AMX AV/IT Administrators Guide
Rev. 1.0 (7/29/2014) www.amx.com Page 23
one active Querier per subnet. In IGMPv2 and v3 if there is more than one Querier then the
Queriers hold an election and the one with the lowest IP address is chosen to be active.
The Querier sends periodic Membership Queries to the All Hosts (224.0.0.1) address. It also
sends out queries to a specific multicast address when it sees an IGMP leave message to check
and see if there is still a listener on the network segment.
The Querier listens for Membership Reports and updates group membership tables used by the
adjacent router to determine if the subnet should have a given multicast forwarded to it. The
Querier removes group from the table after a timeout period if it has not seen a Membership
Report during the period.
IGMP Packets are sent with a TTL of 1 so they are not forwarded through a router.
Group Member: A Group Member is any client that has joined a multicast group. A Group
Member joins the group by sending a membership report to the group multicast address. The
network logs the Membership report in the group membership table (for the router) and the
Switch IGMP cache (for IGMP Snooping) and the Member starts receiving the multicast.
A Group Member responds to IGMP Queries by sending a membership report to the group
multicast address of all groups it is a member of for a general query and a membership report to
the group multicast address for a group query, to keep the tables updated.
In IGMPV2 a Member sends an IGMP Leave to the to the group multicast address when it no
longer wants to receive the multicast.
IGMP Snooping
The purpose of IGMP was originally to prune multicast forwarding at a router level. With the advent
switching instead of hubs the need to prune multicast at a port level was desired. IGMP snooping was
developed for this. In IGMP snooping the network switch listens for the IGMP messages and forwards
the multicast packets from the VLAN only to the Ethernet ports that are sources of IGMP membership
reports and keeps a cache, very much like the IGMP routing table to keep track of the members. Entries
in the cache have a timeout function so if no Membership Reports are received the entry is removed
from the cache.
This also works in switch to switch connections. The switch that supports IGMP snooping must flood all
unrecognized IGMP messages to all other ports, therefore upstream switches receive new Membership
reports to snoop and update their caches.
In the base IGMP protocol a Member responds to a Query after a random amount of time. If a member
hears a response before the timer runs out, they do not respond. In IGMP snooping Membership
Reports are suppressed to Members so all Members respond to all Queries.
PIM
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