18-2
Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 18 Configuring IGMP Snooping
Understanding IGMP Snooping
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.
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 18-2
• Joining a Multicast Group, page 18-3
• Leaving a Multicast Group, page 18-4
• Immediate Leave, page 18-5
• IGMP Configurable-Leave Timer, page 18-5
• IGMP Report Suppression, page 18-5
IGMP Versions
The switch supports IGMP Version 1 and IGMP Version 2, and IGMP Version 3. These versions are
interoperable on the switch. For example, if IGMP snooping is enabled on an IGMPv1 switch and the
switch receives an IGMPv2 report from a host, the switch can forward the IGMPv2 report to the
multicast router.
Note The switches support IGMPv3 snooping based only on the destination multicast MAC address. They do
not support snooping based on the source MAC address or on proxy reports.
An IGMPv3 switch supports Basic IGMPv3 Snooping Support (BISS), which includes support for the
snooping features on IGMPv1 and IGMPv2 switches and for IGMPv3 membership report messages.
BISS constrains the flooding of multicast traffic when your network includes IGMPv3 hosts. It
constrains traffic to approximately the same set of ports as the IGMP snooping feature on IGMPv2 or
IGMPv1 hosts.
An IGMPv3 switch can receive messages from and forward messages to a device running the Source
Specific Multicast (SSM) feature. For more information about source-specific multicast with IGMPv3
and IGMP, see this URL:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121newft/121t/121t5/dtssm5t.htm
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