Enabling IGMP V3 Membership Tracking and Fast Leave for the VLAN

IGMP V3 gives clients membership tracking and fast leave capability. In IGMP V2, only one client on an interface needs to respond to a router's queries. This can leave some clients invisible to the router, making it impossible to track the membership of all clients in a group. When a client leaves the group, the device sends group-specific queries to the interface to see if other clients on that interface need the data stream of the client who is leaving. If no client responds, the device waits a few seconds before it stops the traffic. You can configure the wait time using the ip multicast leave-wait-time command.

IGMP V3 requires every client to respond to queries, allowing the device to track all clients. When tracking is enabled, and an IGMP V3 client sends a leave message and there is no other client, the device immediately stops forwarding traffic to the interface. This feature requires the entire VLAN be configured for IGMP V3 with no IGMP V2 clients. If a client does not send a report during the specified group membership time (the default is 260 seconds), that client is removed from the tracking list.

Every group on a physical port keeps its own tracking record. However, it can only track group membership; it cannot track by (source, group). For example, Client A and Client B belong to group1 but each receives traffic streams from different sources. Client A receives a stream from (source_1, group1) and Client B receives a stream from (source_2, group1). The device still waits for the configured leave-wait-time before it stops the traffic because these two clients are in the same group. If the clients are in different groups, then the waiting period is not applied and traffic is stopped immediately.

The following configuration example enables the tracking and fast leave feature for VLAN 20.

device# configure terminal
device(config)# vlan 20
device(config-vlan-20)# multicast tracking

The membership tracking and fast leave features are supported for IGMP V3 only. If any port or any client is not configured for IGMP V3, then the multicast tracking command is ignored.