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Cisco Catalyst Blade Switch 3130 and 3032 for Dell Software Configuration Guide
OL-12247-04
Chapter 44 Configuring IP Multicast Routing
Understanding Cisco’s Implementation of IP Multicast Routing
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A querier is a network device that sends query messages to discover which network devices are
members of a given multicast group.
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A host is a receiver that sends report messages (in response to query messages) to inform a querier
of a host membership.
A set of queriers and hosts that receive multicast data streams from the same source is called a multicast
group. Queriers and hosts use IGMP messages to join and leave multicast groups.
Any host, regardless of whether it is a member of a group, can send to a group. However, only the
members of a group receive the message. Membership in a multicast group is dynamic; hosts can join
and leave at any time. There is no restriction on the location or number of members in a multicast group.
A host can be a member of more than one multicast group at a time. How active a multicast group is and
what members it has can vary from group to group and from time to time. A multicast group can be active
for a long time, or it can be very short-lived. Membership in a group can constantly change. A group that
has members can have no activity.
IP multicast traffic uses group addresses, which are class D addresses. The high-order bits of a Class D
address are 1110. Therefore, host group addresses can be in the range 224.0.0.0 through
239.255.255.255. Multicast addresses in the range 224.0.0.0 to 224.0.0.255 are reserved for use by
routing protocols and other network control traffic. The address 224.0.0.0 is guaranteed not to be
assigned to any group.
IGMP packets are sent using these IP multicast group addresses:
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IGMP general queries are destined to the address 224.0.0.1 (all systems on a subnet).
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IGMP group-specific queries are destined to the group IP address for which the switch is querying.
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IGMP group membership reports are destined to the group IP address for which the switch is
reporting.
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IGMP Version 2 (IGMPv2) leave messages are destined to the address 224.0.0.2
(all-multicast-routers on a subnet). In some old host IP stacks, leave messages might be destined to
the group IP address rather than to the all-routers address.
IGMP Version 1
IGMP Version 1 (IGMPv1) primarily uses a query-response model that enables the multicast router and
multilayer switch to find which multicast groups are active (have one or more hosts interested in a
multicast group) on the local subnet. IGMPv1 has other processes that enable a host to join and leave a
multicast group. For more information, see RFC 1112.
IGMP Version 2
IGMPv2 extends IGMP functionality by providing such features as the IGMP leave process to reduce
leave latency, group-specific queries, and an explicit maximum query response time. IGMPv2 also adds
the capability for routers to elect the IGMP querier without depending on the multicast protocol to
perform this task. For more information, see RFC 2236.
Understanding PIM
PIM is called
protocol-independent
: regardless of the unicast routing protocols used to populate the
unicast routing table, PIM uses this information to perform multicast forwarding instead of maintaining
a separate multicast routing table.
