Automatic Fast-Leave IGMP

IGMP Operation Presents a “Delayed Leave” Problem. Where multiple IGMP clients are connected to the same port on an IGMP device (switch or router), if only one IGMP client joins a given multicast group, then later sends a Leave Group message and ceases to belong to that group, the IGMP device retains that IGMP client in its IGMP table and continues forwarding IGMP traffic to the IGMP client until the querier triggers confirmation that no other group members exist on the same port. Thus the switch continues to transmit unnecessary multicast traffic through the port until the querier renews its multicast group status.

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

On switches that do not support Data-Driven IGMP, unregistered multicast groups are flooded to the VLAN rather than pruned. In this scenario, Fast-Leave IGMP can actually increase the problem of multicast flooding by removing the IGMP group filter before the querier has recognized the IGMP Leave. The querier continues to transmit the multicast group during this short time and because the group is no longer registered the switch then floods the multicast group to all ports. Because of such multicast flooding, the IGMP Fast-Leave feature is disabled by default on all switches that do not support Data-Driven IGMP. The feature can be enabled on these switches using an SNMP set of the following 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, see the following figure.

Automatic Fast-Leave Operation The Fast-Leave operation applies if a switch port has the following characteristics:

  • Is connected to only one end node.

  • Is an IGMP client (the end node currently belongs to a multicast group).

  • The end node subsequently leaves the multicast group.

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.)

In the following figure, automatic Fast-Leave operates on the switch ports for IGMP clients “3A” and “5A”, but not on the switch port for IGMP clients “7A” and 7B, Server “7C” and printer “7D”.

Automatic Fast-Leave IGMP Criteria

When client “3A” running IGMP is ready to leave the multicast group, it transmits a Leave Group message. Because the switch knows that there is only one end node on port 3, it removes the client from its IGMP table and halts multicast traffic (for that group) to port 3. If the switch is not the querier, it does not wait for the actual querier to verify that there are no other group members on port 3. If the switch itself is the querier, it does not query port 3 for the presence of other group members.


Fast-Leave operation does not distinguish between end nodes on the same port that belong to different VLANs. Thus even if all devices on port 6 in in the preceding figure belong to different VLANs, Fast-Leave does not operate on port 6.