300-420 Question #165: Real Exam Question with Answer & Explanation

Question

Refer to the exhibit. AS65533 and AS65530 are announcing a partial Internet routing table as well as their IP subnets. An architect must create a design that ensures AS64512 become a transit AS. Which filtering solution must the architect choose?

Options

• AMaximum-prefix
• BNo-advertise
• CNext-hop
• Dno-export

Explanation

The architect needs to configure AS64512 to function as a transit AS, ensuring that routes learned from its peers are appropriately filtered for onward advertisement to other external ASs.

Common mistakes.

• A. Maximum-prefix is used to limit the number of prefixes accepted from a BGP peer, protecting the router from routing table overflow, and does not control transit behavior or route propagation policies.
• B. The no-advertise BGP community attribute prevents a route from being advertised to any other BGP peer, whether internal or external, which is a more restrictive action than typically desired for a transit AS.
• C. Modifying the next-hop attribute affects how traffic reaches a destination but is not a filtering solution that controls whether routes are propagated to make an AS a transit network.

Concept tested. BGP community for route propagation control

Reference. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-16-10/bgp-xe-16-10-book/bgp-route-maps-comm-attr.html

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