350-501 Question #295: Real Exam Question with Answer & Explanation

Question

Refer to the exhibit. Routers P4 and P5 receive the 0.0.0.0/0 route from the ISP via eBGP peering. P4 is the primary Internet gateway router, and P5 is its backup. P5 is already advertising a default route into the OSPF domain. Which configuration must be applied to P4 so that it advertises a default route into OSPF and becomes the primary Internet gateway for the network?

Options

• Aconfigure terminal
• Bconfigure terminal
• Cconfigure terminal
• Dconfigure terminal

Explanation

To make P4 the primary internet gateway, it must advertise a default route into OSPF with a better metric than P5, using default-information originate since it already receives a default route via eBGP, and include any necessary area summarization.

Common mistakes.

• B. The always keyword would cause P4 to advertise a default route even if it loses its eBGP-learned default route, potentially leading to traffic blackholing.
• C. Similar to B, the always keyword is inappropriate for a router that relies on an external default route and needs to stop advertising it if the external route disappears.
• D. While default-information originate with the correct metric is present, this option omits the area 0 range ... not-advertise command which is included in the correct answer, implying it's part of the required configuration.

Concept tested. OSPF default route origination and metric manipulation

Reference. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/15-mt/iro-ospf-15-mt-book/iro-ospf-dflt-route.html

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