Dropping Unknown Multicast Flows

Context

Unknown multicast flows are those that do not match any entry in the multicast forwarding table or match multicast forwarding entries with an empty outbound interface list. These flows are not requested by users. The default method that a switch uses to process unknown IPv4 multicast flows depends on whether Layer 2 multicast is enabled and which Layer 2 multicast forwarding mode is used:

If Layer 2 multicast is not enabled on the switch, the switch broadcasts unknown multicast flows in the corresponding VLAN.
If Layer 2 multicast is enabled on the S2720-EI, they broadcast unknown multicast flows in the corresponding VLAN.
If Layer 2 multicast is enabled on other switch models, they broadcast unknown multicast flows in the corresponding VLAN in MAC address-based forwarding mode and drop unknown multicast flows in IP address-based forwarding mode.

If a switch broadcasts unknown multicast flows in a VLAN, you can configure the switch to drop unknown multicast flows, reducing instant bandwidth usage.

Procedure

Run system-view
The system view is displayed.
Run vlan vlan-id
The VLAN view is displayed.
Run multicast drop-unknown
The switch is configured to drop unknown multicast flows.

After the multicast drop-unknown command is configured, the switch drops all unknown IPv4 and IPv6 multicast packets, including the protocol packets that are transparently transmitted within the VLAN and use the reserved multicast address.