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In router high availability, NSF (Non-Stop Forwarding) keeps packet forwarding going, NSR (Non-Stop Routing) preserves routing state on the device, and GR (Graceful Restart) coordinates a restart with neighboring routers. They address different parts of continuity and are often discussed alongside SSO (Stateful Switchover), the processor failover mode.
These acronyms have other meanings in other fields. This article uses their router high-availability meanings; exact behavior and support depend on the vendor, platform, software release, and routing protocol.
How the four mechanisms fit together
A useful way to distinguish them is to ask where the state needed for continuity lives and what is expected to keep working:
| Mechanism | Where continuity is maintained | What it is intended to preserve | Peer participation |
|---|---|---|---|
| SSO | State synchronized to a standby processor | Processor takeover after an active-processor failure | Not, by itself, a peer-facing restart procedure |
| NSF | Forwarding state, including the forwarding information base (FIB) | Packet forwarding while control-plane state is rebuilt or refreshed | Not the defining mechanism |
| GR | Routing information retained or managed across a restart with neighboring peers | Reduced routing disruption while a routing process or router restarts | Yes; peers must support and participate in the relevant protocol procedures |
| NSR | Routing and peering state checkpointed to a standby processor | Routing relationships through a local switchover, without depending on peer interaction in the way GR does | Does not rely on GR-style peer coordination |
The table describes the concepts, not a guarantee that a particular device implements them in the same way. Cisco’s 2007 presentation and a technical overview explain the architectural distinctions, but neither is a current feature matrix or configuration guide: Cisco’s SSO/NSF presentation and the SSO, NSF, NSR and GR overview.
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What each term means
SSO: Stateful Switchover
SSO is a redundant-processor failover mode. State is synchronized to a standby processor so it can take over if the active processor fails. In the Cisco presentation’s described SSO mode, both processors use identical software versions; treat that as a detail of that implementation and date, not a universal requirement.
NSF: Non-Stop Forwarding
NSF concerns the data plane: the router continues forwarding packets while control-plane state is restarted, rebuilt, or refreshed. The forwarding information must remain available during that interval. NSF can therefore complement a processor switchover, but it is not itself a promise that routing adjacencies or sessions will remain uninterrupted.
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GR: Graceful Restart
GR is a protocol mechanism involving neighboring routers. For BGP, IETF RFC 4724 specifies a capability and procedures intended to minimize routing disruption when a BGP session ends and restarts. Peers can retain routing information temporarily; an End-of-RIB marker signals completion of the restarting speaker’s initial route exchange.
Route retention is temporary, not proof that the forwarding path is still usable. GR can help bridge a recoverable restart when forwarding remains valid, but it cannot make an unavailable router or path carry traffic.
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NSR: Non-Stop Routing
NSR checkpoints routing-process and peering state to a standby processor inside the device. The goal is to keep routing relationships through a local switchover without asking neighboring routers to handle the event through GR. Cisco describes this as an “in box” approach; checkpointing routing state adds workload beyond maintaining forwarding state.
Why GR can preserve routes toward a failed router
GR depends on distinguishing a recoverable restart from a failure that has made forwarding impossible. During the restart window, a peer may continue using stale routes. If the router or path is actually down, traffic sent along those routes can be blackholed until stale information is withdrawn or the relevant recovery procedure is abandoned.
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RFC 4724’s route-retention procedures and End-of-RIB marker are designed to reduce disruption and indicate convergence; they do not guarantee traffic continuity when the forwarding device or path is unavailable. See the RFC 4724 specification for the BGP mechanism.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to choose what to investigate
There is no universal choice between GR and NSR, and their availability or interaction varies by implementation. Before relying on one, check the exact protocol, release, platform, topology, and failure mode.
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- For forwarding continuity: determine whether the platform supports NSF and what forwarding state remains available during control-plane recovery.
- For a local processor switchover: check whether SSO and NSR are supported together for the protocol and whether routing and peering state is synchronized as required.
- For peer-assisted restart: verify that each relevant neighbor supports and advertises the needed GR capability, and learn how the implementation handles stale routes and restart completion.
- For mixed deployments: check session-by-session behavior. Cisco’s 2007 presentation says some implementations permit GR and NSR for the same protocol, while a given session in its described arrangement uses either GR or NSR; do not generalize that rule to every vendor or release.
- For operational confidence: test the failure cases that matter in the actual network. Cisco’s presentation notes that time to first packet depends on configuration and platform, so a feature label alone cannot predict interruption.
What not to assume
- NSF, NSR, and GR are not interchangeable names for one feature: they concern forwarding state, local routing state, and peer-coordinated restart behavior, respectively.
- Enabling graceful restart does not guarantee uninterrupted traffic or prove that a route retained by a peer remains usable.
- There is no universal GR timer or command syntax established here. Cisco’s 2007 presentation discusses a 120-second BGP restart timer in its own context; that historical example is not a current or universal default.
- Support claims and configuration guidance must be checked against the current official documentation for the specific vendor, model, software release, and protocol.
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