25-2
Cisco IE 3000 Switch Software Configuration Guide
OL-13018-03
Chapter 25 Configuring IGMP Snooping and MVR
Understanding IGMP Snooping
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 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 page 25-13.
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, page 25-2
• Joining a Multicast Group, page 25-3
• Leaving a Multicast Group, page 25-5
• Immediate Leave, page 25-5
• IGMP Configurable-Leave Timer, page 25-5
• IGMP Report Suppression, page 25-6
IGMP Versions
The switch supports IGMP Version 1, IGMP Version 2, and IGMP Version 3. These versions are
interoperable on the switch. For example, if IGMP snooping is enabled on an IGMPv2 switch and the
switch receives an IGMPv3 report from a host, the switch can forward the IGMPv3 report to the
multicast router.
Note The switch supports IGMPv3 snooping based only on the destination multicast MAC address. It does
not support snooping based on the source MAC address or on proxy reports.
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