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This chapter contains the following sections:
Information about IGMP Snooping
The Internet Group Management Protocol (IGMP) snooping software examines Layer 2 IP multicast traffic within a VLAN to discover the ports where interested receivers reside. Using the port information, IGMP snooping can reduce bandwidth consumption in a multi-access LAN environment to avoid flooding the entire VLAN. The IGMP snooping feature tracks which ports are attached to multicast-capable routers to help the routers forward IGMP membership reports. The IGMP snooping software responds to topology change notifications. By default, IGMP snooping is enabled on the device.
The following figure shows an IGMP snooping switch that sits between the host and the IGMP router. The IGMP snooping switch snoops the IGMP membership reports and Leave messages and forwards them only when necessary to the connected IGMP routers.
The IGMP snooping software operates upon IGMPv1, IGMPv2, and IGMPv3 control plane packets where Layer 3 control plane packets are intercepted and influence the Layer 2 forwarding behavior.
The Cisco Nexus 1000V IGMP snooping implementation has the following proprietary features:
Multicast forwarding based on an IP address rather than a MAC address.
Optimized multicast flooding (OMF) that forwards unknown traffic to routers only and performs no data driven state creation.
For more information about IGMP snooping, see RFC 4541.
IGMPv2 supports the fast leave feature. The fast leave feature does not send last member query messages to hosts. As soon as the software receives an IGMP leave message, the software stops forwarding multicast data to that port.
IGMPv1 does not provide an explicit IGMP leave message, so the software must rely on the membership message timeout to indicate that no hosts remain that want to receive multicast data for a particular group.
Report suppression is not supported and is disabled by default.
IGMPv3 snooping provides constrained flooding based on the group IP information in the IGMPv3 reports. Report suppression is not supported and disabled by default. In addition, explicit tracking is not supported and disabled by default. Instead, the fast leave feature is used for handling leave messages.
IGMP snooping has the following prerequisites:
You are logged in to the switch.
A querier must be running on the uplink switches on the VLANs that contain multicast sources and receivers.
When the multicast traffic does not need to be routed, you must configure an external switch to query membership. On the external switch, define the query feature in a VLAN that contains multicast sources and receivers but no other active query feature. In the Cisco Nexus 1000V, report suppression is not supported and is disabled by default.
When an IGMP snooping query feature is enabled on an upstream switch, it sends out periodic IGMP queries that trigger IGMP report messages from hosts wanting to receive IP multicast traffic. IGMP snooping listens to these IGMP reports to identify accurate forwarding.
Parameters | Default |
---|---|
IGMP snooping |
Enabled |
IGMPv3 Explicit tracking |
Disabled (Not supported.) |
IGMPv2 Fast leave |
Enabled (Cannot be disabled.) |
Last member query interval |
1 second |
Link-local groups suppression |
Enabled |
Snooping querier |
Disabled |
IGMPv1/v2 Report suppression |
Disabled |
IGMPv3 Report suppression |
Disabled |
Configuring IGMP Snooping
You can enable or disable IGMP snooping globally for the VSM. IGMP snooping is enabled globally on the VSM by default. If you enable IGMP snooping globally, you can turn IGMP snooping off individually for each VLAN. If you disable IGMP snooping globally, then IGMP snooping on all VLANs is disabled regardless of their individual settings.
You are logged in to the CLI in EXEC mode.
switch# configure terminal switch(config)# no ip igmp snooping switch(config)# show ip igmp snooping Global IGMP Snooping Information: IGMP Snooping disabled Optimised Multicast Flood (OMF) enabled IGMPv1/v2 Report Suppression disabled IGMPv3 Report Suppression disabled Link Local Groups Suppression enabled VPC Multicast optimization disabled IGMP Snooping information for vlan 1 IGMP snooping disabled Optimised Multicast Flood (OMF) disabled IGMP querier none Switch-querier disabled IGMPv3 Explicit tracking enabled IGMPv2 Fast leave enabled IGMPv1/v2 Report suppression disabled IGMPv3 Report suppression disabled Link Local Groups suppression enabled Router port detection using PIM Hellos, IGMP Queries Number of router-ports: 0 Number of groups: 0 VLAN vPC function disabled Active ports: switch(config)#
Use this procedure to configure IGMP snooping on a VLAN. IGMP snooping is enabled by default for all VLANs in the VSM.
You are logged in to the CLI in EXEC mode.
Note | If IGMP snooping is disabled globally, it takes precedence over the VLAN state. |
Use the following commands to verify the IGMP snooping configuration information.
Command | Purpose |
---|---|
show ip igmp snooping [ vlan vlan-id ] |
Displays IGMP snooping configuration by VLAN. |
show ip igmp snooping groups [ vlan vlan-id ] [ detail ] |
Displays IGMP snooping information about groups by VLAN. |
show ip igmp snooping querier [ vlan vlan-id ] |
Displays IGMP snooping queriers by VLAN. |
show ip igmp snooping mroute [ vlan vlan-id ] |
Displays multicast router ports by VLAN. |
Feature Name |
Release |
Description |
---|---|---|
IGMP Snooping |
Release 5.2(1)SK1(2.1) |
This feature was introduced. |