Multicast stops halfway, switches to unicasts

Cause

Host sending multicast and host receiving multicast are in the same network segment, and IGMP snooping feature is enabled on switch which is performing switching in the network segment, it is possible.

Cause details

If the multicast transmission host and the receiving host are in the same segment, to do the multicast communication, it is not necessary to use IGMP, the setting of IGMP querier (= configuring the PIM) is not done usually in the segment  .

Multicast receiving host will throw the IGMP for the first multicast join.Then switch is, by IGMP snooping function, to allow the multicast communication to that port.

If it is configured "PIM settings" to the router's interface which belongs to the segment, "IGMP querier function" is also enabled, and once the router receives IGMP, do regular keepalive monitoring , and receiving host also sends IGMP periodically.Therefore, the problem does not occur because also regularly receive IGMP switch that is configured with IGMP snooping.However, in this case because it is not a set of PIM, the client does not throw IGMP packet regulary.Only the first one.

Therefore, unawares expired timer of IGMP snooping, it began to reject the multicast communication to that port.

Some application are , as understanding that receiver is not able to receive the multicast, starting to retransmit in unicast.

Solution

Turn OFF the IGMP snooping function on the switch, or it should be the configuration of the IGMP querier (PIM) on the router with the interface of the same segment.

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