IGMP On-Demand

IGMP on-demand helps to maintain IGMP group memberships and frees a multicast device and its connected access device from exchanging a large number of packets.

Background

After IGMP is configured on hosts and the hosts' directly connected multicast device, the hosts can dynamically join multicast groups, and the multicast device can manage multicast group members on the local network.

In some cases, the device directly connected to a multicast device, however, may not be a host but an IGMP proxy-capable access device to which hosts are connected. If you configure only IGMP on the multicast device, access device, and hosts, the multicast and access devices need to exchange a large number of packets.

To resolve this problem, enable IGMP on-demand on the multicast device. The multicast device sends only one general query message to the access device. After receiving the general query message, the access device sends the collected Join and Leave status of multicast groups to the multicast device. The multicast device uses the Join and Leave status of the multicast groups to maintain multicast group memberships on the local network segment.

Benefits

IGMP on-demand reduces packet exchanges between a multicast device and its connected access device and reduces the loads on these devices.

Related Concepts

IGMP on-demand

IGMP on-demand enables a multicast device to send only one IGMP general query message to its connected access device (IGMP proxy-capable) and to use Join/Leave status of multicast groups reported by its connected access device to maintain IGMP group memberships.

Implementation

When a multicast device is directly connected to hosts, the multicast device sends IGMP Query messages to and receives IGMP Report and Leave messages from the hosts to identify the multicast groups that have receivers. The device directly connected to the multicast device, however, may be not a host but an IGMP proxy-capable access device, as shown in Figure 1.

Figure 1 IGMP on-demand

On the network shown in Figure 1:

The provider edge (PE) is a multicast device, and the customer edge (CE) is an access device.

On the network segment a shown in Figure 1, if IGMP on-demand is not enabled on the PE, the PE sends a large number of IGMP Query messages to the CE, and the CE sends a large number of Report and Leave messages to the PE. As a result, lots of PE and CE resources are consumed.
On the network segment b shown in Figure 1, after IGMP on-demand is enabled on the PE, the PE sends only one general query message to the CE. After receiving the general query message from the PE, the CE sends the collected Join and Leave status of IGMP groups to the PE. The CE sends a Report or Leave message for a group to the PE only when the Join or Leave status of the group changes. To be specific, the CE sends an IGMP Report message for a multicast group to the PE only when the first user joins the multicast group and sends a Leave message only when the last user leaves the multicast group.

After you enable IGMP on-demand on a multicast device connected to an IGMP proxy-capable access device, the multicast device implements IGMP in a different way as it implements standard IGMP in the following aspects:

The multicast device interface connected to the access device sends only one IGMP general query message to the access device.
The records about dynamically joined IGMP groups on the multicast device interface connected to the access device do not time out.
The multicast device interface connected to the access device directly deletes the entry for a group only after the multicast device interface receives an IGMP Leave message for the group.