Enabling Fast Multicast Forwarding Path Switching upon STP Topology Changes

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.