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300-410 Practice Question: Is troubleshooting BGP route summarization on a…
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?
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
✓
Use the 'aggregate-address 172.16.0.0 255.255.0.0 summary-only' command under the BGP process.
In BGP, to suppress more specific routes when advertising a summary, the engineer must use the 'aggregate-address' command with the 'summary-only' keyword.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Configure the 'network' command for the summary route and remove the network statements for the specific subnets.
Why it's wrong here
The 'network' command only originates routes already present in the routing table; it cannot suppress the more specific connected or BGP-learned subnets, which continue to be advertised. It is tempting because 'network' statements do define which prefixes a router originates, and would be correct when adding a summary that is not otherwise in the table.
- ✓
Use the 'aggregate-address 172.16.0.0 255.255.0.0 summary-only' command under the BGP process.
Why this is correct
The `summary-only` keyword suppresses all more-specific component routes from being advertised, leaving only 172.16.0.0/16. Without it, `aggregate-address` creates the summary but still advertises the contributing /24s, which is exactly the leak the ISP is seeing. This satisfies the requirement that only the summary route be sent.
- ✗
Apply a route-map to the neighbor to filter out the specific routes using an ACL.
Why it's wrong here
An outbound route-map filtering the /24s suppresses the specifics without creating the aggregate, so the /16 must already exist as a static or summary route. The aggregate-address command with summary-only both originates the summary and hides the components.
- ✗
Configure the 'summary-address' command under the BGP process.
Why it's wrong here
The summary-address command applies to OSPF or EIGRP redistribution, not to BGP's own advertised prefixes, so it cannot suppress the /24s here. It would be right when summarising redistributed routes into OSPF; BGP needs aggregate-address with summary-only.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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