Configuring Multicast Forwarding
IGMP Snooping
Cisco Small Business 300 Series Managed Switch Administration Guide 155
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• Forbidden—Specifies that this port is forbidden from joining this group on
this VLAN.
• None—Indicates that the port is not currently a member of this Multicast
group on this VLAN.
STEP 11 Click Apply. The switch is updated.
IGMP Snooping
To support selective multicast forwarding (IPv4), Bridge Multicast filtering must be
enabled, and IGMP Snooping must be enabled globally and for each relevant
VLAN.
By default, a Layer 2 switch forwards Multicast frames to all ports of the relevant
VLAN, essentially treating the frame as if it were a Broadcast. With IGMP Snooping
the switch forwards multicast frames to ports that have registered multicast
clients.
NOTE The switch supports IGMP Snooping only on static VLANs. It does not support
IGMP Snooping on dynamic VLANs.
When IGMP Snooping is enabled globally or on a VLAN, all IGMP packets are
forwarded to the CPU. The CPU analyzes the incoming packets, and determines
the following:
• Which ports are asking to join which Multicast groups on what VLAN.
• Which ports are connected to Multicast routers (Mrouters) that are
generating IGMP queries.
• Which ports are receiving PIM, DVMRP, or IGMP query protocols.
The are displayed on the
IGMP Snooping Page
.
Ports asking to join a specific Multicast group issue an IGMP report that specifies
which group(s) the host wants to join. This results in the creation of a forwarding
entry in the Multicast forwarding database.
The IGMP Snooping Querier is used to support a Layer 2 Multicast domain of
snooping switches in the absence of a Multicast router. For example, where
Multicast content is provided by a local server, but the router (if one exists) on that
network does not support Multicast.
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