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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 24 Configuring IP Multicast Routing
Cisco Implementation of IP Multicast Routing
Understanding IGMP
To participate in IP multicasting, multicast hosts, routers, and multilayer switches must have IGMP
operating. This protocol is the group membership protocol used by hosts to inform routers and multilayer
switches of the existence of members on their directly connected networks and to allow them to send
and receive multicast datagrams.
Multicast routers and switches learn about group membership when a host joining a new group sends an
IGMP message to the group address declaring its membership.
Using the information obtained through IGMP, routers and switches maintain a list of multicast group
memberships on a per-interface basis. A multicast group membership is active on an interface if at least
one host on that interface has sent an IGMP join message to receive the multicast group traffic.
IGMP Version 1
Most IP stacks in hosts today still use IGMPv1. This version primarily uses a query-response model that
allows the multicast router and multilayer switch to determine which multicast groups are active (have
one or more hosts interested in a multicast group) on the local subnet. In this model, the router or switch
acting as the IGMP querier periodically (every 60 seconds) multicasts an IGMPv1 membership query to
the all-hosts multicast group (224.0.0.1) on the local subnet. All hosts enabled for multicasting listen for
this address and receive the query. A host responds with an IGMPv1 membership report to receive
multicast traffic for a specific group, and routers or switches on the subnet learn where active receivers
are for the multicast groups.
A host can also join a multicast group by sending one or more unsolicited membership reports as shown
in
Figure 24-2
. In this example, Host 3 sends an unsolicited report to receive traffic for multicast
group 224.3.3.3 instead of waiting for the next membership query from Router 1.
A host leaves a multicast group by ceasing to process traffic for the multicast group and to respond to
IGMP queries.
Figure 24-2 IGMPv1 Join Process
IGMPv1 relies on the Layer 3 IP multicast routing protocols (PIM, DVMRP, and so forth) to resolve
which one of multiple multicast routers or multilayer switches on a subnet should be the querier. The
query router sends IGMPv1 queries to determine which multicast groups are active (have one or more
hosts sending unsolicited reports) on the local subnet. In general, a designated router is selected as the
querier.
Host 1
Host 2
Host 3
Router 1
IGMP
Querier
Router 2
IGMPv1
Non-querier
224.3.3.3
Unsolicited report
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