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Multicast Filtering (IGMP)
IGMP (Internet Group Management Protocol) allows hosts and routers to work together to
optimize forwarding of multicast traffic on a network. Without IGMP, all multicast packets
must be forwarded to all network segments. With IGMP, multicast traffic is only forwarded
to those network segments which connect interested hosts.
IGMPv1 provides a basic mechanism for hosts and routers to communicate about multicast
groups. Routers send Query messages and hosts respond with group membership Report
messages.
IGMPv2 adds a maximum response time to the Query and adds a Leave message to the
protocol. IGMPv1 and IGMPv2 should not coexist on the same network. Also, IGMPv2
routers are expected to perform IGMPv1 on segments where IGMPv1 hosts are found.
An IGMP snooping switch performs many of the functions of an IGMP router. In passive
mode, such a switch processes IGMP protocol messages sent by hosts and routers to configure
efficient forwarding of multicast traffic. In active mode, a switch will also send its own queries
to speed network convergence.
Periodically, routers and IGMP snooping switches in active mode send an IGMP Query on
each attached network. (The query interval is generally around 1-2 minutes.) A host that
wishes to be a member of a group sets a timer for a short, random delay when it sees the
Query. If it sees a Report from another host before its timer expires, it cancels the timer and
takes no further action until another Query is seen. If no other Report is seen, a Report is
sent when the timer expires. The router or switch uses the Report to configure multicast
forwarding.
The router or switch keeps track of how long it has been since the last Report on each port
for each group. When the group expires, the router or switch stops forwarding multicast data
to that port. Since the query interval is less than the expiration time, data for active groups
continues to be forwarded without interruption.
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Chapter 4 - Managed Switch Software Setup
Stride Industrial Ethernet Switches User Manual 2nd Ed. Rev. A