Configuring Membership Fast-Update

When deploying a Layer 2 multicast network, ensure that all Layer 2 multicast devices use the same parameter values to calculate the aging time of dynamic group member ports, especially the IGMP snooping general query interval. Otherwise, errors will occur in Layer 2 multicast forwarding.

A router port sends IGMP Report/Leave messages to an upstream Layer 3 device and receives multicast packets from the upstream device. When IGMP snooping is enabled on a device, the device can learn entries of the dynamic router port to monitor the sending of multicast data. When network congestion or flapping occurs, the dynamic router port does not receive General IGMP Query or Protocol Independent Multicast (PIM) Hello messages before it times out. The switch deletes the interface from the router port list, which will cause service interruption. To avoid service interruption, set a longer aging time.

When the switch receives IGMP Leave messages from a group member port, fast leave allows the switch to immediately delete forwarding entries of the member port but does not reset the aging timer.

• Enable fast leave in a VLAN only when each interface in the VLAN is connected to one receiver host.

• Fast leave takes effect for member ports in a VLAN only when the switch can process IGMPv2 messages.

The host-based fast leave function enables a switch to delete a host from the host list of a group immediately after receiving an IGMP Leave message from the host. If the deleted host is the last receiver host connected to the group member port, the switch deletes the forwarding entry of the group from the port, without waiting the aging timer of the port to expire. This conserves bandwidth and system resources.

Host-based fast leave takes effect in a VLAN only when the switch can process IGMPv2 messages in the VLAN.

When a Layer 2 network topology changes, the forwarding path of multicast packets may change. When a fault occurs on a link, the switch sends IGMP Query messages and the multicast members reply with IGMP Report messages. The switch then updates information about multicast member ports based on the IGMP Report messages. In this way, multicast packets can be switched to new forwarding paths quickly.

When a Layer 2 network topology changes, multicast forwarding paths may change. Sending IGMP Query Messages upon Topology Changes enables multicast data flows to be switched to new forwarding paths within a relatively short time. However, the downstream device may not receive Query messages immediately after convergence of the Layer 2 network, because Query messaged are sent after a certain interval (60 seconds by default). As a result, multicast traffic cannot be rapidly switched to the new forwarding paths.

If the Layer 2 network is running the Spanning Tree Protocol (STP), you can enable fast multicast forwarding path switching upon STP topology changes. When the STP topology changes, this function quickly changes the ports in Forwarding state into router ports to direct multicast data flows to new forwarding paths.

• This function takes effect only when STP is used as the loop prevention protocol on a Layer 2 network and the STP operation mode is Multiple Spanning Tree Protocol (MSTP), Rapid Spanning Tree Protocol (RSTP), or STP.
• With this function configured, the switch sets all ports in Forwarding state as router ports when the STP topology changes. Multicast data flows are forwarded to all the router ports before the router ports are aged out. This will cause increase in multicast traffic.

When IGMP snooping is enabled for Dynamic Host Control Protocol (DHCP) snooping users, the switch does not delete the IGMP snooping entry of a user immediately after the user goes offline. The multicast flow requested by the user is still transmitted until the IGMP snooping entry of the user is aged out.

You can configure the switch to delete the IGMP snooping entry of a DHCP snooping user immediately after the user goes offline, so that the multicast flow requested by the user is immediately terminated.

For details about DHCP snooping, see DHCP Snooping Configuration in the S600-E V200R019C10 Configuration Guide - Security.