Cisco Nexus 1000V Layer 2 Switching Configuration Guide, Release 4.2(1)SV2(1.1)
Configuring IGMP Snooping
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Configuring IGMP Snooping

This chapter contains the following sections:

Information about IGMP Snooping

Introduction

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.

Figure 1. IGMP Snooping Switch



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 IP address rather than 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.

IGMPv1 and IGMPv2

If no more than one host is attached to each VLAN switch port, then you can configure the fast leave feature in IGMPv2. 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 time-out 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.


Note

The software ignores the configuration of the last member query interval when you enable the fast leave feature because it does not check for remaining hosts.

IGMPv3

IGMPv3 snooping provides constrained flooding based on the group IP information in the IGMPv3 reports.

By default, the software tracks hosts on each VLAN port. The explicit tracking feature provides a fast leave mechanism. Because every IGMPv3 host sends membership reports, report suppression limits the amount of traffic that the switch sends to other multicast capable routers.

Even though the IGMPv3 membership reports provide a full accounting of group members on a LAN segment, when the last host leaves, the querier sends a membership query. You can configure the parameter last member query interval. If no host responds before the time-out, the software removes the group state. If the querier specifies a mean-response-time (MRT) value in the queries, it overrides the last member query interval configuration.

Prerequisites for IGMP Snooping

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 Cisco Nexus 1000V, report suppression is not supported and is disabled by default.

When an IGMP snooping query feature is enabled, 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.

Default Settings

Table 1 Default IGMP Snooping Settings
Parameters Default

IGMP snooping

Enabled

IGMPv3 Explicit tracking

Enabled

IGMPv2 Fast leave

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

Enabling or Disabling IGMP Snooping Globally for the VSM

Use this procedure to enable or disable IGMP snooping globally for the VSM. IGMP snooping is enabled globally on the VSM (the default). If enabled globally, you can turn it on or off per VLAN.

Before You Begin

You are logged in to the CLI in EXEC mode.

Procedure
     Command or ActionPurpose
    Step 1switch# configure terminal 

    Enters global configuration mode.

     
    Step 2switch(config)# [ no ] ip igmp snooping 

    Enables or disables IGMP snooping in the running configuration for all VLANs. The default is enabled. If you have previously disabled the feature then you can enable it with this command.

     
    Step 3switch(config)# show ip igmp snooping [ vlan vlan-id ]  (Optional)

    Displays the configuration for verification.

    Note   

    If disabled, then IGMP snooping on all VLANs is disabled.

     
    Step 4switch(config)# copy running-config startup-config  (Optional)

    Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

      
    switch# configure terminal
switch(config)# no ip igmp snooping
switch(config)# show ip igmp snooping
Global IGMP Snooping Information:
  IGMP Snooping enabled
  IGMPv1/v2 Report Suppression disabled
  IGMPv3 Report Suppression disabled
  Link Local Groups Suppression enabled

IGMP Snooping information for vlan 1
  IGMP snooping enabled
  IGMP querier none
  Switch-querier disabled
  IGMPv3 Explicit tracking enabled
  IGMPv2 Fast leave disabled
  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
  Active ports:
    
--More--
switch(config)#

    Configuring IGMP Snooping on a VLAN

    Use this procedure to configure IGMP snooping on a VLAN. IGMP snooping is enabled by default for all VLANs in the VSM.

    Before You Begin

    You are logged in to the CLI in EXEC mode.


    Note

    If IGMP snooping is disabled globally, it takes precedence over the VLAN state.

    Procedure
       Command or ActionPurpose
      Step 1switch# configure terminal 

      Enters global configuration mode.

       
      Step 2switch(config)# vlan vlan-id  

      Enters configuration mode for the specified VLAN.

       
      Step 3switch(config-vlan)# [ no ] ip igmp snooping 

      Enables or disables IGMP snooping in the running configuration for the specific VLAN. If IGMP snooping is enabled for the VSM, then IGMP snooping is enabled for the VLAN by default.

      Note   

      IGMP snooping must be enabled globally (the default) in order to toggle it on or off per VLAN. If IGMP snooping is disabled globally, then it cannot be enabled per VLAN.

       
      Step 4switch(config-vlan)# [ no ] ip igmp snooping explicit-tracking  (Optional)

      Tracks IGMPv3 membership reports from individual hosts for each port on a per-VLAN basis in the running configuration.

      The default is enabled.

       
      Step 5switch(config-vlan)# [ no ] ip igmp snooping fast-leave  (Optional)

      Enables fast-leave for the specified VLAN in the running configuration.

      Fast-leave supports IGMPv2 hosts that cannot be explicitly tracked because of the host report suppression mechanism of the IGMPv2 protocol.

      Enables the software to remove the group state when it receives an IGMP Leave report without sending an IGMP query message. This parameter is used for IGMPv2 hosts when no more than one host is present on each VLAN port.

      When you enable fast leave, the IGMP software assumes that no more than one host is present on each VLAN port.

      The default is disabled.

       
      Step 6switch(config-vlan)# [ no ] ip igmp snooping last-member-query-interval seconds  (Optional)

      Sets the interval the software waits after sending an IGMP query to verify that no hosts that want to receive a particular multicast group remain on a network segment. If no hosts respond before the last member query interval expires, the software removes the group from the associated VLAN port.

      Values range from 1 to 25 seconds. The default is 1 second.

       
      Step 7switch(config-vlan)# [ no ] ip igmp snooping mrouter interface type if_id  (Optional)

      Configures a static connection for the VLAN to a multicast router in the running configuration.

      The interface to the router must be in the specified VLAN. You can specify the interface by the type and the number, such as ethernet slot/port.

      vEths are not supported as router ports.

       
      Step 8switch(config-vlan)# [ no ] ip igmp snooping static-group group-ip-addr interface type if_id  (Optional)

      Configures a VLAN Layer 2 port as a static member of a multicast group in the running configuration.

      You can specify the interface by the type and the number, such as ethernet slot/port.

       
      Step 9switch(config-vlan)# [ no ] ip igmp snooping link-local-groups-suppression  (Optional)

      Configures link-local groups suppression. The default is enabled.

      Note   

      You can apply link-local groups suppression to all interfaces in the VSM by entering this command in global configuration mode.

       
      Step 10switch(config-vlan)# show ip igmp snooping [ vlan vlan-id ]  (Optional)

      Displays the configuration for verification.

       
      Step 11switch(config-vlan)# copy running-config startup-config  (Optional)

      (Optional) Saves the running configuration persistently through reboots and restarts by copying it to the startup configuration.

        
      switch# configure terminal
switch(config)# vlan 2
switch(config-vlan)# ip igmp snooping
switch(config-vlan)# ip igmp snooping explicit-tracking
switch(config-vlan)# ip igmp snooping fast-leave
switch(config-vlan)# ip igmp snooping last-member-query-interval 3
switch(config-vlan)# ip igmp snooping mrouter interface ethernet 2/1
switch(config-vlan)# ip igmp snooping static-group 230.0.0.1 interface ethernet 2/1
switch(config-vlan)# ip igmp snooping link-local-groups-suppression
switch(config-vlan)# show ip igmp snooping vlan 2

IGMP Snooping information for vlan 5
  IGMP snooping enabled
  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
  Active ports:
switch(config-vlan)#

      Verifying the IGMP Snooping Configuration

      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.

      show ip igmp snooping explicit-tracking [ vlan vlan-id ]

      Displays IGMP snooping explicit tracking information by VLAN.

      For detailed information about commands and their output, see the Cisco Nexus 1000V Command Reference.

      Example Configuration IGMP Snooping

      This example shows how to enable IP IGMP snooping for the VSM, and make the following optional configurations for VLAN 2:

      • Tracking of IGMPv3 membership reports from individual hosts for each port.
      • A static connection to a multicast router through Ethernet 2/1.
      • Static membership in multicast group 230.0.0.1. 
      switch# configure terminal
switch# ip igmp snooping
switch# vlan 2
switch# ip igmp snooping
switch# ip igmp snooping explicit-tracking
switch# ip igmp snooping mrouter interface ethernet 2/1
switch# ip igmp snooping static-group 230.0.0.1 interface ethernet 2/1
switch# show ip igmp snooping vlan 2
switch# copy running-config startup-config
switch#

      Feature History for IGMP Snooping

      Feature Name

      Releases

      Description

      Link-local suppression

      4.2(1)SV1(4)

      Added support to enable or disable link-local group suppression.

      Report suppression

      4.0(4)SV1(3)

      Removed support for report suppression.

      IGMP Snooping

      4.0(4)SV1(1)

      This feature was introduced.