300-420 Question #307: Real Exam Question with Answer & Explanation

Question

Refer to the exhibit. An engineer is designing a hierarchical ISIS solution for an enterprise customer with these requirements: - Users in areas 25 and 55 send and receive traffic from both backbone areas. - Link flaps in areas 35 and 45 must not impact other areas. - Routers will double within the next 12 months in areas 35 and 45. Which design must the engineer select?

Options

• AA series routers Level 2, B series routers Level 2, and C series routers Level 1
• BA series routers Level 1/2, B series routers Level 1/2, and C series routers Level 1/2
• CA series routers Level 1/2, B series routers Level 2, and C series routers Level 2
• DA series routers Level 1, B series routers Level 1/2, and C series routers Level 2

Explanation

The engineer should select a design where A series routers are Level 1, B series routers are Level 1/2, and C series routers are Level 2 to provide area isolation and a robust backbone.

Common mistakes.

• A. This configuration would place Level 2 routers in areas meant for isolation (35, 45) and Level 1 routers (C series) in what should be a backbone area (55), preventing proper inter-area routing and isolation.
• B. An all Level 1/2 design would propagate link flaps more broadly, failing to meet the requirement for isolating instability in areas 35 and 45 by not creating distinct Level 1 boundaries.
• C. Configuring B and C series as Level 2 means areas 35 and 45 would be part of the Level 2 backbone, which would not isolate link flaps and growth from other areas as required.

Concept tested. Hierarchical IS-IS design levels

Reference. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_isis/configuration/15-s/irs-15-s-book/irs-config-isis.html

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