hardMultiple ChoiceObjective-mapped
300-410 Practice Question: BGP route summarization is causing unexpected…
BGP route summarization is causing unexpected path selection for prefix 172.16.0.0/16. Router R1 (AS 65001) has:
router bgp 65001 neighbor 10.0.0.2 remote-as 65002 network 172.16.0.0 mask 255.255.0.0
aggregate-address 172.16.0.0 255.255.0.0 summary-only !
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?
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
✓
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.
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.
- ✗
The aggregate-address command is missing the as-set keyword, causing the aggregate to have incorrect path attributes.
Why it's wrong here
as-set is not required for this issue; it affects path information.
- ✗
R2's local preference for the /24 is lower than the aggregate, so it prefers the aggregate.
Why it's wrong here
The /24 is not even present in R2's BGP table.
- ✗
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.
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Same concept, more angles
2 more ways this is tested on 300-410
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.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.