Multimedia traffic control with IP multicast (IGMP)

Operation and features


	NOTE: Multicast filtering is not supported on HP switches J9779A, J9780A, J9782A and J9783A.

In a network where IP multicast traffic is transmitted for multimedia applications, you can use a switch to reduce unnecessary per-port bandwidth usage by configuring IGMP (Internet Group Management Protocol) controls. In the factory default state (IGMP disabled), the switch floods all IP multicast traffic it receives on a given VLAN through all ports on that VLAN, except the port on which it received the traffic. This can cause significant and unnecessary bandwidth use in networks employing IP multicast traffic. With IGMP, ports can detect IGMP queries, report packets and manage IP switch multicast traffic.

IGMP is useful in multimedia applications such as LAN TV, desktop conferencing and collaborative computing that have multipoint communication (communication from one-to-many or many-to-many hosts). In such multipoint applications, IGMP is configured on the hosts and multicast traffic is generated by one or more servers (inside or outside the local network). Switches in the network that support IGMP can then be configured to direct the multicast traffic to only the ports where needed. If multiple VLANs are configured, you can configure IGMP by VLAN.

Enabling IGMP allows detection of IGMP queries and report packets to manage IP multicast traffic through the switch. If no other querier is detected, the switch then also functions as the querier. To disable the querier feature, use the IGMP configuration MIB (see “Configuring the querier function”).


	NOTE: IGMP configuration on the switch operates at the VLAN context level. If you are not using VLANs, then configure IGMP in VLAN 1 (the default VLAN) context.

IGMP devices

IGMP device: A switch or router running IGMP traffic control features.
IGMP host: An end-node device running an IGMP (multipoint or multicast communication) application.
Querier: A required IGMP device that facilitates IGMP protocol and traffic flow on a given LAN. This device tracks which ports are connected to devices (IGMP clients) that belong to specific multicast groups and triggers updates of this information.

A querier uses data received from the queries to determine whether to forward or block multicast traffic on specific ports. When the switch has an IP address on a given VLAN, the switch automatically operates as a querier for that VLAN if it does not detect a multicast router or another switch functioning as a querier.

When enabled (the default state), the switch’s querier function eliminates the need for a multicast router. In most cases, HP recommends that you leave this parameter in the default enabled state even if you have a multicast router performing the querier function in your multicast group. For more information, see How IGMP operates.

IGMP operating features

In the factory default configuration, IGMP is disabled. If multiple VLANs are not configured, configure IGMP on the default VLAN (DEFAULT_VLAN; VID = 1); if multiple VLANs are configured, configure IGMP on a per-VLAN basis for every VLAN where this feature is needed.

With the CLI, you can also configure the following options:

Forward with high priority. Disabling this parameter (the default) causes the switch or VLAN to process IP multicast traffic and other traffic, in the order received (usually normal priority). Enabling this parameter causes the switch or VLAN to give higher priority to IP multicast traffic than to other traffic.
Auto/blocked/forward: You can configure individual ports to any of the following states:
- Auto (the default): Causes the switch to interpret IGMP packets and to filter IP multicast traffic based on the IGMP packet information for ports belonging to a multicast group. Thus IGMP traffic is forwarded on a specific port only if an IGMP host or multicast router is connected to the port.
- Blocked: Causes the switch to block IGMP joins arriving on the blocked port. A multicast stream will still flood out a blocked port if no active joins have been received.
- Forward: Causes the switch to forward all IGMP and IP multicast transmissions through the port.
Operation with or without IP addressing: Helps conserve IP addresses by enabling IGMP to run on VLANs that do not have an IP address. See Operation with or without IP addressing .
Querier capability: The switch performs this function for IGMP on VLANs having an IP address when no other device in the VLAN is acting as querier. See Using the switch as querier.

NOTE: Whenever IGMP is enabled, the switch generates an Event Log message indicating whether querier functionality is enabled.

IP multicast traffic groups are identified by IP addresses in the range of 224.0.0.0 to 239.255.255.255 and incoming IGMP packets intended for reserved, or “well-known” multicast addresses automatically flood through all ports (except the port on which the packets entered the switch). For more on this topic, see Excluding multicast addresses from IP multicast filtering .

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