Reference Guide

Internet Group Management Protocol (IGMP) | 405
2. When a router receives a query it compares the IP address of the interface on which it was received
with the source IP address given in the query. If the receiving router IP address is greater than the
source address given in the query, the router stops sending queries. By this method, the router with the
lowest IP address on the subnet is elected querier and continues to send queries.
3. If a specified amount of time elapses during which other routers on the subnet do not receive a query,
those routers assume that the querier is down, and a new querier is elected.
The amount of time that elapses before routers on a subnet assume that the querier is down is the Other
Querier Present Interval. Adjust this value using the command
ip igmp querier-timeout from INTERFACE
mode.
Configuring a Static IGMP Group
Configure a static IGMP group using the command ip igmp static-group. Multicast traffic for static groups
is always forwarded to the subnet even if there are no members in the group.
View the static groups using the command
show ip igmp groups from EXEC Privilege mode. Static groups
have an expiration value of
Never and a Last Reporter value of CLI, as shown in the example in Viewing
IGMP Groups.
Enabling IGMP Immediate-leave
If the querier does not receive a response to a group-specific or group-and-source query, it sends another
(Querier Robustness Value). Then, after no response, it removes the group from the outgoing interface for
the subnet.
IGMP Immediate Leave reduces leave latency by enabling a router to immediately delete the group
membership on an interface upon receiving a Leave message (it does not send any group-specific or
group-and-source queries before deleting the entry). Configure the system for IGMP Immediate Leave
using the command
ip igmp immediate-leave.
View the enable status of this feature using the command
show ip igmp interface from EXEC Privilege
mode, as shown in the example in Selecting an IGMP Version.
IGMP Snooping
Multicast packets are addressed with multicast MAC addresses, which represent a group of devices, rather
than one unique device. Switches forward multicast frames out of all ports in a VLAN by default, even
though there may be only some interested hosts, which is a waste of bandwidth. IGMP Snooping enables
switches to use information in IGMP packets to generate a forwarding table that associates ports with
multicast groups so that when they receive multicast frames, they can forward them only to interested
receivers.
If IGMP snooping is enabled on a VLT unit, the IGMP snooping dynamically learned groups and multicast
router ports are made to learn on the peer by explicitly tunneling the received IGMP control packets.