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352-001 · Question #144

A senior network designer suggests that you should improve network convergence times by reducing BGP timers between your CE router and the PE router of the service provider. Which three factors…

The correct answer is A. access bandwidth B. service provider agreement to support tuned timers C. number of routes on the CE router. When tuning BGP keepalive and hold timers on a CE-PE link, the available bandwidth, service provider cooperation, and CE routing table size are the primary technical constraints that determine safe timer values.

Layer 3 Control Plane

Question

A senior network designer suggests that you should improve network convergence times by reducing BGP timers between your CE router and the PE router of the service provider. Which three factors should you to consider when you adjust the timer values? (Choose three.)

Options

  • Aaccess bandwidth
  • Bservice provider agreement to support tuned timers
  • Cnumber of routes on the CE router
  • Dnumber of VRFs on the PE router
  • Eservice provider scheduling of changes to the PE

How the community answered

(20 responses)
  • A
    70% (14)
  • D
    10% (2)
  • E
    20% (4)

Why each option

When tuning BGP keepalive and hold timers on a CE-PE link, the available bandwidth, service provider cooperation, and CE routing table size are the primary technical constraints that determine safe timer values.

Aaccess bandwidthCorrect

Low access bandwidth can delay or drop BGP keepalive packets, causing false session timeouts, so the timer floor must account for worst-case transmission delay on that link.

Bservice provider agreement to support tuned timersCorrect

The PE router belongs to the service provider, so the SP must agree to and configure matching timer values - mismatched timers default to the higher value, negating the optimization.

Cnumber of routes on the CE routerCorrect

A larger CE routing table means UPDATE processing and route installation take longer during reconvergence, so timers must be generous enough to avoid premature session teardown during heavy BGP processing.

Dnumber of VRFs on the PE router

The number of VRFs on the PE affects overall PE resource utilization but is not a direct input into calculating appropriate BGP timer values for a specific CE-PE session.

Eservice provider scheduling of changes to the PE

SP scheduling of changes is an operational change-management concern, not a technical factor that influences what timer values are technically appropriate for the BGP session.

Concept tested: BGP timer tuning factors for CE-PE convergence

Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-16/irg-xe-16-book/bgp-tuning-and-scalability.html

Topics

#BGP timers#CE-PE design#convergence tuning#timer adjustment

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