Table Of Contents
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.
Feature Specifications for OSPF Shortest Path First Throttling
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Information About OSPF SPF Throttling
To use SPF throttling, you should understand the following concepts:
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
How to Configure OSPF SPF Throttling
Perform the following tasks to configure OSPF SPF throttling:
•Configuring OSPF SPF Throttling (required)
•Verifying SPF Throttle Values (optional)
Configuring OSPF SPF Throttling
2. configure terminal
3. interface type slot/port
4. ip address ip-address mask [secondary]
6. router ospf process-id
7. network network-number [mask | prefix-length]
8. timers throttle spf spf-start spf-hold spf-max-wait
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 ospfRouting Process "ospf 1" with ID 10.10.10.2 and Domain ID 0.0.0.1Supports only single TOS(TOS0) routesSupports opaque LSAIt is an autonomous system boundary routerRedistributing External Routes from,static, includes subnets in redistributionInitial SPF schedule delay 5 msecsMinimum hold time between two consecutive SPFs 1000 msecsMaximum wait time between two consecutive SPFs 90000 msecsMinimum LSA interval 5 secs. Minimum LSA arrival 1 secsLSA group pacing timer 240 secsInterface flood pacing timer 33 msecsRetransmission pacing timer 66 msecsNumber of external LSA 4. Checksum Sum 0x17445Number of opaque AS LSA 0. Checksum Sum 0x0Number of DCbitless external and opaque AS LSA 0Number of DoNotAge external and opaque AS LSA 0Number of areas in this router is 1. 1 normal 0 stub 0 nssaExternal flood list length 0Area BACKBONE(0)Number of interfaces in this area is 2Area has no authenticationSPF algorithm last executed 19:11:15.140 agoSPF algorithm executed 28 timesArea ranges areNumber of LSA 4. Checksum Sum 0x2C1D4Number of opaque link LSA 0. Checksum Sum 0x0Number of DCbitless LSA 0Number of indication LSA 0Number of DoNotAge LSA 0Flood list length 0
Table 1 describes the show ip ospf display fields and their descriptions.
Configuration Examples for OSPF SPF Throttling
This section contains the following examples:
Throttle Timers Example
This example shows a router configured with the start, hold, and maximum interval values for the timers throttle spf command set at 5, 1,000, and 90,000 milliseconds, respectively.router ospf 1router-id 10.10.10.2log-adjacency-changestimers throttle spf 5 1000 90000redistribute static subnetsnetwork 126.96.36.199 0.0.0.255 area 0network 188.8.131.52 0.0.0.255 area 00
For additional information related to OSPF, refer to the following references:
No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature.
MIBs MIBs Link
To obtain lists of supported MIBs by platform and Cisco IOS release, and to download MIB modules, go to the Cisco MIB website on Cisco.com at the following URL:
No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.
This section documents the new timers throttle spf command. All other commands used with this feature are documented in the Cisco IOS Release 12.2T command reference publications.
Obsolete and Replaced Commands
Table 2 lists commands that have been replaced since Cisco IOS Release 12.2.
Table 2 Replaced OSPF Commands
Command in Cisco IOS Release 12.2 Replacement Command in Cisco IOS Release 12.2(11)S
timers throttle spf
timers throttle spf
To turn on OSPF SPF throttling, use the timers throttle spf command in router configuration mode. To turn off SPF throttling, use the no form of this command.
timers throttle spf spf-start spf-hold spf-max-wait
no timers throttle spf spf-start spf-hold spf-max-wait
SPF throttling is not set.
The timers throttle spf command replaces the timers spf-interval command.
The first wait interval between SPF calculations is the amount of time in milliseconds specified by the spf-start argument. Each consecutive wait interval is two times the current hold level in milliseconds until the wait time reaches the maximum time in milliseconds as specified by the spf-maximum argument. Subsequent wait times remain at the maximum until the values are reset or an LSA is received between SPF calculations.
The following example shows a router configured with the start, hold, and maximum interval values for the timers throttle spf command set at 5, 1,000, and 90,000 milliseconds, respectively.router ospf 1router-id 10.10.10.2log-adjacency-changestimers throttle spf 5 1000 90000redistribute static subnetsnetwork 184.108.40.206 0.0.0.255 area 0network 220.127.116.11 0.0.0.255 area 00
This command is replace by the timers throttle spf command. Refer to the timers throttle spf command reference page for instructions on using the new command.