SSO and NSF Considerations for Network Redundancy

350-401 By islever Editorial Last updated

Layer 2 Layer 3

Short answer

AC SSO requires synchronization between supervisors to guarantee continuous connectivity, and must be combined with NSF to support uninterrupted Layer 3 operations.

Question

What are two considerations when using SSO as a network redundancy feature? (Choose two.)
  1. requires synchronization between supervisors in order to guarantee continuous connectivity Correct Answer
  2. the multicast state is preserved during switchover
  3. must be combined with NSF to support uninterrupted Layer 3 operations Correct Answer
  4. both supervisors must be configured separately
  5. must be combined with NSF to support uninterrupted Layer 2 operations

Key Takeaways

The main trap is confusing the layers handled by each feature: SSO handles Layer 2 state preservation, while NSF is the specific add-on needed for Layer 3 routing continuity.

Learn why Stateful Switchover (SSO) synchronizes supervisors for Layer 2 continuity and why Nonstop Forwarding (NSF) is required for uninterrupted Layer 3 operations in Cisco Catalyst platforms.

Common mistake: Candidates often select E, believing NSF is needed for all redundancy. However, NSF is strictly for Layer 3; SSO alone suffices for Layer 2, making E incorrect and C the necessary partner for IP services.

Expert Analysis

Stateful Switchover (SSO) is a high-availability mechanism designed primarily for Layer 2 continuity. It ensures that the standby supervisor maintains an exact copy of the active supervisor’s state, including MAC address tables and spanning-tree topology. Because Layer 2 forwarding relies on these local tables rather than dynamic routing protocols, SSO alone can maintain uninterrupted Layer 2 traffic during a switchover event. This directly validates option A, as synchronization between supervisors is the fundamental requirement for SSO to function.

However, Layer 3 operations rely on dynamic routing protocols (such as OSPF or EIGRP), which require adjacency reconvergence after a supervisor change. SSO does not inherently preserve routing protocol adjacencies or the FIB/CEF tables in a way that allows immediate IP packet forwarding without interruption. To support uninterrupted Layer 3 operations, SSO must be combined with Nonstop Forwarding (NSF). NSF works alongside SSO to allow the data plane to continue forwarding packets while the control plane (routing protocols) rebuilds its neighbor relationships. Therefore, option C is correct because NSF is specifically required to bridge the gap for Layer 3 continuity. Option E is incorrect because SSO handles Layer 2 natively; combining it with NSF is not a prerequisite for Layer 2 integrity. Options B and D are factually inaccurate regarding multicast preservation specifics and configuration requirements respectively.

Official Reference

These links open the official vendor documentation in a new tab.

Exam Strategy

When analyzing redundancy features, always map them to OSI layers. Ask yourself: 'Does this feature handle table synchronization (Layer 2) or route adjacency maintenance (Layer 3)?' If the question asks about uninterrupted IP forwarding, look for NSF paired with SSO. If it asks about MAC/STP states, SSO alone is usually sufficient. Memorize that NSF is the 'Layer 3 enabler' for SSO.

Frequently Asked Questions

Does SSO preserve multicast state?

While SSO synchronizes critical state information, the primary design goal is Layer 2 continuity. Multicast state preservation is often tied to how NSF interacts with the routing process, but SSO's core definition focuses on supervisor state sync.

Why isn't NSF needed for Layer 2?

Layer 2 forwarding uses static MAC tables and STP states. SSO replicates these exactly. Layer 3 requires dynamic routing adjacencies, which take time to rebuild; NSF bridges this gap.

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Discussion 26 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 26

  1. Dudu84 👍 7
    I agree with A+C NSF Operation Cisco NSF always runs with SSO and provides redundancy for Layer 3 traffic. NSF works with SSO to minimize the amount of time that a network is unavailable to its users following a switchover. The main purpose of NSF is to continue forwarding IP packets following a supervisor engine switchover. https://www.cisco.com/en/US/docs/general/Test/dwerblo/broken_guide/nsfsso.html#wp1023627
  2. HungarianDish_111 👍 4
    https://www.cisco.com/en/US/docs/general/Test/dwerblo/broken_guide/nsfsso.html "NSF Operation Cisco NSF always runs with SSO and provides redundancy for Layer 3 traffic."
  3. djedeen 👍 3
    SSO:This type of switchover ensures that Layer 2 traffic is not interrupted NSF: Cisco NSF always runs with SSO and provides redundancy for Layer 3 traffic. NSF works with SSO to minimize the amount of time that a network is unavailable to its users following a switchover. The main purpose of NSF is to continue forwarding IP packets following a supervisor engine switchover.
  4. ihateciscoreally 👍 3
    in reality it is other way around. SSO uses NSF by default and it used for uninterrupted Layer 3 forwarding (SSO guarantees uninterrupted Layer 2 forwarding). you will use NSR to Layer 3 forwarding be operational, because NSF kind of "enables" Layer 3 forwarding, but without NSR it won't work at all.
  5. rogue_user 👍 3
    NSF = Non-stop FORWARDING which means Layer 2. NSR = Non-stop ROUTING.
Show 21 more comments
  1. gcata 👍 3
    AC NSF allow forwarding during route recalculations
  2. diegodavid82 👍 3
    Provided answer is correct
  3. Dryra1n 👍 3
    AC is correct
  4. SheldonC 👍 2
    A & C - From Cisco: [...] SSO also synchronizes critical state information between the RPs so that network state information is dynamically maintained between RPs. <<<<<<<<<< A) IS CORRECT >>>>>>>>>> SSO is generally used with Cisco nonstop forwarding (NSF). Cisco NSF enables forwarding of data packets to continue along known routes while the routing protocol information is being restored following a switchover. With NSF, users are less likely to experience service outages. <<<<<<<<<< C IS CORRECT >>>>>>>>>> SOURCE: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipapp_fhrp/configuration/15-sy/fhp-15-sy-book/fhp-hsrp-sso.pdf
  5. AlbertoStu 👍 2
    Cisco Nonstop Forwarding (NSF) works with the Stateful Switchover (SSO) feature to minimize the amount of time a network is unavailable to users following a switchover. The main objective of NSF SSO is to continue forwarding IP packets following a Route Processor (RP) switchover https://www.cisco.com/c/dam/en/us/td/docs/switches/lan/catalyst9400/software/release/16-6/configuration_guide/stck_mgr_ha/b_166_nsf_sso_9400_cg.html#:~:text=Cisco%20Nonstop%20Forwarding%20(NSF)%20works,Route%20Processor%20(RP)%20switchover.
  6. McBeano 👍 2
    AC - NSF is layer 3 NSF Operation Cisco NSF always runs with SSO and provides redundancy for Layer 3 traffic. NSF works with SSO to minimize the amount of time that a network is unavailable to its users following a switchover. The main purpose of NSF is to continue forwarding IP packets following a supervisor engine switchover.
  7. Asymptote 👍 2
    AC While SSO is happening, routing protocol communication between 2 supervisor stopped because lost of adjacency. It and cause interuption, to avoid this we need NFS. in order for NFS to work properly, both supervisor must NFS.
  8. Rumbrum 👍 2
    NSF only works for L2 uninterrupted failover
  9. kewokil120 👍 2
    just google is non stop forwarding layer 3
  10. ciscolessons 👍 2
    voting. provided answer is correct.
  11. Eddgar0 👍 2
    The provided answer is correct
  12. cloud29 👍 2
    A and C are correct
  13. cerf 👍 2
    AC are the correct
  14. techriese 👍 2
    A + C are correct
  15. Glaudus50 👍 2
    i agree with A+C
  16. Wooker 👍 2
    AC is correct.
  17. Kipyegon 👍 1
    A. Requires synchronization between supervisors to guarantee continuous connectivity. — Stateful Switchover (SSO) synchronizes state information between the active and standby supervisors so the standby can take over if the active supervisor fails. C. Must be combined with NSF to support uninterrupted Layer 3 operations. — Nonstop Forwarding (NSF) works with SSO to maintain Layer 3 packet forwarding during a supervisor switchover while routing protocols re-establish their control-plane adjacencies.
  18. Ayman_B 👍 1
    To support uninterrupted Layer 2 operations in SSO , you have to use additional technologies like (VRRP) or (HSRP) can help to ensure that there is always a active router available to forward packets at the Layer 2 level
  19. andy_doesnt_like_uucp 👍 1
    if A & C , then why not & E as well? A+B imo
  20. DarekD 👍 1
    A&C answer are cirrect
  21. LanreDipeolu 👍 1
    I vote for A+C

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