mediumMultiple Choice
300-410 Practice Question: Is troubleshooting a BGP route summarization issue
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?
⚠ Common exam trap
Candidates often confuse the effect of 'as-set' (which modifies AS_PATH for loop prevention) with 'summary-only' (which suppresses component routes) — candidates often pick as-set thinking it hides the specifics.
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.
The 'aggregate-address' command without keywords advertises both the summary route and all more-specific component routes. Adding the 'summary-only' keyword suppresses the more-specific routes from being advertised to BGP neighbors, leaving only the aggregate. This forces the ISP to use the aggregate since no longer-matching specifics exist.
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
Without keywords, aggregate-address advertises the summary plus all contributing specifics. Adding summary-only suppresses those more specific routes from being advertised, leaving only 192.168.0.0/22, so the ISP neighbour receives and prefers the aggregate. This directly resolves the stated summarisation problem.
- ✗
Remove the network statements for the specific subnets from the BGP process.
Why it's wrong here
Removing the network statements stops the specific prefixes entering the BGP table, but the aggregate is then advertised without any contributing routes, so it is not generated. The aggregate-address command with the summary-only keyword suppresses the more specifics while still advertising the summary, which is the intended fix here.
- ✗
Configure a route-map to set a higher local preference on the aggregate route.
Why it's wrong here
Local preference is not transitive across autonomous systems, so raising it on R1 affects only R1's own path selection, not the ISP's. It is tempting because local preference orders routes within one AS, and would be correct if the neighbour were an iBGP peer inside the same autonomous system.
- ✗
Use the 'aggregate-address' command with the 'as-set' keyword.
Why it's wrong here
The as-set keyword controls AS_PATH loop prevention and attribute aggregation, not suppression of the more specific component routes. It is tempting because as-set is commonly paired with aggregation, and would be correct if the requirement were preserving AS_PATH information rather than hiding specifics.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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