Reference Guide

Internet Group Management Protocol (IGMP) | 397
Disabling Multicast Flooding
If the switch receives a multicast packet that has an IP address of a group it has not learned (unregistered
frame), the switch floods that packet out of all ports on the VLAN.
You can configure the S5000 to only forward unregistered packets to ports on a VLAN that are connected
to multicast routers (mrouter ports) using the command
no ip igmp snooping flood from
CONFIGURATION mode. When flooding is disabled, if there are no such ports in the VLAN connected to
a multicast router, the switch drops the packets.
When you configure
no ip igmp snooping flood, the system drops the packets immediately. The system
does not forward the frames on mrouter ports, even if they are present. Layer 3 multicast must be disabled
(
no ip multicast-routing) in order to disable multicast flooding.
Specifying a Port as Connected to a Multicast Router
You can statically specify a port in a VLAN as connected to a multicast router using the command ip igmp
snooping mrouter
from INTERFACE VLAN mode.
View the ports that are connected to multicast routers using the command
show ip igmp snooping mrouter
from EXEC Privilege mode.
Configuring the Switch as Querier
Hosts that do not support unsolicited reporting wait for a general query before sending a membership
report. When the multicast source and receivers are in the same VLAN, multicast traffic is not routed, and
so there is no querier. You must configure the switch to be the querier for a VLAN so that hosts send
membership reports, and the switch can generate a forwarding table by snooping.
Configure the switch to be the querier for a VLAN by first assigning an IP address to the VLAN interface,
and then using the command
ip igmp snooping querier from INTERFACE VLAN mode.
• IGMP snooping Querier does not start if there is a statically configured multicast router interface in the
VLAN.
• The switch may lose the querier election if it does not have the lowest IP address of all potential
queriers on the subnet.
• When enabled, IGMP snooping Querier starts after one query interval in case no IGMP general query
(with IP SA lower than its VLAN IP address) is received on any of its VLAN members.
Adjusting the Last Member Query Interval
When the querier receives a leave message from a receiver, it sends a group-specific query out of the ports
specified in the forwarding table. If no response is received, it sends another. The amount of time that the
querier waits to receive a response to the initial query before sending a second one is the Last Member
Query Interval (LMQI). The switch waits one LMQI after the second query before removing the
group-port entry from the forwarding table.