Why does an S series switch generates Layer 2 multicast forwarding entries in only one VLAN

14

After an S series switch receives Report messages from the same multicast group in different VLANs, Layer 2 multicast entries are generated in only the VLAN where Report messages are generated first. The causes are as follows:
An S5700SI switch running a version earlier than V200R005 or an S5710-C-LI does not generate the same multicast forwarding entry for a multicast group in multiple VLANs. That is, a multicast group can have a forwarding entry only in one VLAN. To solve this problem, configure a multicast VLAN so that multicast data can be distributed to user VLANs through the multicast VLAN.
- The preceding restriction does not apply to the S5700SI, S5710-X-LI, S5700S-28X-LI-AC, S5700S-52X-LI-AC, S5720S-SI, S5720SI, and E600 running V200R005 and later versions. These switches, however, cannot generate the same multicast forwarding entry for a multicast group in multiple multicast VLANs. That is, a multicast group can have a forwarding entry only in one multicast VLAN.
- The S2700 does not allow an interface to function as the outbound interface of the same multicast forwarding entry in multiple VLAN. That is, one interface can function as the outbound interface of a multicast group in only one VLAN. To solve this problem, add each interface to only one VLAN so that a multicast group has different outbound interfaces in different VLANs.

Other related questions:
Why is no Layer 3 multicast forwarding entry generated on S series switches after users order multicast programs
The possible causes are as follows: - No RP is configured on the PIM-SM network. In this case, configure static or dynamic RP. - The route to the multicast source or RP is unreachable. In this case, configure a unicast routing protocol to enable the switch to learn the routes to the multicast source or RP.

Layer 2 multicast entries and usage of S series switches
For an S series switch, entries fall into protocol and forwarding entries. Protocol entries include router port entries and host port entries. You can run the display igmp-snooping router-port command to check router port entries, run the display igmp-snooping port-info command to check host port entries, and run the display l2-multicast forwarding-table command to check forwarding entries. Protocol entries are triggered by protocol packets or manually generated. They are used to identify the protocol exchange status. Forwarding entries are generated based on protocol entries and are used to guide subsequent multicast data forwarding.

Difference between forwarding modes of Layer 2 multicast on an S series switch
After Layer 2 multicast is enabled on an S series switch, the switch maintains a Layer 2 multicast forwarding table. When receiving a multicast packet, the switch matches the multicast address of the packet with multicast forwarding entries to determine the outbound interface. Whether the switch determines the outbound interface based on the multicast IP address or multicast MAC address depends on the configured Layer 2 multicast forwarding mode. Multiple multicast IP addresses may be mapped to one MAC address. If multicast data is forwarded based on the MAC address, multicast data may be sent to the users who do not order the multicast data. MAC address-based multicast forwarding can improve specifications in certain scenarios. Different models support different Layer 2 multicast forwarding modes: - The S1720, S2700SI, S2710SI, S2700EI, S2720, S2750EI, S5700S-LI, and S5700LI support only MAC address-based multicast forwarding. - The S3700SI and S5710-C-LI support only IP address-based multicast forwarding. - The S5700SI supports only IP address-based multicast forwarding in V200R002 and earlier versions, and supports MAC address-based and IP address-based forwarding modes starting from V200R003. - The S3700EI, S3700HI, S5710-X-LI, S5720S-SI, S5720SI, S5700EI, S5710EI, S5720EI, S5700HI, S5710HI, S5720HI, S6700EI, S6720EI, S7700, S9700, S12700, S9300, and E600 support MAC address-based and IP address-based forwarding modes.

Default Layer 2 multicast forwarding mode on an S series switch
For S series switches excluding the S1700, after Layer 2 multicast is enabled, the S1720, S2710SI, S2700SI, S2700EI, S2720EI, S2750EI, S5700LI, and S5700S-LI (S5700S-28X-LI-AC and S5700S-52X-LI-AC) forward multicast data based on MAC addresses, and other models forward multicast data based on IP addresses by default.

Can an S series switch accurately forward multicast data packets if only Layer 3 multicast is configured
For S series switches, Layer 3 multicast protocols can identify only Layer 3 VLANIF interfaces. If only Layer 3 multicast is configured and Layer 2 multicast is not enabled, multicast data is replicated on a VLANIF interface and broadcast in the corresponding VLAN, but cannot be forwarded to an accurate physical interface. When Layer 2 multicast is enabled in the VLAN corresponding to the VLANIF interface, multicast data can be accurately forwarded through a Layer 2 interface.

If you have more questions, you can seek help from following ways:
To iKnow To Live Chat
Scroll to top