Brocade Multi-Service IronWare Routing Configuration Guide (Supporting R05.6.00) - Manual (Page 221)

Multi-Service IronWare Routing Configuration Guide 193
53-1003033-02
OSPF non-stop routing
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
NetIron XMR and Brocade MLX series devices. 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 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.
TABLE 20 Set Conditions
Set Conditions:
metric metricValue
metric-type type1/type2
tag routeTagValue