300-410 ENARSI: Implementing Cisco Enterprise Advanced Routing and ServicesDemo

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QuestionQ439

Layer 3 Technologies

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Refer to the exhibit. AS 111 must not act as a transit AS, but ISP-1 is receiving ISP-2 routes from AS 111. Which configuration prevents the Customer AS from being used as a transit path on ISP-1?

  • A ip as-path access-list 1 permit.*
  • B ip as-path access-list 1 permit_111_
  • C ip as-path access-list 1 permit ^$
  • D ip as-path access-list 1 permit ^111$
Explanation

An inbound BGP AS-path filter at ISP-1 must permit only prefixes originated by customer AS 111. The expression ^111$ matches an AS path consisting solely of AS 111 and rejects paths containing additional AS numbers, preventing routes learned from ISP-2 from being propagated through the customer.

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