IGMP Snooping Explained [Basic Configuration NX-OS]

  • To learn more about IGMP, please Check IGMP Post

IGMP Snooping Overview:

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. IGMP snooping 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.

This 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.

IGMP communication runs between the receiver and the local router. Layer 2 switches do not participate in IGMP communication. However, they need to be IGMP-aware to effectively forward multicast traffic to interested receivers without flooding it to other ports.

The switch examines the content of the IGMP messages to determine which ports need to receive multicast traffic for a group


IGMP Snooping Querier

When PIM is not enabled on an interface because the multicast traffic does not need to be routed, you must configure an IGMP snooping querier to send membership queries. You define the querier in a VLAN that contains multicast sources and receivers but no other active querier.

The querier can be configured to use any IP address in the VLAN.

As a best practice, a unique IP address, one that is not already used by the switch interface or the Hot Standby Router Protocol (HSRP) virtual IP address, should be configured so as to easily reference the querier.

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 Snooping Configuration

IGMP snooping is enabled by default for all VLANs on Cisco Nexus switches. You can disable IGMP snooping globally or per VLAN:

Switch(config)# no ip igmp snooping 
Switch(config)# vlan vlan-id
Switch(config-vlan)# no ip igmp snooping

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