Brocade Communications Systems ICX 7250 series, Configuration Manual

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If the active management module fails, the standby management module takes over and maintains the current OSPF routes, link-state
advertisements (LSAs), and neighbor adjacencies, so that there is no loss of existing traffic to the OSPF destination.
Limitations of NSR
• Configurations that occur before the switchover are lost due to the CLI synchronization.
• NSR does not support virtual links.
• Changes in the neighbor state or interface state before or during a switchover do not take effect.
• Traffic counters are not synchronized because the neighbor and LSA database counters are recalculated on the standby module
during synchronization.
• LSA acknowledging is delayed because it has to wait until standby acknowledging occurs.
• Depending on the sequence of redistribution or new LSAs (from neighbors), the LSAs accepted within the limits of the database
may change after switchover.
• In NSR hitless failover, after switchover, additional flooding-related protocol traffic is generated to the directly connected
neighbors.
• OSPF startup timers, database overflow, and max-metric, are not applied during NSR switchover.
• Devices may generate OSPF log messages or reset OSPF neighbor timers, but these issues do not cause any OSPF or traffic
disruption.
Synchronization of critical OSPFv2 elements
All types of LSAs and the neighbor information are synchronized to the standby module using the NSR synchronization library and IPC
mechanism to transmit and receive packets.
Link state database synchronization
To ensure non-stop routing, when the active management module fails the standby management module takes over from the active
management module, with the identical OSPF link state database it had before the failure. The next shortest path first (SPF) run after the
switchover yields the same result in routes as the active module had before the failure. The OSPF protocol requires that all devices in the
network have identical databases.
LSA delayed acknowledging
When an OSPF device receives LSAs from its neighbor, it acknowledges the LSAs. After the acknowledgement is received, the neighbor
removes this device from its retransmission list and stops resending the LSAs.
In the case of NSR, the device fails after receiving the LSA from its neighbor and acknowledges that neighbor upon receipt of an LSA.
The LSA synchronization to the standby module is then completed. In this case the standby module, when taking over from the active
module, does not have that LSA in its database and the already acknowledged neighbor does not retransmit that LSA. For this reason,
the NSR-capable device waits for LSA synchronization of the standby module to complete (Sync-Ack) before acknowledging the
neighbor that sent the LSA.
Synchronization of critical OSPFv2 elements
Brocade FastIron Layer 3 Routing Configuration Guide
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