OSPF Shortest Path First Throttling
The OSPF Shortest Path First Throttling feature makes it possible to configure SPF scheduling in millisecond intervals and to potentially delay shortest path first (SPF) calculations during network instability. SPF is scheduled to calculate the Shortest Path Tree (SPT) when there is a change in topology. One SPF run may include multiple topology change events.
The interval at which the SPF calculations occur is chosen dynamically and is 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.
Shortest Path First Calculations
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 one until the wait 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), the wait interval at 1000 milliseconds, and the maximum wait time is set at 90,000 milliseconds.
timers throttle spf 5 1000 90000
Figure 1 shows the intervals at which the SPF calculations occur so long as at least one topology change event is received in a given wait interval.
Figure 1 SPF Calculation Intervals Set by the timers throttle 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. Once the maximum wait time is reached, the wait interval remains the same until the topology stablizes 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 the parameters specified in the timers throttle spf command. Notice in Figure 2 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 2 Timer Intervals Reset after Topology Change Event
Configuring OSPF SPF Throttling
SUMMARY STEPS
1. enable
2. configure terminal
3. interface type slot/port
4. ip address ip-address mask [secondary]
5. exit
6. router ospf process-id
7. network network-number [mask prefix-length]
8. timers throttle spf spf-start spf-hold spf-max-wait
9. end
Verifying SPF Throttle Values
To verify SPF throttle timer values, use the show ip ospf command. The values are displayed in the lines that begin, "Initial SPF schedule delay...," "Minimum hold time between two consecutive SPFs...," and "Maximum wait time between two consecutive SPFs...."
Router# show ip ospf
Routing Process "ospf 1" with ID 10.10.10.2 and Domain ID 0.0.0.1
Supports only single TOS(TOS0) routes
Supports opaque LSA
It is an autonomous system boundary router
Redistributing External Routes from,
static, includes subnets in redistribution
Initial SPF schedule delay 5 msecs
Minimum hold time between two consecutive SPFs 1000 msecs
Maximum wait time between two consecutive SPFs 90000 msecs
Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
LSA group pacing timer 240 secs
Interface flood pacing timer 33 msecs
Retransmission pacing timer 66 msecs
Number of external LSA 4. Checksum Sum 0x17445
Number of opaque AS LSA 0. Checksum Sum 0x0
Number of DCbitless external and opaque AS LSA 0
Number of DoNotAge external and opaque AS LSA 0
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
External flood list length 0
Area BACKBONE(0)
Number of interfaces in this area is 2
Area has no authentication
SPF algorithm last executed 19:11:15.140 ago
SPF algorithm executed 28 times
Area ranges are
Number of LSA 4. Checksum Sum 0x2C1D4
Number of opaque link LSA 0. Checksum Sum 0x0
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
The interval at which the SPF calculations occur is chosen dynamically and is 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.
Shortest Path First Calculations
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 one until the wait 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), the wait interval at 1000 milliseconds, and the maximum wait time is set at 90,000 milliseconds.
timers throttle spf 5 1000 90000
Figure 1 shows the intervals at which the SPF calculations occur so long as at least one topology change event is received in a given wait interval.
Figure 1 SPF Calculation Intervals Set by the timers throttle 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. Once the maximum wait time is reached, the wait interval remains the same until the topology stablizes 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 the parameters specified in the timers throttle spf command. Notice in Figure 2 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 2 Timer Intervals Reset after Topology Change Event
Configuring OSPF SPF Throttling
SUMMARY STEPS
1. enable
2. configure terminal
3. interface type slot/port
4. ip address ip-address mask [secondary]
5. exit
6. router ospf process-id
7. network network-number [mask prefix-length]
8. timers throttle spf spf-start spf-hold spf-max-wait
9. end
Verifying SPF Throttle Values
To verify SPF throttle timer values, use the show ip ospf command. The values are displayed in the lines that begin, "Initial SPF schedule delay...," "Minimum hold time between two consecutive SPFs...," and "Maximum wait time between two consecutive SPFs...."
Router# show ip ospf
Routing Process "ospf 1" with ID 10.10.10.2 and Domain ID 0.0.0.1
Supports only single TOS(TOS0) routes
Supports opaque LSA
It is an autonomous system boundary router
Redistributing External Routes from,
static, includes subnets in redistribution
Initial SPF schedule delay 5 msecs
Minimum hold time between two consecutive SPFs 1000 msecs
Maximum wait time between two consecutive SPFs 90000 msecs
Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
LSA group pacing timer 240 secs
Interface flood pacing timer 33 msecs
Retransmission pacing timer 66 msecs
Number of external LSA 4. Checksum Sum 0x17445
Number of opaque AS LSA 0. Checksum Sum 0x0
Number of DCbitless external and opaque AS LSA 0
Number of DoNotAge external and opaque AS LSA 0
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
External flood list length 0
Area BACKBONE(0)
Number of interfaces in this area is 2
Area has no authentication
SPF algorithm last executed 19:11:15.140 ago
SPF algorithm executed 28 times
Area ranges are
Number of LSA 4. Checksum Sum 0x2C1D4
Number of opaque link LSA 0. Checksum Sum 0x0
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
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