IP Multicast Internet Group Management Protocol (IGMP), which runs at Layer 3 on a multicast device, generates Layer 3 IGMP
queries in subnets where the multicast traffic must be routed. IGMP (on a device) sends out periodic general IGMP queries.
IGMP Snooping is an Ethernet Virtual Circuit (EVC)-based feature set. EVC decouples the concept of VLAN and broadcast domain.
An EVC is an end-to-end representation of a single instance of a Layer 2 service being offered by a provider. In the Cisco
EVC framework, bridge domains are made up of one or more Layer 2 interfaces known as service instances. A service instance
is the instantiation of an EVC on a given port on a given device. A service instance is associated with a bridge domain based
on the configuration.
When you enable EVC-based IGMP snooping on a bridge domain, the bridge domain interface responds at Layer 2 to the IGMP queries
with only one IGMP join request per Layer 2 multicast group. Each bridge domain represents a Layer 2 broadcast domain. The
bridge domain interface creates one entry per subnet in the Layer 2 forwarding table for each Layer 2 multicast group from
which it receives an IGMP join request. All hosts interested in this multicast traffic send IGMP join requests and are added
to the forwarding table entry. During a Layer 2 lookup on a bridge domain to which the bridge domain interface belongs, the
bridge domain forwards the packets to the correct EFP. When the bridge domain interface hears the IGMP Leave group message
from a host, it removes the table entry of the host.
IGMP snooping is
supported on Metro IP and Metro Aggregate licenses on the
Cisco ASR 920 Series
Routers. IGMP snooping is supported with MSTP, REP, and G.8032. IGMP snooping
is also supported on the port-channel interfaces.