C H A P T E R 18 Configuring Igmp Snooping And Filtering - Cisco 4500M Software Manual

Overview of IGMP Snooping
In contrast to IGMPv1 and IGMPv2, IGMPv3 snooping provides immediate-leave processing by default.
It provides Explicit Host Tracking (EHT) and allows network administrators to deploy SSM
functionality on Layer 2 devices that truly support IGMPv3. (See
In subnets where IGMP is configured, IGMP snooping manages multicast traffic at Layer 2. You can
configure interfaces to dynamically forward multicast traffic only to those interfaces that are interested
in receiving it by using the switchport keyword.
IGMP snooping restricts traffic in MAC multicast groups 0100.5e00.0001 to 01-00-5e-ff-ff-ff. IGMP
snooping does not restrict Layer 2 multicast packets generated by routing protocols.
For more information on IP multicast and IGMP, refer to RFC 1112, RFC 2236, RFC 3376 (for
Note
IGMPv3).
IGMP (configured on a router) periodically sends out IGMP general queries. A host responds to these
queries with IGMP membership reports for groups that it is interested in. When IGMP snooping is
enabled, the switch creates one entry per VLAN in the Layer 2 forwarding table for each Layer 2
multicast group from which it receives an IGMP join request. All hosts interested in this multicast traffic
send IGMP membership reports and are added to the forwarding table entry.
Layer 2 multicast groups learned through IGMP snooping are dynamic. However, you can statically
configure Layer 2 multicast groups using the ip igmp snooping static command. If you specify group
membership statically, your setting supersedes any automatic manipulation by IGMP snooping.
Multicast group membership lists can contain both user-defined and IGMP snooping settings.
Groups with IP addresses in the range 224.0.0.0 to 224.0.0.255, which map to the multicast MAC address
range 0100.5E00.0001 to 0100.5E00.00FF, are reserved for routing control packets. These groups are
flooded to all forwarding ports of the VLAN with the exception of 224.0.0.22, which is used for IGMPv3
membership reports.
If a VLAN experiences a spanning-tree topology change, IP multicast traffic floods on all VLAN ports
Note
where PortFast is not enabled, as well as on ports with the no igmp snooping tcn flood command
configured for a period of TCN query count.
For a Layer 2 IGMPv2 host interface to join an IP multicast group, a host sends an IGMP membership
report for the IP multicast group. For a host to leave a multicast group, it can either ignore the periodic
IGMP general queries or it can send an IGMP leave message. When the switch receives an IGMP leave
message from a host, it sends out an IGMP group-specific query to determine whether any devices
connected to that interface are interested in traffic for the specific multicast group. The switch then
updates the table entry for that Layer 2 multicast group so that only those hosts interested in receiving
multicast traffic for the group are listed.
In contrast, IGMPv3 hosts send IGMPv3 membership reports (with the allow group record mode) to join
a specific multicast group. When IGMPv3 hosts send membership reports (with the block group record)
to reject traffic from all sources in the previous source list, the last host on the port will be removed by
immediate-leave if EHT is enabled.
Software Configuration Guide—Release 12.2(25)EW
18-2
Chapter 18 Configuring IGMP Snooping and Filtering
Explicit Host Tracking, page 18-3.)
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