FAQ--- Multicast traffic is broadcasted on the S5700LI switch even though IGMP snooping is enabled

Publication Date:  2016-08-11 Views:  854 Downloads:  0
Issue Description

 Scenario:

As shown in Figure 1 on a pure Layer 2 network, a multicast source (Host B) sends multicast data to multicast group 224.1.1.1 in vlan 10. HostC expects to receive data of the multicast group 224.1.1.1 while HostA is not interested in this traffic. All the hosts run IGMPv2.

 Figure 1 Networking diagram for IGMP snooping querier configuration 


The switch is configured with IGMP snooping and as the IGMP snooping querier of the L2 network. With this configuration the switch would periodically broadcast IGMP Query messages to all the interfaces in the vlan and would be able to establish a L2 multicast forwarding table  so that the traffic sent by the multicast sources will be delivered just to the clients that are joining specific groups. In this specific scenario, the switch should send the multicast traffic just to Host C.

 

After the switch is configured as a igmp snooping querier we can notice that it continues to flood the multicast stream out on every port in VLAN 10 and does not send it just on the interface where Host C is situated.


Configuration:

 #

vlan batch 10

#

igmp-snooping enable

vlan 10

igmp-snooping enable

 igmp-snooping querier enable

#

interface Vlanif10

 ip address 192.168.10.1 255.255.255.224

#

interface GigabitEthernet0/0/1

 description HostA

 port link-type access

 port default vlan 10

#

interface GigabitEthernet0/0/2

 description Host B- Multicast Source

 port link-type access

 port default vlan 10

#

interface GigabitEthernet0/0/3

 description Host C-Receiver

 port link-type access

 port default vlan 10


Solution

This behavior appears on the S2750, S5700S-LI, S5700LI, S5710-X-LI, S5720S-SI  and S5720SI switches even though igmp snooping is enabled because by default they broadcast unknown multicast flows in the corresponding VLAN, regardless of which Layer 2 multicast forwarding mode is used.

 

In order to correct the IGMP snooping behavior on these devices, you would need to configure the switch to drop unknown multicast flows by running  the multicast drop-unknown command in the vlan view:


Example: 

[Huawei]vlan 10

[Huawei-vlan10]

[Huawei-vlan10]multicast drop-unknown


 NOTE:

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

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