IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3850 Switches)
Constraining IP Multicast in Switched Ethernet
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Constraining IP Multicast in Switched Ethernet

Contents

Constraining IP Multicast in Switched Ethernet

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 Constraining IP Multicast in a Switched Ethernet Network

Before using the tasks in this module, you should be familiar with the concepts described in the “IP Multicast Technology Overview” module.

Information About IP Multicast in a Switched Ethernet Network

IP Multicast Traffic and Layer 2 Switches

The default behavior for a Layer 2 switch is to forward all multicast traffic to every port that belongs to the destination LAN on the switch. This behavior reduces the efficiency of the switch, whose purpose is to limit traffic to the ports that need to receive the data. This behavior requires a constraining mechanism to reduce unnecessary multicast traffic, which improves switch performance.

Cisco Group Management Protocol (CGMP), Router Group Management Protocol (RGMP), and IGMP snooping efficiently constrain IP multicast in a Layer 2 switching environment.
  • CGMP and IGMP snooping are used on subnets that include end users or receiver clients.
  • RGMP is used on routed segments that contain only routers, such as in a collapsed backbone.
  • RGMP and CGMP cannot interoperate. However, Internet Group Management Protocol (IGMP) can interoperate with CGMP and RGMP snooping.

CGMP on Catalyst Switches for IP Multicast

CGMP is a Cisco-developed protocol used on device connected to Catalyst switches to perform tasks similar to those performed by IGMP. CGMP is necessary for those Catalyst switches that do not distinguish between IP multicast data packets and IGMP report messages, both of which are addressed to the same group address at the MAC level. The switch can distinguish IGMP packets, but would need to use software on the switch, greatly impacting its performance.

You must configure CGMP on the multicast device and the Layer 2 switches. The result is that, with CGMP, IP multicast traffic is delivered only to those Catalyst switch ports that are attached to interested receivers. All other ports that have not explicitly requested the traffic will not receive it unless these ports are connected to a multicast router. Multicast router ports must receive every IP multicast data packet.

Using CGMP, when a host joins a multicast group, it multicasts an unsolicited IGMP membership report message to the target group. The IGMP report is passed through the switch to the router for normal IGMP processing. The router (which must have CGMP enabled on this interface) receives the IGMP report and processes it as it normally would, but also creates a CGMP Join message and sends it to the switch. The Join message includes the MAC address of the end station and the MAC address of the group it has joined.

The switch receives this CGMP Join message and then adds the port to its content-addressable memory (CAM) table for that multicast group. All subsequent traffic directed to this multicast group is then forwarded out the port for that host.

The Layer 2 switches are designed so that several destination MAC addresses could be assigned to a single physical port. This design allows switches to be connected in a hierarchy and also allows many multicast destination addresses to be forwarded out a single port.

The device port also is added to the entry for the multicast group. Multicast device must listen to all multicast traffic for every group because IGMP control messages are also sent as multicast traffic. The rest of the multicast traffic is forwarded using the CAM table with the new entries created by CGMP.

Related Tasks
Related References

IGMP Snooping

IGMP snooping is an IP multicast constraining mechanism that runs on a Layer 2 LAN switch. IGMP snooping requires the LAN switch to examine, or “snoop,” some Layer 3 information (IGMP Join/Leave messages) in the IGMP packets sent between the hosts and the router. When the switch receives the IGMP host report from a host for a particular multicast group, the switch adds the port number of the host to the associated multicast table entry. When the switch hears the IGMP Leave group message from a host, the switch removes the table entry of the host.

Because IGMP control messages are sent as multicast packets, they are indistinguishable from multicast data at Layer 2. A switch running IGMP snooping must examine every multicast data packet to determine if it contains any pertinent IGMP control information. IGMP snooping implemented on a low-end switch with a slow CPU could have a severe performance impact when data is sent at high rates. The solution is to implement IGMP snooping on high-end switches with special application-specific integrated circuits (ASICs) that can perform the IGMP checks in hardware. CGMP is a better option for low-end switches without special hardware.

Router-Port Group Management Protocol (RGMP)

CGMP and IGMP snooping are IP multicast constraining mechanisms designed to work on routed network segments that have active receivers. They both depend on IGMP control messages that are sent between the hosts and the routers to determine which switch ports are connected to interested receivers.

Switched Ethernet backbone network segments typically consist of several routers connected to a switch without any hosts on that segment. Because routers do not generate IGMP host reports, CGMP and IGMP snooping will not be able to constrain the multicast traffic, which will be flooded to every port on the VLAN. Routers instead generate Protocol Independent Multicast (PIM) messages to Join and Prune multicast traffic flows at a Layer 3 level.

Router-Port Group Management Protocol (RGMP) is an IP multicast constraining mechanism for router-only network segments. RGMP must be enabled on the routers and on the Layer 2 switches. A multicast router indicates that it is interested in receiving a data flow by sending an RGMP Join message for a particular group. The switch then adds the appropriate port to its forwarding table for that multicast group--similar to the way it handles a CGMP Join message. IP multicast data flows will be forwarded only to the interested router ports. When the router no longer is interested in that data flow, it sends an RGMP Leave message and the switch removes the forwarding entry.

If there are any routers that are not RGMP-enabled, they will continue to receive all multicast data.

Related References

How to Constrain Multicast in a Switched Ethernet Network

Configuring Switches for IP Multicast

If you have switching in your multicast network, consult the documentation for the switch you are working with for information about how to configure IP multicast.

Configuring IGMP Snooping

No configuration is required on the router. Consult the documentation for the switch you are working with to determine how to enable IGMP snooping and follow the provided instructions.

Enabling CGMP

CGMP is a protocol used on devices connected to Catalyst switches to perform tasks similar to those performed by IGMP. CGMP is necessary because the Catalyst switch cannot distinguish between IP multicast data packets and IGMP report messages, which are both at the MAC level and are addressed to the same group address.


Note


  • CGMP should be enabled only on 802 or ATM media, or LAN emulation (LANE) over ATM.
  • CGMP should be enabled only on devices connected to Catalyst switches.

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    interface type number

    4.    ip cgmp [proxy | router-only]

    5.    end

    6.    clear ip cgmp [interface-type interface-number]


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 number


    Example:
    Device(config)# interface ethernet 1
     

    Selects an interface that is connected to hosts on which IGMPv3 can be enabled.

     
    Step 4 ip cgmp [proxy | router-only]


    Example:
    Device(config-if)# ip cgmp proxy
     

    Enables CGMP on an interface of a device connected to a Cisco Catalyst 5000 family switch.

    • The proxy keyword enables the CGMP proxy function. When enabled, any device that is not CGMP-capable will be advertised by the proxy router. The proxy router advertises the existence of other non-CGMP-capable devices by sending a CGMP Join message with the MAC address of the non-CGMP-capable device and group address of 0000.0000.0000.
     
    Step 5 end


    Example:
    Device(config-if)# end
     

    Ends the current configuration session and returns to EXEC mode.

     
    Step 6 clear ip cgmp [interface-type interface-number]


    Example:
    Device# clear ip cgmp
     

    (Optional) Clears all group entries from the caches of Catalyst switches.

     
    Related References

    Configuring IP Multicast in a Layer 2 Switched Ethernet Network

    Perform this task to configure IP multicast in a Layer 2 Switched Ethernet network using RGMP.

    SUMMARY STEPS

      1.    enable

      2.    configure terminal

      3.    interface type number

      4.    ip rgmp

      5.    end

      6.    debug ip rgmp

      7.    show ip igmp interface


    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 number


      Example:
      Device(config)# interface ethernet 1
       

      Selects an interface that is connected to hosts.

       
      Step 4 ip rgmp


      Example:
      Device(config-if)# ip rgmp
       

      Enables RGMP on Ethernet, Fast Ethernet, and Gigabit Ethernet interfaces.

       
      Step 5 end


      Example:
      Device(config-if)# end
       

      Ends the current configuration session and returns to EXEC mode.

       
      Step 6 debug ip rgmp


      Example:
      Device# debug ip rgmp
       

      (Optional) Logs debug messages sent by an RGMP-enabled device.

       
      Step 7 show ip igmp interface


      Example:
      Device# show ip igmp interface
       

      (Optional) Displays multicast-related information about an interface.

       
      Related References

      Configuration Examples for Constraining IP Multicast in a Switched Ethernet Network

      Example: CGMP Configuration

      The following example is for a basic network environment where multicast source(s) and multicast receivers are in the same VLAN. The desired behavior is that the switch will constrain the multicast forwarding to those ports that request the multicast stream.

      A 4908G-L3 router is connected to the Catalyst 4003 on port 3/1 in VLAN 50. The following configuration is applied on the GigabitEthernet1 interface. Note that there is no ip multicast-routing command configured because the router is not routing multicast traffic across its interfaces.

      interface GigabitEthernet1
       ip address 192.168.50.11 255.255.255.0
       ip pim dense-mode
       ip cgmp
      Related Tasks

      RGMP Configuration Example

      The following example shows how to configure RGMP on a router:

      ip multicast-routing
      ip pim sparse-mode
      interface ethernet 0
       ip rgmp

      Where to Go Next for Constraining IP Multicast in Switched Ethernet

      You can configure the following:

      • IGMP
      • PIM
      • SSM
      • IP Multicast Routing
      • Service Discovery Gateway

      You can also review the following IP Multicast Optimization processes for your configuration:

      • Optimizing PIM Sparse Mode in a Large IP Multicast Deployment
      • Multicast Subsecond Convergence
      • IP Multicast Load Splitting across Equal-Cost Paths
      • SSM Channel Based Filtering for Multicast
      • PIM Dense Mode State Refresh
      • IGMP State Limit

      Additional References

      The following sections provide references related to constraining IP multicast in a switched Ethernet network.

      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

      IGMP snooping

      The “IGMP Snooping” module of the IP Multicast: IGMP Configuration Guide

      RGMP

      The “Configuring Router-Port Group Management Protocol” module of the IP Multicast: IGMP Configuration Guide

      MIBs

      MIB

      MIBs Link

      None

      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

      Technical Assistance

      Description

      Link

      Technical Assistance Center (TAC) home page, containing 30,000 pages of searchable technical content, including links to products, technologies, solutions, technical tips, and tools. Registered Cisco.com users can log in from this page to access even more content.

      http:/​/​www.cisco.com/​public/​support/​tac/​home.shtml

      Feature History and Information for Constraining IP Multicast in a Switched Ethernet Network

      Release

      Modification

      Cisco IOS XE 3.3SE

      This feature was introduced.