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IGMP Snooping on EX-series Switches Overview

Internet Group Management Protocol (IGMP) snooping regulates multicast traffic in a switched network. With IGMP snooping enabled, a LAN switch monitors the IGMP transmissions between a host (a network device) and a multicast router, keeping track of the multicast groups and associated member ports. The switch uses that information to make intelligent multicast-forwarding decisions and forward traffic to the intended destination interfaces. EX-series switches support IGMPv1 and IGMPv2.

For details on IGMPv1 and IGMPv2, see the following standards:

How IGMP Snooping Works

An EX-series switch usually learns unicast MAC addresses by checking the source address field of the frames it receives. However, a multicast MAC address can never be the source address for a packet. As a result, the switch floods multicast traffic on the VLAN, consuming significant amounts of bandwidth.

IGMP snooping regulates multicast traffic on a VLAN to avoid flooding. When IGMP snooping is enabled, the switch intercepts IGMP packets and uses the content of the packets to build a multicast cache table. The cache table is a database of multicast groups and their corresponding member ports. The cache table is then used to regulate multicast traffic on the VLAN.

When the switch receives multicast packets, it uses the cache table to selectively forward the packets only to the ports that are members of the destination multicast group. Figure 1 shows an example of IGMP traffic flow with IGMP snooping enabled.

Figure 1: IGMP Traffic Flow with IGMP Snooping Enabled

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How Hosts Join and Leave Multicast Groups

Hosts can join multicast groups in either of two ways:

  • By sending an unsolicited IGMP join message to a multicast router that specifies the IP multicast that the host is attempting to join.
  • By sending an IGMP join message in response to a general query from a multicast router.

A multicast router continues to forward multicast traffic to a VLAN provided that at least one host on that VLAN responds to the periodic general IGMP queries. For a host to remain a member of a multicast group, therefore, it must continue to respond to the periodic general IGMP queries.

To leave a multicast group, a host can either not respond to the periodic general IGMP queries, which results in a “silent leave” (the only leave option for hosts connected to switches running IGMPv1), or send a group-specific IGMPv2 leave message.