Understanding Igmp Snooping; Igmp Versions; Chapter 20 Configuring Igmp Snooping And Mvr - Cisco Catalyst 2950 Software Configuration Manual

Understanding IGMP Snooping

Understanding IGMP Snooping
Layer 2 switches can use IGMP snooping to constrain the flooding of multicast traffic by dynamically
configuring Layer 2 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.
For more information on IP multicast and IGMP, see RFC 1112 and RFC 2236.
Note
The multicast router sends out periodic IGMP general queries to all VLANs. When IGMP snooping is
enabled, the switch responds to the router queries with only one join request per MAC multicast group,
and the switch creates one entry per VLAN in the Layer 2 forwarding table for each MAC group from
which it receives an IGMP join request. All hosts interested in this multicast traffic send join requests
and are added to the forwarding table entry.
Layer 2 multicast groups learned through IGMP snooping are dynamic. However, you can statically
configure MAC multicast groups by using the ip igmp snooping vlan static 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
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.
The switches support a maximum of 255 IP multicast groups.
These sections describe characteristics of IGMP snooping on the switch:
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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.
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
20-2
"Configuring the IGMP Snooping Querier" section on page
IGMP Versions, page 20-2
Joining a Multicast Group, page 20-3
Leaving a Multicast Group, page 20-5
Immediate-Leave Processing, page 20-5
IGMP Configurable-Leave Timer, page 20-5
IGMP Report Suppression, page 20-6
IGMP Snooping Querier Configuration Guidelines and Restrictions, page 20-6
Source-Only Networks, page 20-7
Chapter 20 Configuring IGMP Snooping and MVR
20-14.
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