IGMP Snooping Overview

When a device processes a multicast packet, by default, it broadcasts the packets to all ports except the incoming port of a VLAN. Packets are flooded by hardware without going to the CPU. This behavior causes some clients to receive unwanted traffic.

IGMP snooping provides multicast containment by forwarding traffic to only the ports that have IGMP receivers for a specific multicast group (destination address). A device maintains the IGMP group membership information by processing the IGMP reports and leave messages, so traffic can be forwarded to ports receiving IGMP reports.

An IPv4 multicast address is a destination address in the range of 224.0.0.0 to 239.255.255.255. Addresses of 224.0.0.X are reserved. Because packets destined for these addresses may require VLAN flooding, devices do not snoop in the reserved range. Data packets destined to addresses in the reserved range are flooded to the entire VLAN by hardware, and mirrored to the CPU. Multicast data packets destined for the non-reserved range of addresses are snooped. A client must send IGMP reports in order to receive traffic.

An IGMP device's responsibility is to broadcast general queries periodically, and to send group queries when receiving a leave message, to confirm that none of the clients on the port still want specific traffic before removing the traffic from the port. IGMP V2 lets clients specify what group (destination address) receives the traffic, but clients do not specify the source of the traffic. IGMP V3 is for source-specific multicast traffic, adding the capability for clients to INCLUDE or EXCLUDE specific traffic sources. An IGMP V3 device port state could be INCLUDE or EXCLUDE, and there are different types of group records for client reports.

The receivers respond to general or group queries by sending a membership report that contains one or more of the following records associated with a specific group:

  • Current-state record that indicates from which sources the interface wants to receive and not receive traffic. This record contains the source address of interfaces and whether or not traffic will be included (IS_IN) or not excluded (IS_EX) from this source.
  • Filter-mode-change record. If the interface state changes from IS_IN to IS_EX, a TO_EX record is included in the membership report. Likewise, if the interface state changes from IS_EX to IS_IN, a TO_IN record appears in the membership report.
  • An IGMP V2 leave report is equivalent to a TO_IN (empty) record in IGMP V3. This record means that no traffic from this group will be received regardless of the source.
  • An IGMP V2 group report is equivalent to an IS_EX (empty) record in IGMP V3. This record means that all traffic from this group will be received regardless of source.
  • Source-list-change record. If the interface wants to add or remove traffic sources from its membership report, the report can contain an ALLOW record, which includes a list of new sources from which the interface wishes to receive traffic. It can also contain a BLOCK record, which lists the current traffic sources from which the interface wants to stop receiving traffic.

IGMP protocols provide a method for clients and a device to exchange messages, and let the device build a database indicating which port wants what traffic. The protocols do not specify forwarding methods. They require IGMP snooping or multicast protocols such as PIM to handle packet forwarding. PIM can route multicast packets within and outside a VLAN, while IGMP snooping can switch packets only within a VLAN.

If a VLAN is not IGMP snooping-enabled, it floods multicast data and control packets to the entire VLAN in hardware. When snooping is enabled, IGMP packets are trapped to the CPU. Data packets are mirrored to the CPU in addition to being VLAN flooded. The CPU then installs hardware resources, so that subsequent data packets can be switched to desired ports in hardware without going to the CPU. If there is no client report or port to queriers for a data stream, the hardware resource drops it.