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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter21 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, refer to RFC 1112 and RFC 2236.
The multicast router sends out periodic IGMP general que ries to al l V LA Ns. W he n IG MP sno oping 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 a ddr ess sta tica lly, 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.
The switches support a maximum of 255 IP multicast group s.
These sections describe characteristics of IGMP snooping on the switch:
•IGMP Versions, page 21-2
•Joining a Multicast Group, page 21-3
•Leaving a Multicast Group, page 21-4
•Immediate-Leave Processing, page 21-5
•IGMP Report Suppression, page 21-5
•Source-Only Networks, page 21-5
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.
Note IGMPv3 join and leave messages are not supported on switches running IGMP filtering or MVR.