Cisco Nexus 6000 Series NX-OS Layer 2 Switching Configuration Guide, Release 6.x
Configuring IGMP Snooping
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Configuring IGMP Snooping

This chapter contains the following sections:

Information About IGMP Snooping

The IGMP snooping software examines IGMP protocol messages within a VLAN to discover which interfaces are connected to hosts or other devices interested in receiving this traffic. Using the interface information, IGMP snooping can reduce bandwidth consumption in a multiaccess LAN environment to avoid flooding the entire VLAN. The IGMP snooping feature tracks which ports are attached to multicast-capable routers to help it manage the forwarding of IGMP membership reports. The IGMP snooping software responds to topology change notifications.


Note


IGMP snooping is supported on all Ethernet interfaces. The term snooping is used because Layer 3 control plane packets are intercepted and influence Layer 2 forwarding decisions.


Cisco NX-OS supports IGMPv2 and IGMPv3. IGMPv2 supports IGMPv1, and IGMPv3 supports IGMPv2. Although not all features of an earlier version of IGMP are supported, the features related to membership query and membership report messages are supported for all IGMP versions.

The following figure shows an IGMP snooping switch that is located 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 Cisco NX-OS IGMP snooping software supports 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 http:/​/​tools.ietf.org/​wg/​magma/​draft-ietf-magma-snoop/​rfc4541.txt.

IGMPv1 and IGMPv2

Both IGMPv1 and IGMPv2 support membership report suppression, which means that if two hosts on the same subnet want to receive multicast data for the same group, the host that receives a member report from the other host suppresses sending its report. Membership report suppression occurs for hosts that share a port.

If no more than one host is attached to each VLAN switch port, 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 timeout to indicate that no hosts remain that want to receive multicast data for a particular group.


Note


Cisco NX-OS 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

The IGMPv3 snooping implementation on the switch forwards IGMPv3 reports to allow the upstream multicast router to do source-based filtering.

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, a report suppression feature limits the amount of traffic the switch sends to other multicast-capable routers. When report suppression is enabled, and no IGMPv1 or IGMPv2 hosts request the same group, the software provides proxy reporting. The proxy feature builds the group state from membership reports from the downstream hosts and generates membership reports in response to queries from upstream queriers.

Even though the IGMPv3 membership reports provide a full accounting of group members on a LAN segment, when the last host leaves, the software sends a membership query. You can configure the parameter last member query interval. If no host responds before the timeout, the software removes the group state.

IGMP Snooping Querier

When there is no multicast router in the VLAN to originate the queries, you must configure an IGMP snooping querier to send membership queries.

When an IGMP snooping querier is enabled, it sends out periodic IGMP queries that trigger IGMP report messages from hosts that want to receive IP multicast traffic. IGMP snooping listens to these IGMP reports to establish appropriate forwarding.

IGMP Forwarding

The Cisco Nexus device supports snooping based on (S,G)/(*,G) IP addresses. Multicast MAC aliasing does not apply for Cisco Nexus devices and the snooped entries are programmed in the FIB tables and not in MAC table.

When a host connected to the switch wants to join an IP multicast group, it sends an unsolicited IGMP join message, specifying the IP multicast group to join. Alternatively, when the switch receives a general query from a connected router, it forwards the query to all interfaces, physical and virtual, in the VLAN. Hosts that want 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.

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 to only those hosts listed in the forwarding table for that multicast group.

When hosts want to leave a multicast group, they can either 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 determine 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 (S,G) or (*,G) 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.

Configuring IGMP Snooping Parameters

To manage the operation of the IGMP snooping process, you can configure the optional IGMP snooping parameters described in the following table.

Table 1 IGMP Snooping Parameters

Parameter

Description

IGMP snooping

Enables IGMP snooping on a per-VLAN basis. The default is enabled.

Note   

If the global setting is disabled, all VLANs are treated as disabled, whether they are enabled or not.

Explicit tracking

Tracks IGMPv3 membership reports from individual hosts for each port on a per-VLAN basis. The default is enabled.

Fast leave

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. The default is disabled.

Last member query interval

Sets the interval that 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.

Snooping querier

Configures a snooping querier on an interface when there is no multicast router in the VLAN to generate queries. The default is disabled.

Report suppression

Limits the membership report traffic sent to multicast-capable routers. When you disable report suppression, all IGMP reports are sent as is to multicast-capable routers. The default is enabled.

Multicast router

Configures a static connection to a multicast router. The interface to the router must be in the selected VLAN.

Multicast router vpc-peer-link

Configures a static connection to a virtual port channel (vPC) peer link.

By default, the vPC peer link is considered a multicast router port and the multicast packet is sent to the peer link for each receiver VLAN.

To send the multicast traffic over a vPC peer link to each receiver VLAN that has orphan ports, use the no ip igmp snooping mrouter vpc-peer-link command. If you use the no ip igmp snooping mrouter vpc-peer-link command, the multicast traffic is not sent over to a peer link for the source VLAN and receiver VLAN unless there is an orphan port in the VLAN. The IGMP snooping mrouter VPC peer link should also be globally disabled on the peer VPC switch.

Static group

Configures an interface that belongs to a VLAN as a static member of a multicast group.

You can disable IGMP snooping either globally or for a specific VLAN.

Procedure
      Command or Action Purpose
    Step 1 switch# configure terminal 

    Enters global configuration mode.

     
    Step 2 switch(config)# ip igmp snooping  

    Globally enables IGMP snooping. The default is enabled.

    Note   

    If the global setting is disabled, all VLANs are treated as disabled, whether they are enabled or not.

     
    Step 3 switch(config)# vlan configuration vlan-id  

    Enters VLAN configuration mode.

     
    Step 4 switch(config-vlan)# ip igmp snooping  

    Enables IGMP snooping for the current VLAN. The default is enabled.

    Note   

    If IGMP snooping is enabled globally, this command is not required.

     
    Step 5 switch(config-vlan)# ip igmp snooping explicit-tracking  

    Tracks IGMPv3 membership reports from individual hosts for each port on a per-VLAN basis. The default is enabled on all VLANs.

     
    Step 6 switch(config-vlan)# ip igmp snooping fast-leave  

    Supports IGMPv2 hosts that cannot be explicitly tracked because of the host report suppression mechanism of the IGMPv2 protocol. 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 for all VLANs.

     
    Step 7 switch(config-vlan)# ip igmp snooping last-member-query-interval seconds  

    Removes the group from the associated VLAN port if no hosts respond to an IGMP query message before the last member query interval expires. Values range from 1 to 25 seconds. The default is 1 second.

     
    Step 8 switch(config-vlan)# ip igmp snooping querier IP-address  

    Configures a snooping querier when you do not enable PIM because multicast traffic does not need to be routed. The IP address is used as the source in messages. The default is disabled.

     
    Step 9 switch(config-vlan)# ip igmp snooping report-suppression  

    Limits the membership report traffic sent to multicast-capable routers. When you disable report suppression, all IGMP reports are sent as is to multicast-capable routers. The default is enabled.

     
    Step 10 switch(config-vlan)# ip igmp snooping mrouter interface interface  

    Configures a static connection to a multicast router. The interface to the router must be in the selected VLAN. You can specify the interface by type and number.

     
    Step 11 switch(config-vlan)# ip igmp snooping mrouter vpc-peer-link  

    Configures a static connection to a virtual port channel (vPC) peer link. By default, the vPC peer link is considered as a multicast router port and the multicast packet is sent to the peer link for each receiver VLAN. To send the multicast traffic over a vPC peer link to each receiver VLAN that has orphan ports, use the no ip igmp snooping mrouter vpc-peer-link command. The IGMP snooping mrouter VPC peer link should also be globally disabled on the peer VPC switch.

     
    Step 12 switch(config-vlan)# ip igmp snooping static-group group-ip-addr [source source-ip-addr] interface interface  

    Configures an interface belonging to a VLAN as a static member of a multicast group. You can specify the interface by type and number.

     

    This example shows how to configure IGMP snooping parameters for a VLAN:

    switch# configure terminal
    switch(config)# vlan configuration 5
    switch(config-vlan)# ip igmp snooping last-member-query-interval 3
    switch(config-vlan)# ip igmp snooping querier 172.20.52.106
    switch(config-vlan)# ip igmp snooping explicit-tracking
    switch(config-vlan)# ip igmp snooping fast-leave
    switch(config-vlan)# ip igmp snooping report-suppression
    switch(config-vlan)# ip igmp snooping mrouter interface ethernet 1/10
    switch(config-vlan)# ip igmp snooping mrouter vpc-peer-link
    switch(config-vlan)# ip igmp snooping static-group 230.0.0.1 interface ethernet 1/10
    switch(config-vlan)# end
    
    

    This example shows how to configure a static connection to a vPC peer link and how to remove the static connection to a vPC peer link:

    switch(config)# ip igmp snooping mrouter vpc-peer-link
    switch(config)# no ip igmp snooping mrouter vpc-peer-link
    Warning: IGMP Snooping mrouter vpc-peer-link should be globally disabled on peer VPC switch as well.
    switch(config)#
    

    Verifying IGMP Snooping Configuration

    Use the following commands to verify the IGMP snooping configuration.

    Command

    Description

    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 mrouter [[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.

    This example shows how to verify the IGMP snooping parameters:

    switch# show ip igmp snooping
    Global IGMP Snooping Information:
      IGMP Snooping enabled
    IGMP Snooping information for vlan 1
      IGMP snooping enabled
      IGMP querier none
      Switch-querier disabled
      Explicit tracking enabled
      Fast leave disabled
      Report suppression enabled
      Router port detection using PIM Hellos, IGMP Queries
      Number of router-ports: 0
      Number of groups: 0
    IGMP Snooping information for vlan 5
    IGMP snooping enabled
      IGMP querier present, address: 192.0.2.1, version: 3
      Querier interval: 125 secs
      Querier last member query interval: 10 secs
      Querier robustness: 2
      Switch-querier enabled, address 192.0.2.1, currently running
      Explicit tracking enabled
      Fast leave enabled
      Report suppression enabled
      Router port detection using PIM Hellos, IGMP Queries
      Number of router-ports: 1
      Number of groups: 1