350-501 Question #73: Real Exam Question with Answer & Explanation

Question

What is the main difference between the OSPF NSR and OSPF NSF mechanisms?

Options

• ANSR does not use as much memory resource as NSF does.
• BWith NSR, OSPF on the newly active standby RP initially has no state information.
• COn NSR, the neighbors must be NSR-aware.
• DWith NSR, the router that performs the switchover preserves the state internally.

Explanation

The main difference is that OSPF Nonstop Routing (NSR) preserves OSPF state internally during a switchover, unlike OSPF Nonstop Forwarding (NSF) which relies on neighbor assistance for state recovery.

Common mistakes.

• A. NSR typically uses more memory resources than NSF because it needs to maintain a synchronized copy of the entire control plane state on the standby RP.
• B. This statement is incorrect; with NSR, the newly active standby RP has complete state information because it has been synchronized with the active RP.
• C. NSR does not require neighbors to be NSR-aware; it is an internal mechanism, while NSF often requires neighbors to be NSF-aware for graceful restart signaling.

Concept tested. OSPF NSR vs. NSF differences

Reference. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/15-mt/iro-15-mt-book/iro-nsr-ospf.html

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