IP Multicast Overview
XSR User’s Guide 7-3
• Addresses between 239.0.0.0 and 239.255.255.255 should not be forwarded beyond an
organization's intranet.
• Addresses between 232.0.0.0 and 232.255.255.255 are set aside especially for a Source-Specific
Multicast service (SSM).
IP multicast enables multiple hosts to receive packets wrapped with the same MAC address: the
IP multicast addresses are mapped directly into MAC addresses. In turn, network interface cards
can receive packets destined to different MAC addresses. The MAC address range from
0100.5e00.0000 through 0100.5e7f.ffff is the available range of Ethernet MAC addresses for IP
multicast. The mapping rule for Ethernet is to place the lower 23 bits of the IP multicast group
address into these available 23 bits in the Ethernet address. For example, the MAC address on
Ethernet for IP multicast group 226.129.1.1 is 0100.5e01.0101, as illustrated in Figure 7-1.
Figure 7-1 Sample IP Multicast Address Mapped to MAC Address
Outlining IGMP Versions
IGMP allows hosts and routers to report their IP multicast group memberships to neighboring
multicast routers. In some circumstances, an IP multicast router may itself be a member of one or
more multicast groups, in which case it performs both the multicast router role (to collect
membership data) and the group member role (to inform itself and other neighboring multicast
routers of its memberships). Presently, three versions of IGMP exist:
•IGMPv1(RFC-1112) was the first widely-deployed version and the first to become an Internet
standard. It is supported on Windows 95.
• IGMPv2 (RFC-2236) added support for low leave latency, that is, a reduction in the period
between the moment the last host leaves a group and when the routing protocol is notified
that there are no more members). It also allows tuning the burstiness of IGMP traffic on a
subnet. This version is supported on the latest service pack for Windows, newer Windows
releases, and most UNIX systems.
•IGMPv3 (DRAFT-IETF-IDMR-IGMP-V3-07) added support for source filtering, permitting a
system to report interest in receiving packets only from a specific source addressed, or from
all but specific source addresses, sent to a particular multicast address. It is supported on
FreeBSD patch, Linux patch, and Windows XP.
Comparing Multicast Distribution Trees
To reach packet receivers, multicast packets flow through Multicast Distribution Trees (MDTs)
which are created by multicast-capable routers residing between the source and its receivers.
Multicast group members can join or leave at any time, so distribution trees are dynamically
updated. When all active receivers on a particular branch stop requesting traffic for a particular
multicast group, the routers prune that branch from the distribution tree and stop forwarding it
traffic. If one receiver on that branch later becomes active requesting multicast traffic, the router
will modify the distribution tree on the fly and start re-forwarding traffic. Distrib ution trees are
maintained by multicast routing protocols such as PIM.
226.
0100.5e
10000001 00000001
23 bits
00000001
000000010000000100000001