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Catalyst 3750 SwitchSoftware Configuration Guide
OL-8550-09
Chapter24 Configuring IGMP Snooping and MVR
Understanding IGMP Snooping
Understanding IGMP Snooping
Layer 2 switches can use IGMP snooping to constrain the flooding of multicast traffic by dynamically
configuring Layer2 interfaces so that multicast traffic is forwarded to only those interfaces associated
with IP multicast devices. As the name implies, IGMP snooping requires the LAN switch to snoop on
the IGMP transmissions between the host and the router and to keep track of multicast groups and
member ports. When the switch receives an IGMP report from a host for a particular multicast group,
the switch adds the host port number to the forwarding table entry; when it receives an IGMP Leave
Group message from a host, it removes the host port from the table entry. It also periodically deletes
entries if it does not receive IGMP membership reports from the multicast clients.
Note For more information on IP multicast and IGMP, see RFC 1112 and RFC 2236.
The multicast router (which could be a Catalyst 3750 switch with the IP services image on the stack
master) sends out periodic general queries to all VLANs. All hosts interested in this multicast traffic
send join requests and are added to the forwarding table entry. The switch creates one entry per VLAN
in the IGMP snooping IP multicast forwarding table for each group from which it receives an IGMP join
request.
The switch supports IP multicast group-based bridging, rather than MAC-addressed based groups. With
multicast MAC address-based groups, if an IP address being configured translates (aliases) to a
previously configured MAC address or to any reserved multicast MAC addresses (in the range
224.0.0.xxx), the command fails. Because the switch uses IP multicast groups, there are no address
aliasing issues.
The IP multicast groups learned through IGMP snooping are dynamic. However, you can statically
configure multicast groups by using the ip igmp snooping vlan vlan-id static ip_address interface
interface-id global configuration command. If you specify group membership for a multicast group
address statically, your setting supersedes any automatic manipulation by IGMP snooping. Multicast
group membership lists can consist of both user-defined and IGMP snooping-learned settings.
You can configure an IGMP snooping querier to support IGMP snooping in subnets without multicast
interfaces because the multicast traffic does not need to be routed. For more information about the IGMP
snooping querier, see the “Configuring the IGMP Snooping Querier” section on page24-15.
If a port spanning-tree, a port group, or a VLAN ID change occurs, the IGMP snooping-learned multicast
groups from this port on the VLAN are deleted.
These sections describe IGMP snooping characteristics:
•IGMP Versions, page24-3
•Joining a Multicast Group, page24-3
•Leaving a Multicast Group, page24-5
•Immediate Leave, page24-6
•IGMP Configurable-Leave Timer, page24-6
•IGMP Report Suppression, page 24-6
•IGMP Snooping and Switch Stacks, page24-7