MLD snooping

MLD snooping is an IPv6 multicast constraining mechanism running on Layer 2 devices. When MLD snooping is enabled on a VLAN, the switch examines the MLD messages between hosts and multicast routers and learns which hosts are interested in receiving traffic for a multicast group. Based on the learning, the switch forwards multicast traffic only to those interfaces in the VLAN that are connected to the interested receivers instead of flooding traffic to all the interfaces.

The following figure shows an example of this scenario. On the left side of the figure, IPv6 multicast packets are transmitted when MLD snooping is not enabled. All the hosts that are interested and not interested receive the IP Multicast traffic consuming bandwidth. Whereas, on the right side of the figure, when MLD snooping is enabled and IPv6 multicast packets are transmitted, only the interested hosts receive the IP multicast packets.

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IPv6 multicast packet transmission when MLD snooping is enabled and not enabled

The following figure shows IPv6 multicast packets transmitted when MLD v2 snooping is enabled and not enabled.

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IPv6 multicast packet transmission when MLD v2 snooping is enabled and not enabled

MLD snooping configuration guidelines and restrictions

You can perform the following configurations to manage and control IPv6 multicast groups using the MLD snooping feature:

  • Enable or disable MLD snooping on each VLAN. MLD snooping can be enabled on a maximum of 512 VLANs.

  • Enable IGMP snooping and MLD snooping on the same VLAN.

Limitations

Following are the limitations for MLD snooping configuration:

  • The maximum (S,G,V) entries supported in the IPv6 multicast routing table (L3_ENTRY_IPV6_MULTICAST) is 512.

    MLD snooping shares the (S,G,V) entries with IGMP snooping, where the (S,G,V) entries number = (G,V) MLD_V1 type entries number + (S,G,V) MLD_V2 type entries number + (*,G,V) MLD_V2 type entries number + number of groups without (*,G,V) registered listeners.

  • IPv6 MLD proxy functionality is not supported.

  • Multicast Flood Control (MFC) is not supported.

  • Static mrouter ports cannot be configured.

  • IPv6 MLD send query functionality is not supported.

  • Configure static router ports is not supported.

  • Multicast Listener Discovery (MLDv1/v2) is not supported with vIST/SMLT.

  • Multicast Listener Discovery (MLDv1/v2) is not supported with Fabric Connect.

  • Protocol Independent Multicast (PIM) with IPv6 is not supported vIST/SMLT.

  • Protocol Independent Multicast (PIM) with IPv6 is not supported with Fabric Connect (SPBM).

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