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Implementing OSPF on Cisco IOS XR Software
Information About Implementing OSPF on Cisco IOS XR Software
RC-140
Cisco IOS XR Routing Configuration Guide
The interval at which the SPF calculations occur is chosen dynamically and based on the frequency of
topology changes in the network. The chosen interval is within the boundary of the user-specified value
ranges. If network topology is unstable, SPF throttling calculates SPF scheduling intervals to be longer
until topology becomes stable.
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
Figure 8 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 8 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 9 that a topology change event was
received after the start of the maximum wait time interval and that the SPF intervals have been reset.
Figure 9 Timer Intervals Reset After Topology Change Event
Nonstop Forwarding for OSPF Version 2
Cisco IOS XR 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
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5 ms
1000 ms
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32000 ms
64000 ms
90000 ms
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64000 ms
90000 ms
Topology change event
SPF scheduled at
start interval
5 ms
1000 ms
2000 ms
4000 ms
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16000 ms
90000 ms