OSPF Shortest Path First Throttling

The OSPF Shortest Path First Throttling feature makes it possible to configure shortest path first (SPF) scheduling in millisecond intervals and to potentially delay 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 the network topology is unstable, SPF throttling calculates SPF scheduling intervals to be longer until the topology becomes stable.

Information About OSPF SPF Throttling

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

The figure below 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 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 the parameters specified in the timers throttle spf command. Notice in the figure below 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 a Topology Change Event

How to Configure OSPF SPF Throttling

Configuring OSPF SPF Throttling

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. router ospf process-id
  4. timers throttle spf spf-start spf-hold spf-max-wait
  5. end

DETAILED STEPS

  Command or Action Purpose

Step 1

enable

Example:


Router> enable

Enables higher privilege levels, such as privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Router# configure terminal

Enters global configuration mode.

Step 3

router ospf process-id

Example:


Router(config)# router ospf 1

Configures an OSPF routing process.

Step 4

timers throttle spf spf-start spf-hold spf-max-wait

Example:


Router(config-router)# timers throttle spf 10 4800 90000

Sets OSPF throttling timers.

Step 5

end

Example:


Router(config-router)# end

Exits configuration mode.

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

Configuration Example for OSPF SPF Throttling

Example Throttle Timers

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 1
 router-id 10.10.10.2
 log-adjacency-changes
 timers throttle spf 5 1000 90000 
 redistribute static subnets
 network 21.21.21.0 0.0.0.255 area 0
 network 22.22.22.0 0.0.0.255 area 00

Additional References

The following sections provide references related to OSPF Shortest Path First Throttling.

Related Documents

Related Topic

Document Title

Configuring OSPF

"Configuring OSPF"

OSPF commands

Cisco IOS IP Routing: OSPF Command Reference

Cisco IOS master command list, all releases

Cisco IOS Master Command List, All Releases

Standards

Standards

Title

None

--

MIBs

MIBs

MIBs Link

No new or modified MIBs are supported by this feature, and support for existing MIBs has not been modified by this feature.

To locate and download MIBs for selected platforms, Cisco IOS XE releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs

RFCs

RFCs

Title

None

--

Technical Assistance

Description

Link

The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

http://www.cisco.com/cisco/web/support/index.html

Feature Information for OSPF Shortest Path First Throttling

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Table 1. Feature Information for OSPF Shortest Path First Throttling

Feature Name

Releases

Feature Information

OSPF Shortest Path First Throttling

Cisco IOS XE Release 2.1

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.

The following commands are introduced or modified in the feature documented in this module:

  • timer spf-interval

  • timers throttle spf

Table 2. Feature Information for OSPF Shortest Path First Throttling

Feature Name

Releases

Feature Information

OSPF Shortest Path First Throttling

Cisco IOS XE Release 17.4

This feature was introduced.