300-420 Question #119: Real Exam Question with Answer & Explanation

Question

A network engineer must segregate three interconnected campus networks using IS-IS routing. A two-layer hierarchy must be used to support large routing domains and to avoid more specific routes from each campus network being advertised to other campus network routers automatically. Which two actions does the engineer take to accomplish this segregation? (Choose two.)

Options

• ADesignate two IS-IS routers as BDR routers at the edge of each campus, and configure one BDR
• BDesignate two IS-IS routers from each campus to act as Level 1/Level 2 backbone routers at the
• CAssign the same IS-IS NET value for each campus, and configure internal campus routers with
• DUtilize different MTU values for each campus network segment. Level 2 backbone routers must
• EAssign a unique IS-IS NET value for each campus, and configure internal campus routers with

Explanation

To segregate three interconnected campus networks using a two-layer IS-IS hierarchy and prevent specific routes from being advertised across campuses, the engineer must configure unique IS-IS NET values for each campus and designate specific routers as Level 1/Level 2 routers.

Common mistakes.

• A. Designating BDR (Backup Designated Router) is an OSPF concept and not applicable to IS-IS for hierarchical segregation; IS-IS uses Designated IS (DIS).
• C. Assigning the same IS-IS NET value for each campus would place them in the same IS-IS area, which contradicts the goal of segregating the campus networks.
• D. Utilizing different MTU values for network segments primarily relates to fragmentation and path MTU discovery, not to the segregation of routing domains or controlled route advertisement in IS-IS.

Concept tested. IS-IS hierarchical routing and area segregation

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

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