IGMP Snooping

A multicast address is a single IP data packet set that represents a network host group. Multicast addresses are available to process datagrams or frames intended to be multicast to a designated network service. Multicast addressing is applied in the link layer (Layer 2 of the OSI Model) and the Internet layer (Layer 3 of the OSI Model) for IP versions 4 (IPv4) and 6 (IPv6).

Multicast addresses in IPV4 are defined using leading address bits of 1110, which originate from the classful network design of the early Internet when this group of addresses was designated as Class D.

IPv4 multicast packets are delivered using the Ethernet MAC address range 01:00:5e:00:00:00–01:00:5e:7f:ff:ff. This range has 23 bits of available address space. The first octet (01) includes the broadcast/multicast bit. The lower 23 bits of the 28-bit multicast IP address are mapped into the 23 bits of available Ethernet address space. This means that there is ambiguity in delivering packets. If two hosts on the same subnet each subscribe to a different multicast group whose address differs only in the first 5 bits, Ethernet packets for both multicast groups will be delivered to both hosts, requiring the network software in the hosts to discard the unrequired packets.

To support selective IPv4 Multicast forwarding, bridge Multicast filtering must be enabled (in Multicast Properties). The IGMP Snooping must be enabled globally and for each relevant VLAN in the IGMP Snooping page.

To enable IGMP Snooping and identify the device as an IGMP Snooping Querier on a VLAN, follow these steps:

Procedure

Step 1

Click Multicast > IGMP Snooping.

Step 2

Enter the following information:

  • IGMP Snooping Version—Select either IGMPv2 or IGMPv3.

  • Report Suppression—Enable or disable IGMP report suppression. Disabling this feature will forward all IGMP reports to Multicast routers.

Note

IGMP report suppression is supported only when the Multicast query has IGMPv1 and IGMPv2 reports. This feature is not supported when the query includes IGMPv3 reports.

The switch uses IGMP report suppression to forward only one IGMP report per Multicast router query to Multicast devices. When IGMP report suppression is enabled, the switch sends the first IGMP report from all hosts for a group to all Multicast routers. The switch does not send the remaining IGMP reports for the group to the Multicast routers. This feature prevents duplicate reports from being sent to the Multicast devices.

The switch always forwards only the first IGMPv1 or IGMPv2 report from all hosts for a group to all Multicast routers, regardless of the Multicast router query also includes requests for IGMPv3 reports.

Step 3

Click Apply.

Step 4

To configure IGMP on an interface, select a static VLAN and click Edit. Enter the following fields:

OptionDescription

VLAN ID

Select The VLAN Id from the dropdown list.

IGMP Snooping Status

Select to enable IGMP Snooping on the VLAN. The device monitors network traffic to determine which hosts have asked to be sent Multicast traffic.

Multicast Router Ports Auto Learn

Select to enable Auto Learn of the Multicast router.

Query Robustness

Enter the robustness variable value to be used if this switch is the elected querier.

Query Interval

Enter the interval between the general queries to be used if this switch is the elected. querier.

Query Max Response Interval

Enter the delay used to calculate the maximum response code inserted into the periodic general queries.

Last Member Query Counter

Number of IGMP group-specific queries sent before the device assumes that there are no more members for the group, if the device is the elected querier.

Last Member Query Interval

Enter the maximum response delay to be used if the switch cannot read maximum response time value from group specific queries sent by the elected querier.

Immediate Leave

Select to enable the switch to remove an interface that sends a leave message from the forwarding table without first sending out MAC-based general queries to the interface. When an IGMP Leave Group message is received from a host, the system removes the host port from the table entry. After it relays the IGMP queries from the Multicast router, it deletes entries periodically if it doesn’t receive any IGMP membership reports from the Multicast clients. When enabled, this feature reduces the time it takes to block unnecessary IGMP traffic sent to a device port.

IGMP Querier Status

Select to enable this feature. This feature is required if there’s no Multicast router.

IGMP Querier Version

Select the IGMP version to be used if the device becomes the elected querier. Select IGMPv3 if there are switches and/or Multicast routers in the VLAN that perform source-specific IP Multicast forwarding. Otherwise, select IGMPv2.
Step 5

Click Apply. The Running Configuration file is updated.

Note

Changes in IGMP Snooping timers configuration, such as: Query Robustness, Query Interval etc. don’t take effect on timers which already created.