300-420 Question #234: Real Exam Question with Answer & Explanation

Question

An engineer must design a routing solution for a company that is single-homed to an ISP. The company's goal is to run BGP between the CE and the PE devices. To support running BGP, the company obtained a public AS number and IP subnet from ARIN. Which solution must the engineer select?

Options

• A- The customer announces the default route to the ISP.
• B- The customer announces the public IP subnet to the ISP.
• C- The ISP announces the customer public IP subnet.
• D- The customer announces the public IP subnet to the ISP.

Explanation

For a company single-homed to an ISP using BGP with a public AS and IP subnet, the customer must announce its public IP subnet to the ISP.

Common mistakes.

• A. The customer announcing a default route to the ISP is typically incorrect; the customer usually receives a default route from the ISP to reach the internet, rather than advertising one.
• B. This choice is functionally identical to option D, which is the correct action.
• C. The ISP does not announce the customer's public IP subnet to the customer; the customer announces it to the ISP, and the ISP then advertises it to the broader internet.

Concept tested. BGP advertisement in single-homed network

Reference. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/15-s/irg-15-s-book/irg-basic-bgp.html

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