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352-001 · Question #706

352-001 Question #706: Real Exam Question with Answer & Explanation

The correct answer is C: Implement a delay between successive IS-IS LSP packet transmissions on routers with long-haul. IS-IS LSP pacing - controlling the delay between successive LSP transmissions on routers with flapping long-haul links - reduces CPU load during burst flapping events without delaying the initial LSP generation that drives fast convergence.

Question

A large service provider network has a single-level IS-IS network with 500 routers. This network has short- haul and long-haul links. Periodically, long-haul links bounce for a short period of time, causing 10 to 20 flaps in a few minutes. The probable cause is local road construction. Although fast convergence is important, a design engineer is concerned about taxing CPU cycles on the older routers. Which solution protects the CPU of the older routers during the short periods of excessive flapping, yet has no impact on fast convergence for all interface failures?

Options

  • AModify the length of time that an LSP remains in the router database without being refreshed on
  • BModify hello timers on routers with short-haul links
  • CImplement a delay between successive IS-IS LSP packet transmissions on routers with long-haul
  • DImplement LSP generation throttling on routers with long-haul links

Explanation

IS-IS LSP pacing - controlling the delay between successive LSP transmissions on routers with flapping long-haul links - reduces CPU load during burst flapping events without delaying the initial LSP generation that drives fast convergence.

Common mistakes.

  • A. Modifying the LSP lifetime controls how long LSPs remain in the database before requiring a refresh, which has no direct effect on CPU load during rapid link flapping events.
  • B. Modifying hello timers on short-haul link routers changes neighbor adjacency detection speed on those links and does not address the CPU overload caused by flapping long-haul links elsewhere.
  • D. LSP generation throttling delays how quickly a new LSP is created after a topology change, which directly slows down initial convergence for all interface failures and violates the requirement of no impact on fast convergence.

Concept tested. IS-IS LSP pacing to protect CPU during link flapping

Reference. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_isis/configuration/xe-16/iri-xe-16-book/iri-lsp-gen-throttle.html

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