Ospf Non-Stop Routing; Synchronization Of Critical Ospf Elements; Link State Database Synchronization - Brocade Communications Systems FastIron SX 800 Configuration Manual

OSPF non-stop routing

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OSPF non-stop routing

The graceful restart feature supported by open shortest path first (OSPF) maintains area topology and dataflow. Though the network
requires neighboring routers to support graceful restart and perform hitless failover, the graceful restart feature may not be supported by
all routers in the network. To eliminate this dependency, the non-stop routing (NSR) feature is supported on Brocade devices. NSR does
not require support from neighboring routers to perform hitless failover. NSR does not support virtual link, so traffic loss is expected
while performing hitless failover.
NSR does not require support from neighboring
routers to perform hitless failover.
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.

Synchronization of critical OSPF 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

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 to ensure non-stop routing. The next shortest path first (SPF) run after
switchover yields the same result in routes as the active module had before the failure and OSPF protocol requires that all routers in the
network to have identical databases.
LSA delayed acknowledging
When an OSPF router receives LSAs from its neighbor, it acknowledges the LSAs. After the acknowledgement is received, the neighbor
removes this router from its retransmission list and stops resending the LSAs.
In the case of NSR, the router fails after receiving the LSA from its neighbor and has acknowledged that neighbor upon receipt of an
LSA, and the LSA synchronization to the standby module is 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
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FastIron Ethernet Switch Layer 3 Routing
53-1003627-04