Multicast
Multicast Forwarding
Cisco 500 Series Stackable Managed Switch Administration Guide Release 1.3 268
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The device can forward Multicast streams based on one of the following options:
• Multicast MAC Group Address
• IP Multicast Group Address (G)
• A combination of the source IP address (S) and the destination IP Multicast
Group Address (G) of the Multicast packet.
One of these options can be configured per VLAN.
The system maintains lists of Multicast groups for each VLAN, and this manages
the Multicast information that each port should receive. The Multicast groups and
their receiving ports can be configured statically or learned dynamically using
IGMP or Multicast Listener Discovery (MLD) protocols snooping.
Multicast registration is the process of listening and responding to Multicast
registration protocols. The available protocols are IGMP for IPv4 and MLD for IPv6.
When IGMP/MLD snooping is enabled in a device on a VLAN, it analyzes the IGMP/
MLD packets it receives from the VLAN connected to the device and Multicast
routers in the network.
When a device learns that a host is using IGMP/MLD messages to register to
receive a Multicast stream, optionally from a specific source, the device adds the
registration to its Multicast Forwarding Data Base (MFDB).
IGMP/MLD snooping can effectively reduce Multicast traffic from streaming
bandwidth-intensive IP applications. A device 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 device, and also reduces
the workload of 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
• A simple IGMP Snooping Querier
An IGMP Querier is required to facilitate the IGMP protocol on a given subnet. In
general, a Multicast router is also an IGMP Querier. When there are multiple IGMP
Queriers in a subnet, the queriers elect a single querier as the primary querier.
The device can be configured to be an IGMP Querier as a backup querier, or in
situation where a regular IGMP Querier does not exist. The device is not a full
capability IGMP Querier.
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