IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3850 Switches)
IP Multicast Optimization: Multicast Subsecond Convergence
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IP Multicast Optimization: Multicast Subsecond Convergence

Contents

IP Multicast Optimization: Multicast Subsecond Convergence

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release.

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

Prerequisites for Multicast Subsecond Convergence

Service providers must have a multicast-enabled core in order to use the Cisco Multicast Subsecond Convergence feature.

Restrictions for Multicast Subsecond Convergence

Devices that use the subsecond designated router (DR) failover enhancement must be able to process hello interval information arriving in milliseconds. Devices that are congested or do not have enough CPU cycles to process the hello interval can assume that the Protocol Independent Multicast (PIM) neighbor is disconnected, although this may not be the case.

Information About Multicast Subsecond Convergence

Benefits of Multicast Subsecond Convergence

  • The scalability components improve on the efficiency of handling increases (or decreases) in service users (receivers) and service load (sources or content).
  • New algorithms and processes (such as aggregated join messages, which deliver up to 1000 individual messages in a single packet) reduce the time to reach convergence by a factor of 10.
  • Multicast subsecond convergence improves service availability for large multicast networks.
  • Multicast users such as financial services firms and brokerages receive better quality of service (QoS), because multicast functionality is restored in a fraction of the time previously required.

Multicast Subsecond Convergence Scalability Enhancements

The Multicast Subsecond Convergence feature provides scalability enhancements that improve on the efficiency of handling increases (or decreases) in service users (receivers) and service load (sources or content). Scalability enhancements in this release include the following:

  • Improved Internet Group Management Protocol (IGMP) and PIM state maintenance through new timer management techniques
  • Improved scaling of the Multicast Source Discovery Protocol (MSDP) Source-Active (SA) cache

The scalability enhancements provide the following benefits:

  • Increased potential PIM multicast route (mroute), IGMP, and MSDP SA cache state capacity
  • Decreased CPU usage

PIM Router Query Messages

Multicast subsecond convergence allows you to send PIM router query messages (PIM hellos) every few milliseconds. The PIM hello message is used to locate neighboring PIM devices. Before the introduction of this feature, the device could send the PIM hellos only every few seconds. By enabling a device to send PIM hello messages more often, this feature allows the device to discover unresponsive neighbors more quickly. As a result, the device can implement failover or recovery procedures more efficiently.

Reverse Path Forwarding

Unicast Reverse Path Forwarding (RPF) helps to mitigate problems caused by the introduction of malformed or forged IP source addresses into a network by discarding IP packets that lack a verifiable IP source address. Malformed or forged source addresses can indicate denial-of-service (DoS) attacks based on source IP address spoofing.

RPF uses access control lists (ACLs) in determining whether to drop or forward data packets that have malformed or forged IP source addresses. An option in the ACL commands allows system administrators to log information about dropped or forwarded packets. Logging information about forged packets can help in uncovering information about possible network attacks.

Per-interface statistics can help system administrators quickly discover the interface serving as the entry point for an attack on the network.

RPF Checks

PIM is designed to forward IP multicast traffic using the standard unicast routing table. PIM uses the unicast routing table to decide if the source of the IP multicast packet has arrived on the optimal path from the source. This process, the RPF check, is protocol-independent because it is based on the contents of the unicast routing table and not on any particular routing protocol.

Triggered RPF Checks

Multicast subsecond convergence provides the ability to trigger a check of RPF changes for mroute states. This check is triggered by unicast routing changes. By performing a triggered RPF check, users can set the periodic RPF check to a relatively high value (for example, 10 seconds) and still fail over quickly.

The triggered RPF check enhancement reduces the time needed for service to be restored after disruption, such as for single service events (for example, in a situation with one source and one receiver) or as the service scales along any parameter (for example, many sources, many receivers, and many interfaces). This enhancement decreases in time-to-converge PIM (mroute), IGMP, and MSDP (SA cache) states.

RPF Failover

In an unstable unicast routing environment that uses triggered RPF checks, the environment could be constantly triggering RPF checks, which places a burden on the resources of the device. To avoid this problem, use the ip multicast rpf backoff command to prevent a second triggered RPF check from occurring for the length of time configured. That is, the PIM “backs off” from another triggered RPF check for a minimum amount of milliseconds as configured by the user.

If the backoff period expires without further routing table changes, PIM then scans for routing changes and accordingly establishes multicast RPF changes. However, if more routing changes occur during the backoff period, PIM doubles the backoff period to avoid overloading the device with PIM RPF changes while the routing table is still converging.

Topology Changes and Multicast Routing Recovery

The Multicast Subsecond Convergence feature set enhances both enterprise and service provider network backbones by providing almost instantaneous recovery of multicast paths after unicast routing recovery.

Because PIM relies on the unicast routing table to calculate its RPF when a change in the network topology occurs, unicast protocols first need to calculate options for the best paths for traffic, and then multicast can determine the best path.

Multicast subsecond convergence allows multicast protocol calculations to finish almost immediately after the unicast calculations are completed. As a result, multicast traffic forwarding is restored substantially faster after a topology change.

How to Configure Multicast Subsecond Convergence

Modifying the Periodic RPF Check Interval

Perform this optional task to modify the intervals at which periodic RPF checks occur.


Note


Cisco recommends that you do not change the default values for the ip rpf intervalcommand. The default values allow subsecond RPF failover. The default interval at which periodic RPF checks occur is 10 seconds.


SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    ip multicast rpf interval seconds [list access-list | route-map route-map]


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example:
    Device> enable
     

    Enables privileged EXEC mode.

    • Enter your password if prompted.
     
    Step 2 configure terminal


    Example:
    Device# configure terminal
     

    Enters global configuration mode.

     
    Step 3 ip multicast rpf interval seconds [list access-list | route-map route-map]


    Example:
    Device(config)# ip multicast rpf interval 10
     

    Configures the periodic RPF check intervals to occur at a specified interval, in seconds.

     
    Related Concepts

    Configuring PIM RPF Failover Intervals

    Perform this optional task to configure the intervals at which PIM RPF failover will be triggered by changes in the routing tables.


    Note


    Cisco recommends that you do not modify the default values for the ip multicast rpf backoff command. The default values allow subsecond RPF failover.


    SUMMARY STEPS

      1.    enable

      2.    configure terminal

      3.    ip multicast rpf backoff minimum maximum [disable]


    DETAILED STEPS
       Command or ActionPurpose
      Step 1 enable


      Example:
      Device> enable
       

      Enables privileged EXEC mode.

      • Enter your password if prompted.
       
      Step 2 configure terminal


      Example:
      Device# configure terminal
       

      Enters global configuration mode.

       
      Step 3 ip multicast rpf backoff minimum maximum [disable]


      Example:
      Device(config)# ip multicast rpf backoff 100 2500
       

      Configures the minimum and the maximum backoff intervals.

       
      Related Concepts

      Modifying the PIM Router Query Message Interval

      Perform this task to modify the PIM router query message interval.

      SUMMARY STEPS

        1.    enable

        2.    configure terminal

        3.    interface type slot / subslot / port

        4.    ip pim query-interval period [msec]


      DETAILED STEPS
         Command or ActionPurpose
        Step 1 enable


        Example:
        Device> enable
         

        Enables privileged EXEC mode.

        • Enter your password if prompted.
         
        Step 2 configure terminal


        Example:
        Device# configure terminal
         

        Enters global configuration mode.

         
        Step 3 interface type slot / subslot / port


        Example:
        Device(config)# interface gigabitethernet 1/0/0
         

        Specifies the interface and enters interface configuration mode.

         
        Step 4 ip pim query-interval period [msec]


        Example:
        Device(config-if)# ip pim query-interval 45
         

        Configures the frequency at which multicast routers send PIM router query messages.

         
        Related Concepts

        Verifying Multicast Subsecond Convergence Configurations

        Perform this task to display detailed information about and to verify information regarding the Multicast Subsecond Convergence feature.

        SUMMARY STEPS

          1.    enable

          2.    show ip pim interface type number

          3.    show ip pim neighbor


        DETAILED STEPS
          Step 1   enable


          Example:
          Device> enable

          Enables privileged EXEC mode.

          • Enter your password if prompted.
          Step 2   show ip pim interface type number

          Use this command to display information about interfaces configured for PIM.

          The following is sample output from the show ip pim interface command:



          Example:
          Device# show ip pim interface GigabitEthernet 1/0/0
          Address          Interface                Ver/   Nbr    Query  DR     DR
                                                    Mode   Count  Intvl  Prior
          172.16.1.4       GigabitEthernet1/0/0     v2/S   1      100 ms 1      172.16.1.4
          
          Step 3   show ip pim neighbor

          Use this command to display the PIM neighbors discovered by the Cisco IOS XE software.

          The following is sample output from the show ip pim neighbor command:



          Example:
          Device# show ip pim neighbor
          PIM Neighbor Table
          Neighbor          Interface                Uptime/Expires    Ver   DR
          Address                                                            Prio/Mode
          172.16.1.3        GigabitEthernet1/0/0     00:03:41/250 msec v2    1 / S

          Configuration Examples for Multicast Subsecond Convergence

          Example Modifying the Periodic RPF Check Interval

          In the following example, the ip multicast rpf interval has been set to 10 seconds. This command does not show up in show running-config output unless the interval value has been configured to be the nondefault value.

          !
          ip multicast-routing 
          ip multicast rpf interval 10 
          .
          .
          .
          interface Ethernet0/0
           ip address 172.16.2.1 255.255.255.0
          .
          .
          .
          ip pim sparse-mode
          !
          Related Concepts

          Example Configuring PIM RPF Failover Intervals

          In the following example, the ip multicast rpf backoff command has been configured with a minimum backoff interval value of 100 and a maximum backoff interval value of 2500. This command does not show up in show running-config command output unless the interval value has been configured to be the nondefault value.

          !
          ip multicast-routing 
          .
          .
          .
          ip multicast rpf backoff 100 2500
          !
          !
          interface Ethernet0/0
           ip address 172.16.2.1 255.255.255.0
          .
          .
          .
          ip pim sparse-mode
          !
          Related Concepts

          Modifying the PIM Router Query Message Interval Example

          In the following example, the ip pim query-interval command has been set to 100 milliseconds. This command does not show up in show running-config command output unless the interval value has been configured to be the nondefault value.

          !
          interface gigabitethernet0/0/1
           ip address 172.16.2.1 255.255.255.0
          ip pim query-interval 100 msec 
          ip pim sparse-mode
          Related Concepts

          Additional References

          Related Documents

          Related Topic

          Document Title

          Cisco IOS commands

          Cisco IOS Master Commands List, All Releases

          Cisco IOS IP SLAs commands

          Cisco IOS IP Multicast Command Reference

          PIM Sparse Mode concepts and configuration

          “Configuring Basic IP Multicast” module or “Configuring IP Multicast in IPv6 Networks” module

          MIBs

          MIB

          MIBs Link

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

          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

          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 History and Information for Multicast Subsecond Convergence

          Release

          Modification

          Cisco IOS XE 3.3SE

          This feature was introduced.