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300-410 R1 and R2 are eBGP peers Practice Question
R1 and R2 are eBGP peers. R1 advertises a summary route 10.0.0.0/8 via aggregate-address 10.0.0.0 255.0.0.0 summary-only. R2 receives the summary but also expects to receive more specific routes (e.g., 10.1.0.0/16) for traffic engineering. R2's BGP table shows only the summary, and the more specific routes are missing. R1's configuration includes: router bgp 65001, network 10.1.0.0 mask 255.255.0.0, and aggregate-address 10.0.0.0 255.0.0.0 summary-only. What is the root cause?
⚠ Common exam trap
Watch out — candidates often confuse the as-set keyword (which affects AS_PATH loop prevention) with the summary-only keyword (which controls suppression of more-specific routes) — candidates often pick as-set thinking it 'includes' the more 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
✓
The summary-only keyword suppresses all more specific routes, including the network 10.1.0.0/16, from being advertised to R2.
The aggregate-address command with the summary-only keyword advertises only the aggregate 10.0.0.0/8 to BGP peers and suppresses all more-specific component routes (such as 10.1.0.0/16) from being advertised. This is the documented behavior of summary-only in Cisco IOS: it creates the aggregate and filters the contributing routes from outbound updates. Removing summary-only (or using suppress-map/as-set) would allow the more specifics to be advertised alongside the summary.
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, including the network 10.1.0.0/16, from being advertised to R2.
Why this is correct
The summary-only keyword on aggregate-address suppresses advertisement of all component routes, so 10.1.0.0/16 is filtered from R2's BGP updates. Removing summary-only, or using suppress-map or unsuppress-map, restores the more specific prefixes required for traffic engineering.
- ✗
The network 10.1.0.0/16 is not in the routing table of R1, so it cannot be advertised.
Why it's wrong here
The network statement is present, so R1 does have 10.1.0.0/16 in its BGP table; the summary-only keyword suppresses advertisement of all more specifics. It tempts because a missing network statement genuinely prevents advertisement, which would be the cause if the prefix were absent from R1's routing table.
- ✗
R2 must have a route-map to accept the more specific route.
Why it's wrong here
Inbound route-maps filter what a peer sends; R1 never advertises the more specifics because summary-only suppresses them, so no R2 policy can restore them. It tempts because route-maps do control prefix acceptance, and would be correct if R1 were advertising the specifics and R2 were filtering them.
- ✗
The aggregate-address should be configured with the as-set keyword to include more specifics.
Why it's wrong here
The as-set keyword only adds AS-path information to the aggregate for loop prevention; it does not cause more specific routes to be advertised. It tempts because as-set appears alongside aggregate-address commands, and would matter for summarisation where path attributes of contributing routes must be preserved.
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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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