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Layer 2 switches can use IGMP snooping to constrain the flooding of multicast traffic by dynamically configuring Layer 2 interfaces so that multicast traffic is forwarded to only those interfaces associated with IP multicast devices. As the name implies, IGMP snooping requires the LAN switch to snoop on the IGMP transmissions between the host and the router and to keep track of multicast groups and member ports. When the switch receives an IGMP report from a host for a particular multicast group, the switch adds the host port number to the forwarding table entry; when it receives an IGMP Leave Group message from a host, it removes the host port from the table entry. It also periodically deletes entries if it does not receive IGMP membership reports from the multicast clients.

Note	For more information on IP multicast and IGMP, see RFC 1112 and RFC 2236.

The multicast router sends out periodic general queries to all VLANs. All hosts interested in this multicast traffic send join requests and are added to the forwarding table entry. The switch creates one entry per VLAN in the IGMP snooping IP multicast forwarding table for each group from which it receives an IGMP join request.

The switch supports IP multicast group-based bridging, instead of MAC-addressed based groups. With multicast MAC address-based groups, if an IP address being configured translates (aliases) to a previously configured MAC address or to any reserved multicast MAC addresses (in the range 224.0.0.xxx), the command fails. Because the switch uses IP multicast groups, there are no address aliasing issues.

The IP multicast groups learned through IGMP snooping are dynamic. However, you can statically configure multicast groups by using the ip igmp snooping vlan vlan-id static ip_address interface interface-id global configuration command. If you specify group membership for a multicast group address statically, your setting supersedes any automatic manipulation by IGMP snooping. Multicast group membership lists can consist of both user-defined and IGMP snooping-learned settings.

Note	You can manage IP multicast group addresses through features such as IGMP snooping and Multicast VLAN Registration (MVR), or by using static IP addresses. For information about MVR, see the next chapter.

You can configure an IGMP snooping querier to support IGMP snooping in subnets without multicast interfaces because the multicast traffic does not need to be routed.

If a port spanning tree, a port group, or a VLAN ID change occurs, the IGMP snooping-learned multicast groups from this port on the VLAN are deleted.

The switch supports IGMP Version 1, IGMP Version 2, and IGMP Version 3. These versions are interoperable on the switch. For example, if IGMP snooping is enabled on an IGMPv2 switch and the switch receives an IGMPv3 report from a host, the switch can forward the IGMPv3 report to the multicast router.

Note	The switch supports IGMPv3 snooping based only on the destination multicast MAC address. It does not support snooping based on the source MAC address or on proxy reports.

An IGMPv3 switch supports Basic IGMPv3 Snooping Support (BISS), which includes support for the snooping features on IGMPv1 and IGMPv2 switches and for IGMPv3 membership report messages. BISS constrains the flooding of multicast traffic when your network includes IGMPv3 hosts. It constrains traffic to approximately the same set of ports as the IGMP snooping feature on IGMPv2 or IGMPv1 hosts.

Note	IGMPv3 join and leave messages are not supported on switches running IGMP filtering or MVR.

An IGMPv3 switch can receive messages from and forward messages to a device running the Source Specific Multicast (SSM) feature.

Figure 1. Initial IGMP Join Message.

When a host connected to the switch wants to join an IP multicast group and it is an IGMP version 2 client, it sends an unsolicited IGMP join message, specifying the IP multicast group to join. Alternatively, when the switch receives a general query from the router, it forwards the query to all ports in the VLAN. IGMP version 1 or version 2 hosts wanting to join the multicast group respond by sending a join message to the switch. The switch CPU creates a multicast forwarding-table entry for the group if it is not already present. The CPU also adds the interface where the join message was received to the forwarding-table entry. The host associated with that interface receives multicast traffic for that multicast group.

Router A sends a general query to the switch, which forwards the query to ports 2 through 5, which are all members of the same VLAN. Host 1 wants to join multicast group 224.1.2.3 and multicasts an IGMP membership report (IGMP join message) to the group. The switch CPU uses the information in the IGMP report to set up a forwarding-table entry that includes the port numbers connected to Host 1 and to the router.

The switch hardware can distinguish IGMP information packets from other packets for the multicast group. The information in the table tells the switching engine to send frames addressed to the 224.1.2.3 multicast IP address that are not IGMP packets to the router and to the host that has joined the group.

Figure 2. Second Host Joining a Multicast Group. If another host (for example, Host 4) sends an unsolicited IGMP join message for the same group, the CPU receives that message and adds the port number of Host 4 to the forwarding table. Because the forwarding table directs IGMP messages only to the CPU, the message is not flooded to other ports on the switch. Any known multicast traffic is forwarded to the group and not to the CPU.

The router sends periodic multicast general queries, and the switch forwards these queries through all ports in the VLAN. Interested hosts respond to the queries. If at least one host in the VLAN wants to receive multicast traffic, the router continues forwarding the multicast traffic to the VLAN. The switch forwards multicast group traffic only to those hosts listed in the forwarding table for that IP multicast group maintained by IGMP snooping.

When hosts want to leave a multicast group, they can silently leave, or they can send a leave message. When the switch receives a leave message from a host, it sends a group-specific query to learn if any other devices connected to that interface are interested in traffic for the specific multicast group. The switch then updates the forwarding table for that MAC group so that only those hosts interested in receiving multicast traffic for the group are listed in the forwarding table. If the router receives no reports from a VLAN, it removes the group for the VLAN from its IGMP cache.

The switch uses IGMP snooping Immediate Leave to remove from the forwarding table an interface that sends a leave message without the switch sending group-specific queries to the interface. The VLAN interface is pruned from the multicast tree for the multicast group specified in the original leave message. Immediate Leave ensures optimal bandwidth management for all hosts on a switched network, even when multiple multicast groups are simultaneously in use.

Immediate Leave is only supported on IGMP version 2 hosts. IGMP version 2 is the default version for the switch.

Note	You should only use the Immediate Leave feature on VLANs where a single host is connected to each port. If Immediate Leave is enabled in VLANs where more than one host is connected to a port, some hosts might inadvertently be dropped.

You can configure the time that the switch waits after sending a group-specific query to determine if hosts are still interested in a specific multicast group. The IGMP leave response time can be configured from 100 to 5000 milliseconds. The timer can be set either globally or on a per-VLAN basis. The VLAN configuration of the leave time overrides the global configuration.

Note	IGMP report suppression is supported only when the multicast query has IGMPv1 and IGMPv2 reports. This feature is not supported when the query includes IGMPv3 reports.

The switch uses IGMP report suppression to forward only one IGMP report per multicast router query to multicast devices. When IGMP router suppression is enabled (the default), the switch sends the first IGMP report from all hosts for a group to all the multicast routers. The switch does not send the remaining IGMP reports for the group to the multicast routers. This feature prevents duplicate reports from being sent to the multicast devices.

If the multicast router query includes requests only for IGMPv1 and IGMPv2 reports, the switch forwards only the first IGMPv1 or IGMPv2 report from all hosts for a group to all the multicast routers.

If the multicast router query also includes requests for IGMPv3 reports, the switch forwards all IGMPv1, IGMPv2, and IGMPv3 reports for a group to the multicast devices.

If you disable IGMP report suppression, all IGMP reports are forwarded to the multicast routers.

IGMP snooping functions across the switch stack; that is, IGMP control information from one switch is distributed to all switches in the stack. Regardless of the stack member through which IGMP multicast data enters the stack, the data reaches the hosts that have registered for that group.

If a switch in the stack fails or is removed from the stack, only the members of the multicast group that are on that switch will not receive the multicast data. All other members of a multicast group on other switches in the stack continue to receive multicast data streams. However, multicast groups that are common for both Layer 2 and Layer 3 (IP multicast routing) might take longer to converge if the active switch is removed.

In some environments, for example, metropolitan or multiple-dwelling unit (MDU) installations, you might want to control the set of multicast groups to which a user on a switch port can belong. You can control the distribution of multicast services, such as IP/TV, based on some type of subscription or service plan. You might also want to limit the number of multicast groups to which a user on a switch port can belong.

With the IGMP filtering feature, you can filter multicast joins on a per-port basis by configuring IP multicast profiles and associating them with individual switch ports. An IGMP profile can contain one or more multicast groups and specifies whether access to the group is permitted or denied. If an IGMP profile denying access to a multicast group is applied to a switch port, the IGMP join report requesting the stream of IP multicast traffic is dropped, and the port is not allowed to receive IP multicast traffic from that group. If the filtering action permits access to the multicast group, the IGMP report from the port is forwarded for normal processing. You can also set the maximum number of IGMP groups that a Layer 2 interface can join.

IGMP filtering controls only group-specific query and membership reports, including join and leave reports. It does not control general IGMP queries. IGMP filtering has no relationship with the function that directs the forwarding of IP multicast traffic. The filtering feature operates in the same manner whether CGMP or MVR is used to forward the multicast traffic.

IGMP filtering applies only to the dynamic learning of IP multicast group addresses, not static configuration.

With the IGMP throttling feature, you can set the maximum number of IGMP groups that a Layer 2 interface can join. If the maximum number of IGMP groups is set, the IGMP snooping forwarding table contains the maximum number of entries, and the interface receives an IGMP join report, you can configure an interface to drop the IGMP report or to replace the randomly selected multicast entry with the received IGMP report.

Note	IGMPv3 join and leave messages are not supported on switches running IGMP filtering.

1.    configure terminal


2.    ip igmp snooping


3.    end


4.    copy running-config startup-config

            
            
Switch# configure terminal

            
            
Switch(config)# ip igmp snooping

            
            
Switch(config)# end

            
            
Switch# copy running-config startup-config 

1.    configure terminal


2.    ip igmp snooping vlan vlan-id


3.    end


4.    copy running-config startup-config

            
            
Switch# configure terminal

            
            
Switch(config)# ip igmp snooping vlan 7

            
            
Switch(config)# end

            
            
Switch# copy running-config startup-config

1.    configure terminal


2.    ip igmp snooping vlan vlan-id mrouter learn { cgmp | pim-dvmrp }


3.    end


4.    show ip igmp snooping


5.    copy running-config startup-config

            
            
Switch# configure terminal

Switch(config)# ip igmp snooping vlan 1 mrouter learn cgmp

            
            
Switch(config)# end

            
            
Switch# show ip igmp snooping

            
            
Switch# copy running-config startup-config

To add a multicast router port (add a static connection to a multicast router), use the ip igmp snooping vlan mrouter global configuration command on the switch.

Note	Static connections to multicast routers are supported only on switch ports.

1.    configure terminal


2.    ip igmp snooping vlan vlan-id mrouter interface interface-id


3.    end


4.    show ip igmp snooping mrouter [ vlan vlan-id]


5.    copy running-config startup-config

            
            
Switch# configure terminal

            
            
Switch(config)# ip igmp snooping vlan 5 mrouter interface gigabitethernet1/0/1 

            
            
Switch(config)# end

            
            
Switch# show ip igmp snooping mrouter vlan 5

            
            
Switch# copy running-config startup-config

1.    configure terminal


2.    ip igmp snooping vlan vlan-id static ip_address interface interface-id


3.    end


4.    show ip igmp snooping groups


5.    copy running-config startup-config

            
            
Switch# configure terminal

            
            
Switch(config)# ip igmp snooping vlan 105 static 230.0.0.1 interface gigabitethernet1/0/1

            
            
Switch(config)# end

            
            
Switch# show ip igmp snooping groups

            
            
Switch# copy running-config startup-config

When you enable IGMP Immediate Leave, the switch immediately removes a port when it detects an IGMP Version 2 leave message on that port. You should only use the Immediate-Leave feature when there is a single receiver present on every port in the VLAN.

Note	Immediate Leave is supported only on IGMP Version 2 hosts. IGMP Version 2 is the default version for the switch.

1.    configure terminal


2.    ip igmp snooping vlan vlan-id immediate-leave


3.    end


4.    show ip igmp snooping vlan vlan-id


5.    copy running-config startup-config

            
            
Switch# configure terminal

            
            
Switch(config)# ip igmp snooping vlan 21 immediate-leave 

            
            
Switch(config)# end

            
            
Switch# show ip igmp snooping vlan 21 

            
            
Switch# copy running-config startup-config

1.    configure terminal


2.    ip igmp snooping last-member-query-interval time


3.    ip igmp snooping vlan vlan-id last-member-query-interval time


4.    end


5.    show ip igmp snooping


6.    copy running-config startup-config

            
            
Switch# configure terminal

            
            
Switch(config)# ip igmp snooping last-member-query-interval 1000

            
            
Switch(config)# ip igmp snooping vlan 210 last-member-query-interval 1000

            
            
Switch(config)# end

            
            
Switch# show ip igmp snooping

            
            
Switch# copy running-config startup-config

1.    configure terminal


2.    ip igmp snooping tcn flood query count count


3.    end


4.    show ip igmp snooping


5.    copy running-config startup-config

            
            
Switch# configure terminal

            
            
Switch(config)# ip igmp snooping tcn flood query count 3

            
            
Switch(config)# end

            
            
Switch# show ip igmp snooping

            
            
Switch# copy running-config startup-config

1.    configure terminal


2.    ip igmp snooping tcn query solicit


3.    end


4.    show ip igmp snooping


5.    copy running-config startup-config

            
            
Switch# configure terminal

            
            
Switch(config)# ip igmp snooping tcn query solicit

            
            
Switch(config)# end

            
            
Switch# show ip igmp snooping

            
            
Switch# copy running-config startup-config

1.    configure terminal


2.    interface interface-id


3.    no ip igmp snooping tcn flood


4.    end


5.    show ip igmp snooping


6.    copy running-config startup-config

            
            
Switch# configure terminal

            
            
Switch(config)# interface gigabitethernet 1/0/1

            
            
Switch(config-if)# no ip igmp snooping tcn flood

            
            
Switch(config)# end

            
            
Switch# show ip igmp snooping

            
            
Switch# copy running-config startup-config

1.    configure terminal


2.    ip igmp snooping querier


3.    ip igmp snooping querier address ip_address


4.    ip igmp snooping querier query-interval interval-count


5.    ip igmp snooping querier tcn query [ count count | interval interval]


6.    ip igmp snooping querier timer expiry timeout


7.    ip igmp snooping querier version version


8.    end


9.    show ip igmp snooping vlan vlan-id


10.    copy running-config startup-config

            
            
Switch# configure terminal

            
            
Switch(config)# ip igmp snooping querier

            
            
Switch(config)# ip igmp snooping querier address 172.16.24.1 

            
            
Switch(config)# ip igmp snooping querier query-interval 30

            
            
Switch(config)# ip igmp snooping querier tcn query interval 20

            
            
Switch(config)# ip igmp snooping querier timer expiry 180

            
            
Switch(config)# ip igmp snooping querier version 2

            
            
Switch(config)# end

            
            
Switch# show ip igmp snooping vlan 30

            
            
Switch# copy running-config startup-config

Note	IGMP report suppression is supported only when the multicast query has IGMPv1 and IGMPv2 reports. This feature is not supported when the query includes IGMPv3 reports.

IGMP report suppression is enabled by default. When it is enabled, the switch forwards only one IGMP report per multicast router query. When report suppression is disabled, all IGMP reports are forwarded to the multicast routers.

1.    configure terminal


2.    no ip igmp snooping report-suppression


3.    end


4.    show ip igmp snooping


5.    copy running-config startup-config

            
            
Switch# configure terminal

            
            
Switch(config)# no ip igmp snooping report-suppression 

            
            
Switch(config)# end

            
            
Switch# show ip igmp snooping 

            
            
Switch# copy running-config startup-config 

1.    configure terminal


2.    ip igmp profile profile number


3.    permit | deny

4.    range ip multicast address


5.    end


6.    show ip igmp profile profile number


7.    copy running-config startup-config

            
            
Switch# configure terminal

            
            
Switch(config)# ip igmp profile 3

            
            
Switch(config-igmp-profile)# permit

            
            
Switch(config-igmp-profile)# range 229.9.9.0

            
            
Switch(config-igmp-profile)# end

            
            
Switch# show ip igmp profile 3

            
            
Switch# copy running-config startup-config

1.    configure terminal


2.    interface interface-id


3.    ip igmp filter profile number


4.    end


5.    show running-config interface interface-id


6.    copy running-config startup-config

            
            
Switch# configure terminal

            
            
Switch(config)# interface gigabitethernet1/0/1

            
            
Switch(config-if)# ip igmp filter 321

            
            
Switch(config-if)# end

            
            
Switch# show running-config interface gigabitethernet1/0/1

            
            
Switch# copy running-config startup-config

1.    configure terminal


2.    interface interface-id


3.    ip igmp max-groups number


4.    end


5.    show running-config interface interface-id


6.    copy running-config startup-config

            
            
Switch# configure terminal

            
            
Switch(config)# interface gigabitethernet1/0/2

            
            
Switch(config-if)# ip igmp max-groups 20

            
            
Switch(config-if)# end

            
            
Switch# show running-config interface gigabitethernet1/0/1

            
            
Switch# copy running-config startup-config

1.    configure terminal


2.    interface interface-id


3.    ip igmp max-groups action { deny | replace}


4.    end


5.    show running-config interface interface-id


6.    copy running-config startup-config

            
            
Switch# configure terminal

            
            
Switch(config)# interface gigabitethernet 1/0/1

            
            
Switch(config-if)# ip igmp max-groups action replace

            
            
Switch(config-if)# end

            
            
Switch# show running-config interface gigabitethernet1/0/1

            
            
Switch# copy running-config startup-config