Configuring Multicast Forwarding
MLD Snooping
Cisco 500 Series Stackable Managed Switch Administration Guide
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When a switch learns that a host is using IGMP/MLD messages to register to
receive a Multicast stream, optionally from a specific source, the switch adds the
registration in its Multicast Forwarding Data Base.
IGMP/MLD snooping can effectively reduce Multicast traffic from streaming
bandwidth-intensive IP applications. A switch using IGMP/MLD snooping only
forwards Multicast traffic to the hosts interested in that traffic. This reduction of
Multicast traffic reduces the packet processing at the switch, and also reduces the
workload at the end hosts, since they do not have to receive and filter all of the
Multicast traffic generated in the network.
The following versions are supported:
•
IGMP v1/v2/ v3
•
MLD v1/v2
To support selective Multicast forwarding (IPv6), Bridge Multicast filtering must be
enabled, and MLD Snooping must be enabled globally and for each relevant
VLAN.
NOTE
The switch supports MLD Snooping only on static VLANs. It does not support MLD
Snooping on dynamic VLANs
The switch uses this feature to build Multicast membership lists. It uses the lists to
forward Multicast packets only to switch ports where there are host nodes that are
members of the Multicast groups. The switch does not support MLD Querier.
Hosts use the MLD protocol to report their participation in Multicast sessions.
The switch supports two versions of MLD snooping:
•
MLDv1 snooping detects MLDv1 control packets, and sets up traffic
bridging, based on IPv6 destination Multicast addresses.
•
MLDv2 snooping uses MLDv2 control packets to forward traffic based on
the source IPv6 address, and the destination IPv6 Multicast address.
The actual MLD version is selected by the Multicast router in the network.
In an approach similar to IGMP snooping, MLD frames are snooped as they are
forwarded by the switch from stations to an upstream Multicast router and vice
versa. This facility enables a switch to conclude the following:
•
On which ports stations interested in joining a specific Multicast group are
located
•
On which ports Multicast routers sending Multicast frames are located
