Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 280)

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 reason, the NSR-capable router waits for LSA synchronization of the standby module to complete
(Sync-Ack) and then acknowledges the neighbor that sent the LSA.
LSA syncing and packing
When the LSA processing is completed on the active management module and the decision is made
to install the LSA in its link state database (LSDB), OSPF synchronizes that LSA to the standby
module. OSPF checks the current state of the database entry whether or not it is marked for deletion.
After checking the database state, OSPF packs the LSA status and other necessary information
needed for direct installation in the standby OSPF LSDB along with the LSA portion. When the LSA
reaches the standby module, OSPF checks the database entry state in the buffer and takes
appropriate action, such as adding, overwriting, updating, or deleting the LSA from the LSDB.
Neighbor router synchronization
When the neighbor router is added in the active management module, it is synchronized and added to
the standby module. When the neighbor is deleted in the active module, it is synchronized to the
standby and deleted in the standby. When the neighbor router state becomes 2WAY or FULL, the
neighbor router is synchronized to the standby module.The following attributes of the neighbor router
is synchronized to the standby module:
• Neighbor router id
• Neighbor router ip address
• Destination router or backup destination router information
• Neighbor state 2WAY or FULL
• MD5 information
• Neighbor priority
Synchronization of critical OSPF elements
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