IGMP snooping provides a way to constrain multicast traffic at Layer 2. By snooping the IGMP membership reports sent by hosts in the bridge domain, the IGMP snooping application can set up Layer 2 multicast forwarding tables to deliver traffic only to ports with at least one interested member, significantly reducing the volume of multicast traffic.

Configured at Layer 3, IGMP provides a means for hosts in an IPv4 multicast network to indicate which multicast traffic they are interested in and for routers to control and limit the flow of multicast traffic in the network at Layer 3.

IGMP snooping uses the information in IGMP membership report messages to build corresponding information in the forwarding tables to restrict IP multicast traffic at Layer 2. The forwarding table entries are in the form <Route, OIF List>, where:

• Route is a <*, G> route or <S, G> route, where * is any source, G is group and S is the source.

• OIF List comprises all bridge ports that have sent IGMP membership reports for the specified route plus all multicast router (mrouter) ports in the bridge domain.

Implemented in a multicast network, IGMP snooping has the following attributes:

• In its basic form, it reduces bandwidth consumption by reducing multicast traffic that would otherwise flood an entire VPLS bridge domain.

• With the use of some optional configurations, it provides security between bridge domains by filtering the IGMP reports received from hosts on one bridge port and preventing leakage towards the hosts on other bridge ports.

• Using optional configurations, reduces the traffic impact on upstream IP multicast routers by suppressing IGMP membership reports (IGMPv2) or by acting as an IGMP proxy reporter (IGMPv3) to the upstream IP multicast router.

All high availability features apply to the IGMP snooping processes with no additional configuration beyond enabling IGMP snooping. The following high availability features are supported:

• Process restarts

• RP Failover

• Stateful Switch-Over (SSO)

• Non-Stop Forwarding (NSF)—Forwarding continues unaffected while the control plane is restored following a process restart or route processor (RP) failover.

• Line card online insertion and removal (OIR)

IGMP snooping operates at the bridge domain level. When IGMP snooping is enabled on a bridge domain, the snooping functionality applies to all ports under the bridge domain, including:

• Physical ports under the bridge domain.

• Ethernet flow points (EFPs)—An EFP can be a VLAN, VLAN range, list of VLANs, or an entire interface port.

• Ethernet bundles—Ethernet bundles include IEEE 802.3ad link bundles and Cisco EtherChannel bundles. From the perspective of the IGMP snooping application, an Ethernet bundle is just another EFP. The forwarding application in the Cisco NCS 540 Series Routers randomly nominates a single port from the bundle to carry the multicast traffic.

IGMP snooping for bridge domains without Bridged Virtual Interface (BVI) is supported with the following design consideration:

You must configure the multicast-source ipv4 command in the source switch where IGMP snooping is enabled as seen in the following example:

l2vpn 
bridge group 1 
bridge-domain 1 
multicast-source ipv4 
igmp snooping profile grp1 
! 
interface TenGigE0/0/0/3.31 //Source 
! 
interface TenGigE0/0/0/3.32 
! 
routed interface BVI1 

IGMP snooping classifies each port (for example, EFPs, PWs, physical ports, or EFP bundles) as one of the following:

• Multicast router ports (mrouter ports)—These are ports to which a multicast-enabled router is connected. Mrouter ports are usually dynamically discovered, but may also be statically configured. Multicast traffic is always forwarded to all mrouter ports, except when an mrouter port is the ingress port.

• Host ports—Any port that is not an mrouter port is a host port.

The following tables describe traffic handling behaviors by IGMP snooping and host ports.

By default, IGMP snooping supports IGMPv2 and IGMPv3. The version of the IGMP querier discovered in the bridge domain determines the operational version of the snooping processes. If you change the default, configuring IGMP snooping to support a minimum version of IGMPv3, IGMP snooping ignores any IGMPv2 queriers.

To create a profile, use the igmp snooping profile command in global configuration mode.

To attach a profile to a bridge domain, use the igmp snooping profile command in l2vpn bridge group bridge domain configuration mode. To attach a profile to a port, use the igmp snooping profile command in the interface configuration mode under the bridge domain. To detach a profile, use the no form of the command in the appropriate configuration mode.

When you detach a profile from a bridge domain or a port, the profile still exists and is available for use at a later time. Detaching a profile has the following results:

• If you detach a profile from a bridge domain, IGMP snooping is deactivated in the bridge domain.

• If you detach a profile from a port, IGMP snooping configuration values for the port are instantiated from the bridge domain profile.

You cannot make changes to an active profile. An active profile is one that is currently attached.

• If the active profile is configured under the bridge, you must detach it from the bridge, and reattach it.

• If the active profile is configured under a specific bridge port, you must detach it from the bridge port, and reattach it.

Another way to do this is to create a new profile incorporating the desired changes and attach it to the bridges or ports, replacing the existing profile. This deactivates IGMP snooping and then reactivates it with parameters from the new profile.

The minimum-version command determines which IGMP versions are supported by IGMP snooping in the bridge domain:

• When minimum-version is 2, IGMP snooping intercepts IGMPv2 and IGMPv3 messages. This is the default value.

• When minimum-version is 3, IGMP snooping intercepts only IGMPv3 messages and drops all IGMPv2 messages.

IGMPv1 is not supported. The scope for this command is the bridge domain. The command is ignored in a profile attached to a port.

The group membership interval (GMI) controls when IGMP snooping expires stale group membership states. The show igmp snooping group command shows groups with an expiry time of 0 until that stale state is cleaned up following the next query interval.

The GMI is calculated as:

GMI = (robustness-variable * query-interval) + maximum-response-time

where:

• maximum-response-time (MRT) is the amount of time during which receivers are required to report their membership state.

• robustness-variable is an integer used to influence the calculated GMI.

• query-interval is the amount of time between general queries.

Values for the components in the GMI are obtained as follows:

• MRT is advertised in the general query, for both IGMPv2 and IGMPv3.

• If the querier is running IGMPv2, IGMP snooping uses the IGMP-snooping-configured values for the robustness-variable and query-interval. These parameter values must match the configured values for the querier. In most cases, if you are interacting with other Cisco routers, you should not need to explicitly configure these values—the default values for IGMP snooping should match the default values of the querier. If they do not, use the querier robustness-variable and querier query-interval commands to configure matching values.

• IGMPv3 general queries convey values for robustness-variable and query-interval (QRV and QQI, respectively). IGMP snooping uses the values from the query, making the IGMP snooping GMI exactly match that of the querier.