How does an S series switch process unknown multicast flows

Unknown multicast flows are the multicast data flows that have no forwarding entries in the chip or the flows that are not requested by users. When Layer 2 multicast is not enabled on a switch, an S series switch broadcasts unknown multicast flows in a VLAN. After Layer 2 multicast is enabled, processing on other models excluding the S1700 is as follows:

If Layer 2 multicast is enabled on the E600, S1720, S2700SI, S2710SI, S2700EI, S2720, S2750EI, S5700S-LI, S5700LI, S5710-C-LI, S5710-X-LI, S5700SI, S5720S-SI, and S5720SI, they broadcast unknown multicast flows in the corresponding VLAN, regardless of which Layer 2 multicast forwarding mode is used.
- Other switches process unknown multicast flows based on the Layer 2 multicast forwarding mode. If the IP address-based forwarding mode is used, the switches drop unknown multicast flows. If the MAC address-based forwarding mode is used, the switches broadcast unknown multicast flows in a VLAN.

To prevent broadcast of unknown multicast packets in a VLAN, run the multicast drop-unknown command. This command configures a switch to drop unknown multicast packets. After this command is configured, the switch also discards protocol packets that are transparently transmitted within the VLAN and use the reserved multicast address.

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