Nonstop Forwarding For Ospf Version 2 - Cisco ASR 9000 Series Configuration Manual

Implementing OSPF

Nonstop Forwarding for OSPF Version 2

SPF calculations occur at the interval set by the timers throttle spf command. The wait interval indicates the
amount of time to wait until the next SPF calculation occurs. Each wait interval after that calculation is twice
as long as the previous interval until the interval reaches the maximum wait time specified.
The SPF timing can be better explained using an example. In this example, the start interval is set at
5 milliseconds (ms), initial wait interval at 1000 ms, and maximum wait time at 90,000 ms.
timers spf 5 1000 90000
This figure shows the intervals at which the SPF calculations occur as long as at least one topology change
event is received in a given wait interval.
Figure 20: SPF Calculation Intervals Set by the timers spf Command
Notice that the wait interval between SPF calculations doubles when at least one topology change event is
received during the previous wait interval. After the maximum wait time is reached, the wait interval remains
the same until the topology stabilizes and no event is received in that interval.
If the first topology change event is received after the current wait interval, the SPF calculation is delayed by
the amount of time specified as the start interval. The subsequent wait intervals continue to follow the dynamic
pattern.
If the first topology change event occurs after the maximum wait interval begins, the SPF calculation is again
scheduled at the start interval and subsequent wait intervals are reset according to the parameters specified in
the timers throttle spf command. Notice in Figure 21: Timer Intervals Reset After Topology Change Event,
on page 348that a topology change event was received after the start of the maximum wait time interval and
that the SPF intervals have been reset.
Figure 21: Timer Intervals Reset After Topology Change Event
Nonstop Forwarding for OSPF Version 2
Cisco IOS XR Software NSF for OSPF Version 2 allows for the forwarding of data packets to continue along
known routes while the routing protocol information is being restored following a failover. With NSF, peer
networking devices do not experience routing flaps. During failover, data traffic is forwarded through intelligent
line cards while the standby Route Processor (RP) assumes control from the failed RP. The ability of line
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
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