IGMP Snooping

IGMP Snooping

Introduction

IGMP Snooping allows the switch to forward multicast traffic only to those ports that request it. IGMP Snooping prevents multicast traffic from being flooded to all data ports. The switch learns which server hosts are interested in receiving multicast traffic, and forwards it only to ports connected to those servers.

The following topics are discussed in this chapter:

•Overview

•IGMPv3

•FastLeave

•IGMP Filtering

•Static Multicast Router

•IGMP Snooping Configuration example

Overview

Internet Group Management Protocol (IGMP) is used by IP Multicast routers to learn about the existence of host group members on their directly attached subnet (see RFC 2236). The IP Multicast routers get this information by broadcasting IGMP Query Reports and listening for IP hosts reporting their host group memberships. This process is used to set up a client/server relationship between an IP Multicast source that provides the data streams and the clients that want to receive the data.

IGMP Snooping conserves bandwidth. With IGMP Snooping, the switch learns which ports are interested in receiving multicast data, and forwards multicast data only to those ports. In this way, other ports are not burdened with unwanted multicast traffic.

The switch currently supports snooping for IGMP version 1, version 2, and version 3.

The switch can sense IGMP Membership Reports from attached host servers and act as a proxy to set up a dedicated path between the requesting host and a local IP Multicast router. After the pathway is established, the switch blocks the IP Multicast stream from flowing through any port that does not connect to a host member, thus conserving bandwidth.

The client-server path is set up as follows:

•An IP Multicast Router (Mrouter) sends Membership Queries to the switch, which forwards them to all ports in a given VLAN.

•Hosts that want to receive the multicast data stream send Membership Reports to the switch, which sends a proxy Membership Report to the Mrouter.

•The switch sets up a path between the Mrouter and the host, and blocks all other ports from receiving the multicast.

•Periodically, the Mrouter sends Membership Queries to ensure that the host wants to continue receiving the multicast. If the host fails to respond with a Membership Report, the Mrouter stops sending the multicast to that path.

118