KB.15.15

Figure 2 With MLD snooping, traffic is sent to MLD hosts

Switch

Source

Listener

(MLD host)

Listener

(MLD host)

MLD snooping enabled

MLD snooping operates on a single VLAN (though there can be multiple VLANs, each running

MLD snooping). Cross-VLAN traffic is handled by a multicast router.

Forwarding in MLD snooping

When MLD snooping is active, a multicast packet is handled by the switch as shown in the following

list.

The packet is:

• Forwarded to ports that have nodes that have joined the packet's multicast address (that is,

MLD hosts on that address packet)

• Forwarded toward the querier—If the switch is not the querier, the packet is forwarded out

the port that leads to the querier.

• Forwarded toward any multicast routers—If there are multicast routers on the VLAN, the packet

is forwarded out any port that leads to a router.

• Forwarded out administratively forwarded ports—The packet is forwarded through all ports

set administratively to forward mode. (See the description of forward modes, below.)

• Dropped for all other ports.

Each individual port's forwarding behavior can be explicitly set using a CLI command to one of

these modes:

Auto (the default mode)

The switch forwards packets through this port based on the MLD rules and the packet's multicast

address. In most cases, this means that the switch forwards the packet only if the port connects

to a node that is joined to the packet's multicast address (that is, to an MLD host). There is

seldom any reason to use a mode other than "auto" in normal operation (though some

diagnostics may make use of "forward" or "block" mode).

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