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414 | Internet Group Management Protocol
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• 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.
Adjust the Last Member Query Interval using the command
ip igmp snooping last-member-query-interval
from INTERFACE VLAN mode.
Fast Convergence after MSTP Topology Changes
When a port transitions to the Forwarding state as a result of an STP or MSTP topology change, FTOS
sends a general query out of all ports except the multicast router ports. The host sends a response to the
general query and the forwarding database is updated without having to wait for the query interval to
expire.
When an IGMP snooping switch is not acting as a Querier it sends out the general query, in response to the
MSTP triggered link-layer topology change, with the source IP address of 0.0.0.0 to avoid triggering
Querier election.
Designating a Multicast Router Interface
You can designate an interface as a multicast router interface with the command ip igmp snooping
mrouter interface
. FTOS also has the capability of listening in on the incoming IGMP General Queries
and designate those interfaces as the multicast router interface when the frames have a non-zero IP source
address. All IGMP control packets and IP multicast data traffic originating from receivers is forwarded to
multicast router interfaces.