Enterasys Networks X-PeditionTM Outlining Igmp Versions, Comparing Multicast Distribution Trees

IP Multicast Overview

•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

226.

0100.5e

10000001	00000001	00000001
	23 bits
00000001	00000001	00000001

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. Distribution trees are maintained by multicast routing protocols such as PIM.

XSR User’s Guide 7-3