300-420 Question #256: Real Exam Question with Answer & Explanation

Question

Refer to the exhibit. A customer has two eBGP peerings from a single CE router toward two service providers. The customer has hired an architect to design a solution to ensure certain traffic enters the customer's network through interface gig0/0. Which solution must the architect include in the design?

Options

• ABreak aggregated routes into longer prefixes and advertise to the preferred service provider.
• BAdvertise a lower MED value toward the less preferred service provider.
• CPrepend additional AS on the AS path toward the preferred service provider.
• DSet a higher local preference to the preferred service provider path.

Explanation

To ensure certain inbound traffic enters a customer's network via a preferred eBGP peering, the architect must advertise more specific routes to the preferred service provider.

Common mistakes.

• B. MED (Multi-Exit Discriminator) influences how other ASes enter your AS, with a lower MED being preferred; advertising a lower MED to the less preferred service provider would encourage traffic through the non-preferred path.
• C. AS path prepending makes a path appear longer and thus less preferred; prepending additional ASes on the AS path toward the preferred service provider would discourage inbound traffic from using that path.
• D. Local Preference is a BGP attribute used to influence outbound traffic from your AS to choose a specific exit path and has no effect on how external ASes route traffic into your network.

Concept tested. BGP inbound traffic engineering with prefix specificity

Reference. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-16/irg-xe-16-book/bgp-inbound-traffic-engineering.html

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