Constraining IP Multicast in a Switched Ethernet Network

Constraining IP Multicast in a Switched
Ethernet Network
Last Updated: January 3, 2012
This module describes how to configure routers 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 router-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.
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Finding Feature Information, page 1
Prerequisites for Constraining IP Multicast in a Switched Ethernet Network, page 2
Information About IP Multicast in a Switched Ethernet Network, page 2
How to Constrain Multicast in a Switched Ethernet Network, page 4
Configuration Examples for Constraining IP Multicast in a Switched Ethernet Network, page 7
Additional References, page 7
Feature Information for Constraining IP Multicast in a Switched Ethernet Network, page 8
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. 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 document.
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.
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IP Multicast Traffic and Layer 2 Switches
Prerequisites for Constraining IP Multicast in a Switched Ethernet Network
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
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IP Multicast Traffic and Layer 2 Switches, page 2
CGMP on Catalyst Switches for IP Multicast, page 2
IGMP Snooping, page 3
Router-Port Group Management Protocol (RGMP), page 3
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.
Three methods that efficiently constrain IP multicast in a Layer 2 switching environment are described in
the following sections:
Note
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 routers 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 routers 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
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IGMP Snooping
Information About IP Multicast in a Switched Ethernet Network
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 router port also is added to the entry for the multicast group. Multicast routers 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.
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 routeronly 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.
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Configuring Switches for IP Multicast
How to Constrain Multicast in a Switched Ethernet Network
How to Constrain Multicast in a Switched Ethernet Network
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Configuring Switches for IP Multicast, page 4
Configuring IGMP Snooping, page 4
Enabling CGMP on a Router, page 4
Configuring IP Multicast in a Layer 2 Switched Ethernet Network, page 5
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 on a Router
CGMP is a protocol used on routers 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
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CGMP should be enabled only on 802 or ATM media, or LAN emulation (LANE) over ATM.
CGMP should be enabled only on routers 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]
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Configuring IP Multicast in a Layer 2 Switched Ethernet Network
How to Constrain Multicast in a Switched Ethernet Network
DETAILED STEPS
Command or Action
Step 1 enable
Purpose
Enables privileged EXEC mode.
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Enter your password if prompted.
Example:
Router> enable
Step 2 configure terminal
Enters global configuration mode.
Example:
Router# configure terminal
Step 3 interface type number
Selects an interface that is connected to hosts on which IGMPv3 can be
enabled.
Example:
Router(config)# interface ethernet 1
Step 4 ip cgmp [proxy | router-only]
Enables CGMP on an interface of a router connected to a Cisco Catalyst
5000 family switch.
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Example:
Router(config-if)# ip cgmp proxy
Step 5 end
The proxy keyword enables the CGMP proxy function. When
enabled, any router that is not CGMP-capable will be advertised by
the proxy router. The proxy router advertises the existence of other
non-CGMP-capable routers by sending a CGMP Join message with
the MAC address of the non-CGMP-capable router and group
address of 0000.0000.0000.
Ends the current configuration session and returns to EXEC mode.
Example:
Router(config-if)# end
Step 6 clear ip cgmp [interface-type interfacenumber]
(Optional) Clears all group entries from the caches of Catalyst switches.
Example:
Router# clear ip cgmp
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.
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Configuring IP Multicast in a Layer 2 Switched Ethernet Network
How to Constrain Multicast in a Switched Ethernet Network
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
Step 1 enable
Purpose
Enables privileged EXEC mode.
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Enter your password if prompted.
Example:
Router> enable
Step 2 configure terminal
Enters global configuration mode.
Example:
Router# configure terminal
Step 3 interface type number
Selects an interface that is connected to hosts.
Example:
Router(config)# interface ethernet 1
Step 4 ip rgmp
Enables RGMP on Ethernet, Fast Ethernet, and Gigabit Ethernet interfaces.
Example:
Router(config-if)# ip rgmp
Step 5 end
Example:
Router(config-if)# end
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Ends the current configuration session and returns to EXEC mode.
CGMP Configuration Example
Configuration Examples for Constraining IP Multicast in a Switched Ethernet Network
Command or Action
Purpose
Step 6 debug ip rgmp
(Optional) Logs debug messages sent by an RGMP-enabled router.
Example:
Router# debug ip rgmp
Step 7 show ip igmp interface
(Optional) Displays multicast-related information about an interface.
Example:
Router# show ip igmp interface
Configuration Examples for Constraining IP Multicast in a
Switched Ethernet Network
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CGMP Configuration Example, page 7
CGMP Configuration Example
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.
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CGMP Configuration Example
Feature Information for Constraining IP Multicast in a Switched Ethernet Network
Related Documents
Related Topic
Document Title
IP multicast commands: complete command syntax, Cisco IOS IP Multicast Command Reference
command mode, command history, defaults, usage
guidelines, and examples
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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
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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.
CGMP Configuration Example
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