Multimedia Traffic Control with IP Multicast (IGMP)

How IGMP Operates

Leave. The Querier will continue to transmit the multicast group during this short time, and because the group is no longer registered the switch will then flood the multicast group to all ports.

On ProCurve switches that do support Data-Driven IGMP (“Smart” IGMP), when unregistered multicasts are received the switch automatically filters (drops) them. Thus, the sooner the IGMP Leave is processed, the sooner this multicast traffic stops flowing.

Because of the multicast flooding problem mentioned above, the IGMP FastLeave feature is disabled by default on all ProCurve switches that do not support Data-Driven IGMP. (See table 4-2,above.) The feature can be enabled on these switches via an SNMP set of this object:

hpSwitchIgmpPortForceLeaveState.< vid >.< port number>

However, this is not recommended as this will increase the amount of multicast flooding during the period between the client's IGMP Leave and the Querier's processing of that Leave. For more on this topic, refer to “Forced Fast-Leave IGMP” on page 4-18.

ProCurve recommends that the following settings be used.

Use Delayed Group Flush on the Series 2600 switches whenever Fast Leave or Forced Fast Leave are set on a port (see page 4-17).

Forced fast leave can be used when there are multiple devices attached to a port.

Automatic Fast-Leave Operation. If a switch port is:

a.Connected to only one end node

b.The end node currently belongs to a multicast group; i.e. is an IGMP client

c.The end node subsequently leaves the multicast group

Then the switch does not need to wait for the Querier status update interval, but instead immediately removes the IGMP client from its IGMP table and ceases transmitting IGMP traffic to the client. (If the switch detects multiple end nodes on the port, automatic Fast-Leave does not activate—regardless of whether one or more of these end nodes are IGMP clients.)

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