IP Routing: OSPF Configuration Guide, Cisco IOS Release 15S
OSPF Shortest Path First Throttling
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OSPF Shortest Path First Throttling

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

Feature History

Release

Modification

12.2(14)S

This feature was introduced.

12.0(23)S

This feature was integrated into Cisco Release 12.0(23)S.

12.2(15)T

This feature was integrated into Cisco IOS Release 12.2(15)T.

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Information About OSPF SPF Throttling

Shortest Path First Calculations

SPF calculations occur at the interval set by the timers throttle spfcommand. 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 spfcommand. 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 Topology Change Event

How to Configure OSPF SPF Throttling

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


DETAILED STEPS
     Command or ActionPurpose
    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 interface type slot / port


    Example:
    Router(config)# interface ethernet 1/1/1
     

    Enters interface configuration mode for the interface specified.

     
    Step 4 ip address ip-address mask [secondary]


    Example:
    Router(config-if)# ip address 192.168.0.2 255.255.255.0
     

    Sets a primary or secondary IP address for an interface.

     
    Step 5 exit


    Example:
    router# exit
     

    Exits interface configuration mode.

     
    Step 6 router ospf process-id


    Example:
    Router(config)# router ospf 1
     

    Configures an OSPF routing process.

     
    Step 7 network network-number [mask | prefix-length]


    Example:
    Router(config-router)# network 192.168.0.0 0.0.255.255 area 0
     

    Configures the subnet number and mask for a Dynamic Host Configuration Protocol (DHCP) address pool on a Cisco IOS DHCP Server.

     
    Step 8 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 9 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
    

    The table below describes the show ip ospf display fields and their descriptions.

    Table 1 show ip ospf Field Descriptions

    Field

    Description

    Routing process "ospf 201" with ID 192.42.110.200

    Process ID and OSPF router ID.

    Supports ...

    Number of types of service supported (Type 0 only).

    It is ...

    Possible types are internal, area border, or autonomous system boundary.

    Summary Link update interval

    Specifies summary update interval in hours:minutes:seconds, and time until next update.

    External Link update interval

    Specifies external update interval in hours:minutes:seconds, and time until next update.

    Redistributing External Routes from

    Lists of redistributed routes, by protocol.

    SPF calculations

    Lists start, hold, and maximum wait interval values in milliseconds.

    Number of areas

    Number of areas in router, area addresses, and so on.

    SPF algorithm last executed

    Shows the last time an SPF calculation was performed in response to topology change event records.

    Link State Update Interval

    Specifies router and network link-state update interval in hours:minutes:seconds, and time until next update.

    Link State Age Interval

    Specifies max-aged update deletion interval, and time until next database cleanup, in hours:minutes:seconds.

    Configuration Examples for OSPF SPF Throttling

    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 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

    For additional information related to OSPF, refer to the following references:

    Related Documents

    Related Topic

    Document Title

    OSPF commands

    Cisco IOS IP Routing: OSPF Command Reference

    OSPF configuration tasks

    "Configuring OSPF" module in the Cisco IOS IP Routing Protocols Configuration Guide

    OSPFv3 Fast Convergence: LSA and SPF Throttling

    OSPFv3 Fast Convergence: LSA and SPF Throttling” module

    Standards

    Standards

    Title

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

    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 releases, and feature sets, use Cisco MIB Locator found at the following URL:

    http:/​/​www.cisco.com/​go/​mibs

    RFCs

    RFCs

    Title

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

    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