IP Multicast Addressing and Protocols
IP Multicast Stub Routing in AOS
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Copyright © 2005 ADTRAN, Inc.
61200890L1-29.3A
IP Multicast Addressing and Protocols
Multicast Addressing
The IPV4 address scheme (layer 3) sets aside Class D addresses for use in IP multicast. RFC1112 discusses
multicast addressing in detail. The Class D range is from 224.0.0.0 through 239.255.255.255. The range
from 224.0.0.0 through 224.0.0.255 is reserved for local administrative or maintenance use and is usually
limited to the scope of a single subnet. In other words, processes on devices connected to the same
segment use these addresses to communicate with each other.
Examples in this range include:
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The all-hosts address (224.0.0.1)
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The all-routers address (224.0.0.2)
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Routing protocols such as RIP V2 (224.0.0.9) and OSPF (224.0.0.5 and 6)
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Multicast IP routing protocols such as DVMRP (224.0.0.4) and PIM (224.0.0.13)
The range from 224.0.1.0 through 239.255.255.255 is used for IP multicast where a source sends content to
multiple receivers via a multicast-enabled network, as this document describes.
For layer 2, a specific range of ethernet addresses for IP multicast use has been set aside
(01-00-5E-xx-xx-xx). RFC1112 discusses the technique used to map IP layer 3 multicast addresses into
Ethernet layer 2 multicast addresses. Use of layer 2 multicast addresses is critical to network performance
since it allows a device’s network interface to listen for a specific set of addresses in hardware instead of
having to listen to all addresses and sort through them in software.
The benefit of using a multicast address instead of a broadcast address is that only devices running a
process that uses a given multicast address need listen for the address. Other devices are not interrupted
when a multicast address is transmitted. With broadcast addressing, all attached devices are interrupted to
listen to a broadcast packet, whether they need it or not.
All devices that wish to receive the same IP multicast content are referred to as a group. The multicast IP
address that a specific content is being sent to is referred to as the group address.
IGMP – Internet Group Management Protocol
The IGMP protocol allows a device to notify a directly-connected multicast router that it wishes to join a
specific group and therefore receive packets sent to that group address. It also allows a router to query
attached segments (subnets) to determine whether any group members remain. If no remaining group
members are detected, streams to that group are no longer forwarded to that segment. In IGMP V1, when a
device wishes to leave a group, it ceases to respond to the router’s query. When no devices respond on a
given segment, the router stops forwarding that group to that segment. This causes some lag time between
when the last device stops needing a stream and when the router stops sending the stream. To reduce this
lag time and make better use of network resources, IGMP V2 introduced a specific leave message and
process that expedites termination of a stream to an interface when the last member leaves.
Multicast Routing Protocols
Multicast routing is a complex topic, and its details are beyond the scope of this document. To summarize,
whereas IGMP is typically used by an end device to signal a directly-connected router that it wishes to join
a specific multicast group, a multicast routing protocol allows a router to pass this information on to other
