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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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Chapter20 Configuring IGMP Snooping and MVR
Understanding IGMP Snooping
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 forwar ding tab le en try. The switch cr eates on e
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 M AC-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 m ult icas t gr oup
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 ro uted. F or more inform ation about the I GMP
snooping querier, see the “Configuring the IGMP Snooping Querier” section on page20-13.
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 20-2
•Joining a Multicast Group, page 20-3
•Leaving a Multicast Group, page 20-5
•Immediate Leave, page 20-5
•IGMP Configurable-Leave Timer, page 20-5
•IGMP Report Suppression, page 20-6
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 ena ble d on an I GM Pv2 swi tch and the
switch receives an IGMPv3 report from a host, the switch can forward the IGMPv3 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 incl udes supp ort for t he
snooping features on IGMPv1 and IGMPv2 switches and for IGMPv 3 m embe rsh ip r eport me ssage s.
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.