why this is correct
Correct. The summary-only keyword suppresses the advertisement of more specific routes, leaving only the aggregate route.
Related concept
OSPF neighbours must agree on key parameters.
✗
Remove the network statements for the specific subnets from the BGP process.
Why it's wrong here
Incorrect. Removing network statements may cause the specific routes to not be injected, but the aggregate may still not be preferred if the specific routes are learned via other means.
✗
Configure a route-map to set a higher local preference on the aggregate route.
Why it's wrong here
Incorrect. Local preference is used for outbound traffic; it does not affect what the ISP prefers.
✗
Use the 'aggregate-address' command with the 'as-set' keyword.
Why it's wrong here
Incorrect. The as-set keyword preserves AS_PATH information but does not suppress specific routes.

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

Keyword trap
Incorrect. The as-set keyword preserves AS_PATH information but does not suppress specific routes.

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?

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: Add the 'summary-only' keyword to the aggregate-address command. — By default, BGP prefers more specific routes over less specific ones. To make the aggregate route preferred, the engineer can use the 'summary-only' keyword to suppress specific routes, or use attributes like AS_PATH to make the aggregate more attractive.

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

What is the key concept behind this question?

OSPF neighbours must agree on key parameters.

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