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Configuring IP Multicast Routing
The Cisco IP Multicast Routing Implementation
IPC-401
Cisco IOS IP Configuration Guide
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 transmitted using IP multicast group addresses as follows:
• IGMP general queries are destined to the address 224.0.0.1 (all systems on a subnet).
• IGMP group-specific queries are destined to the group IP address for which the router is querying.
• IGMP group membership reports are destined to the group IP address for which the router is
reporting.
• IGMP Version 2 (IGMPv2) Leave messages are destined to the address 224.0.0.2 (all routers on a
subnet).
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Note that in some old host IP stacks, Leave messages might be destined to the group IP address
rather than to the all-routers address.
IGMP Versions
IGMP messages are used primarily by multicast hosts to signal their interest in joining a specific
multicast group and to begin receiving group traffic.
The original IGMP Version 1 Host Membership model defined in RFC 1112 is extended to significantly
reduce leave latency and provide control over source multicast traffic by use of Internet Group
Management Protocol, Version 2.
• IGMP Version 1
Provides for the basic Query-Response mechanism that allows the multicast router to determine
which multicast groups are active and other processes that enable hosts to join and leave a multicast
group. RFC 1112 defines Host Extensions for IP Multicasting.
• IGMP Version 2
Extends IGMP allowing such features as the IGMP leave process, group-specific queries, and an
explicit maximum query response time. IGMP Version 2 also adds the capability for routers to elect
the IGMP querier without dependence on the multicast protocol to perform this task. RFC 2236
defines Internet Group Management Protocol, Version 2.
• IGMP Version 3
Provides for “source filtering” which enables a multicast receiver host to signal to a router which
groups it wants to receive multicast traffic from, and from which sources this traffic is expected.
PIM
The PIM protocol maintains the current IP multicast service mode of receiver-initiated membership. It
is not dependent on a specific unicast routing protocol.
PIM is defined in RFC 2362, Protocol-Independent Multicast-Sparse Mode (PIM-SM): Protocol
Specification. PIM is defined in the following Internet Engineering Task Force (IETF) Internet drafts:
• Protocol Independent Multicast (PIM): Motivation and Architecture
• Protocol Independent Multicast (PIM), Dense Mode Protocol Specification
• Protocol Independent Multicast (PIM), Sparse Mode Protocol Specification
• draft-ietf-idmr-igmp-v2-06.txt, Internet Group Management Protocol, Version 2
• draft-ietf-pim-v2-dm-03.txt, PIM Version 2 Dense Mode