Igmp Snooping; Igmp Snooping Configuration - Cisco 3200 Series Hardware Reference Manual

Appendix C Switch Port Functionality
Verifying Storm Control
Use the show storm-control command to view switch port characteristics, including the storm control
levels set on the interface.
To verify storm-control statistics on an interface, use the following commands, beginning in privileged
EXEC mode:
Command
show interface [ interface-id ]
counters broadcast
show interface [ interface-id ]
counters multicast
show interface [ interface-id ]
counters unicast

IGMP Snooping

Internet Group Management Protocol (IGMP) snooping allows the switch to "listen in" on the IGMP
conversation between hosts and routers. When a switch "hears" an IGMP report from a host for a given
multicast group, the switch adds the host's port number to the Group Destination Address (GDA) list for
that group. And, when the switch hears an IGMP leave, it removes the host's port from the
content-addressable memory (CAM) table entry.
The purpose of IGMP snooping is to restrain multicast traffic in a switched network. By default, a LAN
switch floods multicast traffic within the broadcast domain, and this can consume a lot of bandwidth if
many multicast servers are sending streams to the segment.
Multicast traffic is flooded because a switch usually learns MAC addresses by looking into the source
address field of all the frames it receives. But, since a multicast MAC address is never used as source
address for a packet and since the addresses do not appear in the MAC address table, the switch has no
method for learning the addresses.

IGMP Snooping Configuration

IGMP snooping is enabled by default on a VLAN. Multicast routing has to be enabled on the router
first and then PIM (Multicast routing protocol) has to be enabled on the VLAN interface so that the
switch acknowledges the IGMP join and leave messages which are sent from the hosts connected to the
switch. For example:
Router(config)# ip multicast-routing
Router(config-if)# interface VLAN1
To verify multicasting support, use the show ip igmp group command:
Router# show ip igmp group
OL-5816-10
ip-address 192.168.10.1 255.255.255.0
ip pim sparse-mode
Purpose
Verifies the broadcast suppression discard counter for all
interfaces or a specific interface. Verify the number of packets
discarded.
Verifies the multicast suppression discard counter for all
interfaces or a specific interface. Verify the number of packets
discarded.
Verifies the unicast suppression discard counter for all
interfaces or a specific interface. Verify the number of packets
discarded.
Cisco 3200 Series Router Hardware Reference
IGMP Snooping
C-15