Appendix C Switch Port Functionality

IGMP Snooping

Verifying Storm Control

Use the show storm-controlcommand 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

Purpose

 

 

show interface [interface-id]

Verifies the broadcast suppression discard counter for all

counters broadcast

interfaces or a specific interface. Verify the number of packets

 

discarded.

 

 

show interface [interface-id]

Verifies the multicast suppression discard counter for all

counters multicast

interfaces or a specific interface. Verify the number of packets

 

discarded.

 

 

show interface [interface-id]

Verifies the unicast suppression discard counter for all

counters unicast

interfaces or a specific interface. Verify the number of packets

 

discarded.

 

 

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 ip-address 192.168.10.1 255.255.255.0 ip pim sparse-mode

To verify multicasting support, use the show ip igmp group command:

Router# show ip igmp group

Cisco 3200 Series Router Hardware Reference

 

OL-5816-10

C-15

 

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Cisco Systems 3200 manual Igmp Snooping Configuration, Verifying Storm Control

3200 specifications

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