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352-001 · Question #247

What is the best practice for tuning routing protocol hello and dead timers when deploying IGP non-stop forwarding (NSF)?

The correct answer is A. The hello and dead timers should be tuned to allow NSF to continue forwarding after an initial. When deploying IGP Non-Stop Forwarding, hello and dead timers must be set long enough to prevent neighbors from tearing down adjacencies before the NSF switchover completes.

Design Considerations

Question

What is the best practice for tuning routing protocol hello and dead timers when deploying IGP non-stop forwarding (NSF)?

Options

  • AThe hello and dead timers should be tuned to allow NSF to continue forwarding after an initial
  • BNSF-independent timers should be used so that routing protocol timers have no effect.
  • CThe hello and dead timers should be tuned so the link failure is detected before NSF has the
  • DThe routing protocol hello and dead timers should be decreased to the minimum.

How the community answered

(25 responses)
  • A
    76% (19)
  • B
    8% (2)
  • C
    12% (3)
  • D
    4% (1)

Why each option

When deploying IGP Non-Stop Forwarding, hello and dead timers must be set long enough to prevent neighbors from tearing down adjacencies before the NSF switchover completes.

AThe hello and dead timers should be tuned to allow NSF to continue forwarding after an initialCorrect

NSF relies on neighboring routers holding adjacencies during a route processor failover or routing process restart so that forwarding continues uninterrupted. Timers tuned to outlast the NSF convergence window prevent neighbors from declaring the device dead prematurely. Timers set too aggressively will cause adjacency teardown before NSF finishes, defeating its purpose.

BNSF-independent timers should be used so that routing protocol timers have no effect.

Routing protocol timers directly govern whether a neighbor tears down an adjacency during an NSF event - there are no timers fully decoupled from this relationship.

CThe hello and dead timers should be tuned so the link failure is detected before NSF has the

Tuning timers so that link failure is detected before NSF completes defeats the entire purpose of NSF, which requires adjacencies to be held up long enough for the forwarding state to be restored.

DThe routing protocol hello and dead timers should be decreased to the minimum.

Decreasing timers to the minimum would cause neighbors to expire adjacencies rapidly, preventing NSF from completing before the neighbor relationship is torn down.

Concept tested: NSF hello and dead timer tuning best practice

Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_eigrp/configuration/xe-16/ire-xe-16-book/ire-nsf-cisco.html

Topics

#NSF non-stop forwarding#hello timers#IGP graceful restart#routing protocol timers

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