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300-410 Practice Question: Route summarization blocks specific routes in an…
Route summarization blocks specific routes in an MPLS network. Router R1 is a PE router that summarizes routes from a VRF into BGP. Configuration: 'router bgp 65000 address-family ipv4 vrf CUSTOMER network 10.0.0.0 255.255.252.0' and 'aggregate-address 10.0.0.0 255.255.252.0 summary-only'. A specific prefix 10.0.3.0/24 is not reachable from the MPLS core. 'show ip bgp vpnv4 vrf CUSTOMER 10.0.3.0/24' output: 'Not in table'. 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 more specific routes, so 10.0.3.0/24 is not advertised.
The 'summary-only' keyword in aggregate-address suppresses all more specific routes, including 10.0.3.0/24, from being advertised. This causes the specific prefix to be missing from the BGP table, leading to unreachability if the summary route does not have a valid next-hop or if the specific route is needed for traffic engineering.
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 more specific routes, so 10.0.3.0/24 is not advertised.
Why this is correct
With summary-only, only the aggregate is advertised; specifics are suppressed, causing loss of reachability to 10.0.3.0/24.
- ✗
The VRF CUSTOMER does not have the route 10.0.3.0/24 in its routing table.
Why it's wrong here
The route is expected to be present.
- ✗
The aggregate-address command is missing the 'as-set' keyword, causing routing loops.
Why it's wrong here
Missing as-set affects AS path, not route suppression.
- ✗
The network statement for 10.0.0.0/22 is missing; need to advertise the aggregate.
Why it's wrong here
Aggregate-address does not require network statement.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
MPLS Layer 3 VPN
A technology that uses Multiprotocol Label Switching to create secure, scalable virtual private networks that connect multiple sites at the network layer, where the service provider manages routing between customer sites.
Key term
BGP Route Aggregation
BGP Route Aggregation is a technique that combines multiple specific network routes into a single summary route to reduce the size of routing tables and improve network efficiency.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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