Nonstop Forwarding with Stateful Switchover (NSF/SSO) are redundancy mechanisms for intra-chassis route processor failover.
SSO synchronizes Layer 2 protocol state, hardware L2/L3 tables (MAC, FIB, adjacency table), configuration, ACL and QoS tables.
NSF gracefully restarts routing protocol neighbor relationships after an SSO fail-over
1. Newly active redundant route processor continues forwarding traffic using synchronized HW forwarding tables.
2. NSF capable routing protocol (eg: OSPF) requests graceful neighbor restart.Routing neighbors reform with no traffic loss.
3. Cisco and RFC3623 as per standard.
4. Graceful Restart capability must always be enabled for all protocols. This is only necessary on routers with dual processors that will be performing switch overs.
5. Graceful Restart awareness is on by default for non-TCP based interior routing protocols (OSPF,ISIS and EIGRP). These protocols will start operating in GR mode as soon as one side is configured.
6. TCP based protocols (BGP) must enable GR on both sides of the session and the session must be reset to enable GR. The information enabling GR is sent in the Open message for these protocols.
Nonstop Routing (NSR) is a stateful redundancy mechanism for intra chassis route processor (RP) failover.
NSR , unlike NSF with SSO, 1. Allows routing process on active RP to synchronize all necessary data and states with routing protocol process on standy RP.
2. When switchover occurs, routing process on newly active RP has all necessary data and states to continue running without requiring any help from its neighbor(s).
3. When switchover occurs, routing process on newly active RP has all necessary data and states to continue running without requiring any help from its neighbor(s).
4. Standards are not necessary as NSR does NOT require additional communication with protocol peers
5. NSR is desirable in cases where routing protocol peer doesn’t support Cisco or IETF standards to support Graceful Restart capabilities exchange.
6. NSR uses more system resources due to information transfer to standby processor.
Deployment Considerations for SSO/NSF and NSR
1. From a high level, you need to protect the interfaces (SSO), the forwarding plane (NSF) and the control plane (GR or NSR).
2. Enabling SSO also enables NSF.
3. Each routing protocol peer needs to be examined to ensure that both its capability has been enabled and that its peer has awareness enabled.
4. While configuring OSPF with NSF, make sure all the peer devices that participate in OSPF must be made OSPF NSF-aware.
5. While configuring OSPF with Non Stop Routing (NSR), it doesn’t require peer devices be NSR capable. It only requires more system resources. Both NSF and NSR could be active at same time but NSF is used as fallback.
Read More About OSPF Design Consideration
Design Considerations:OSPF Network Stability
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OSPF with Nonstop Forwarding
router ospf 1
nsf [cisco | ietf]
OSPF with Nonstop Routing
nsr process-failures switchover
router ospf 1