In Layer 2 switches, multicast traffic is treated like an unknown MAC address or broadcast frame, which causes the incoming frame to be flooded out (broadcast) on every port within a VLAN. Although this is acceptable behavior for unknowns and broadcast frames, this flooded multicast traffic may result in wasted bandwidth on network segments and end stations, as IP multicast hosts can join and be interested in only specific multicast groups.
IGMP snooping entails using information in Layer 3 protocol headers of multicast control messages to determine the processing at Layer 2. By doing so, an IGMP snooping switch provides the benefit of conserving bandwidth on those segments of the network in which no node has expressed interest in receiving packets addressed to the group address.
IGMP snooping can be enabled in the context of VPLS services. The IGMP snooping feature allows for optimization of the multicast data flow to only those SAPs and SDPs that are members of the group. The system builds a database of group members per service by listening to IGMP queries and reports from each SAP and SDP:
When the switch receives an IGMP report from a host for a multicast group, the switch adds the host port number to the forwarding table entry.
When it receives an IGMP leave message from a host, it removes the host port from the table entry, if no other group members are present. It also deletes entries if it does not receive periodic IGMP membership reports from the multicast clients.
The following are IGMP snooping features:
IGMP v1, v2, and v3 are supported (RFC 1112, Host Extensions for IP Multicasting, and RFC 2236, Internet Group Management Protocol, Version 2). On 7210 SAS-K 2F6C4T and 7210 SAS-K 3SFP+ 8C, IGMP v1, v2, v3 is supported with both with L2 uplinks and MPLS uplinks.
IGMP snooping can be enabled and disabled on individual VPLS service instances.
IGMP snooping can be configured on individual SAPs and SDPs that are part of a VPLS service. When IGMP snooping is enabled on a VPLS service, all its contained SAPs and SDPs automatically have snooping enabled.
Fast leave terminates the multicast session immediately, rather than using the standard group-specific query to check if other group members are present on the network.
SDPs and SAPs can be statically configured as multicast router ports. This allows the operator to control the set of ports to which IGMP membership reports are forwarded.
Static multicast group membership on a per SAP and SDP basis can be configured.
The maximum number of multicast groups (static and dynamic) that a SAP and SDP can join can be configured. An event is generated when the limit is reached.
The maximum number of multicast groups (static and dynamic) that a VPLS instance simultaneously supports can be configured.
Proxy summarization of IGMP messages reduces the number of IGMP messages processed by upstream devices in the network.
IGMP filtering allows a subscriber to a service or the provider to block, receive, or transmit permission (or both) to individual hosts or a range of hosts. The following types of filters can be defined:
Filter group membership that report from a particular host or range of hosts. This filtering is performed by importing an appropriately-defined routing policy into the SAP and SDP.
Filters that prevent a host from transmitting multicast streams into the network. The operator can define a data-plane filter (ACL) that drops all multicast traffic, and apply this filter to a SAP and SDP.