Published On: August 5ᵗʰ, 2019 19:02

IP Multicast: IGMP Configuration Guide, Cisco IOS Release 15SY

Constraining IP Multicast in a Switched Ethernet Network

Last Updated: October 16, 2012

This module describes how to configure devices to use the Cisco Group Management Protocol (CGMP) in switched Ethernet networks to control multicast traffic to Layer 2 switch ports and the Router-Port Group Management Protocol (RGMP) to constrain IP multicast traffic on routing device-only network segments.

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.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.

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.

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.

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.

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 Action Purpose
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.

 

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 Action Purpose
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.

 

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

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 Information for Constraining IP Multicast in a Switched Ethernet Network

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 Constraining IP Multicast in a Switched Ethernet Network

Feature Name

Releases

Feature Configuration Information

CGMP - Cisco Group Management Protocol

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For information about feature support in Cisco IOS software, use Cisco Feature Navigator.

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