MLD Snooping

Without MLD with Snooping enabled, when a layer 2 device receives a multicast it would get flooded out all ports (similar to an IPv4 broadcast). Bandwidth would be wasted sending the multicast to devices that don't want it. These same devices would also need to use CPU power to process and discard the packets. This is an inefficient use of network resources and is a security risk.

To prevent this unnecessary traffic, layer 2 devices can use MLD Snooping to listen to MLD Report messages. This is done so the layer 2 device can learn which ports want to listen to what specific multicast groups.

Since MLD Query messages can cause excessive reports MLD Snooping should be used with the Suppression feature. With Suppression enabled the end devices do not need to process and respond to query messages – the layer 2 device will do this for them.

Pro Tip: MLD Queries must be sent at a rate that is aligned to the snooping table aging time!

Other facts:

  • Note that MLD Snooping ignores solicited node multicasts because they are used for address resolution and Duplicate Address Detection (DAD).
  • At the time of this writing, there is no MLD Guard feature.

MLD Snooping Querier

MLD Snooping Querier is for use in LANs that have a multicast server and do not have a multicast capable router. In this scenario, a layer 2 switch can be configured as a Querier. There are scenarios that a layer 2 switch can be configured to act as an MLD Snooping Backup Querier.

Note that a MLD Snooping Querier / Backup Querier is not a fully capable MLD Querier.

A layer 2 device that has MLD Snooping Querier enabled, will listen for MLD Queries for 60 seconds before assuming the role. If the device does receive an MLD Query from another device, the devices will hold a Querier Election to see which device will assume the Querier role. The device with the lowest link-local address wins the election. Consider this in the design!

RFC 4551: Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches

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