300-320 Question #617: Real Exam Question with Answer & Explanation

The correct answer is C: 13. To prevent your AS from being used as a transit, you must filter outbound BGP advertisements to only allow routes that originate within your own AS, using an AS path access list that matches locally originated prefixes.

Question

Refer to the exhibit You must deny your service providers from using your network as a transit AS The solution must ensure that the network remains reachable. Which AS path access list do you use in the localonly route map?

Exhibit

Options

Explanation

To prevent your AS from being used as a transit, you must filter outbound BGP advertisements to only allow routes that originate within your own AS, using an AS path access list that matches locally originated prefixes.

Common mistakes.

A. Access list 11 does not match the correct AS path pattern needed to distinguish locally originated routes from transit routes, allowing incorrect prefixes through or blocking legitimate ones.
B. Access list 12 matches an incorrect pattern that either permits transit routes or denies locally originated routes, failing to meet the no-transit requirement.
D. Access list 14 uses a regex pattern that does not correctly filter AS paths to only local origination, either permitting transit traffic or breaking reachability of your own prefixes.

Concept tested. BGP AS path ACL to prevent transit AS routing

Reference. https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13754-41.html

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