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Understanding IGMP Snooping

Snooping is a general way for Layer 2 devices, such as Juniper Networks MX Series Ethernet Services Routers, to implement a series of procedures to “snoop” at the Layer 3 packet content to determine which actions are to be taken to process or forward a frame. More specific forms of snooping, such as Internet Group Membership Protocol (IGMP ) snooping or Protocol Independent Multicast (PIM) snooping, are used with multicast.

Layer 2 devices (LAN switches or bridges) handle multicast packets and the frames that contain them much in the same way the Layer 3 devices (routers) handle broadcasts. So, a Layer 2 switch processes an arriving frame having a multicast destination media access control (MAC) address by forwarding a copy of the packet (frame) onto each of the other network interfaces of the switch that are in a forwarding state.

However, this approach (sending multicast frames everywhere the device can) is not the most efficient use of network bandwidth, particularly for IPTV applications. IGMP snooping functions by “snooping” at the IGMP packets received by the switch interfaces and building a multicast database similar to that a multicast router builds in a Layer 3 network. Using this database, the switch can forward multicast traffic only onto downstream interfaces with interested receivers, and this technique allows more efficient use of network bandwidth.

You configure IGMP snooping for each bridge on the router. A bridge instance without qualified learning has just one learning domain. For a bridge instance with qualified learning, snooping will function separately within each learning domain in the bridge. That is, IGMP snooping and multicast forwarding will proceed independently in each learning domain in the bridge.

This discussion focuses on bridge instances without qualified learning (those forming one learning domain on the device). Therefore, all the interfaces mentioned are logical interfaces of the bridge or VPLS instance.

• Bridge or VPLS instance interfaces are either multicast-router interfaces or host-side interfaces.
• IGMP snooping supports proxy mode or without-proxy mode.

Note: When integrated routing and bridging (IRB) is used, if the router is an IGMP querier, any leave message received on any Layer 2 interface will cause a group-specific query on all Layer 2 interfaces (as a result of this practice, some corresponding reports might be received on all Layer 2 interfaces). However, if some of the Layer 2 interfaces are also router (Layer 3) interfaces, reports and leaves from other Layer 2 interfaces will not be forwarded on those interfaces.

If an IRB interface is used as an outgoing interface in a multicast forwarding cache entry (as determined by the routing process), then the output interface list is expanded into a subset of the Layer 2 interface in the corresponding bridge. The subset is based on the snooped multicast membership information, according to the multicast forwarding cache entry installed by the snooping process for the bridge.

If no snooping is configured, the IRB output interface list is expanded to all Layer 2 interfaces in the bridge.

The Junos OS does not support IGMP snooping in a VPLS configuration on a virtual switch. This configuration is disallowed in the CLI.