Quick Reference Guide

466 PowerConnect B-Series TI24X Configuration Guide
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Overview of IP multicasting
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Mapping of IPv4 Multicast group addresses to
Ethernet MAC addresses
The IANA owns a block of Ethernet MAC addresses for Multicast usage that are in the range
0100.5e00.0000 through 0100.5e7F.FFFF. For a given IPv4 Multicast group, there is a simple way
of obtaining the appropriate Ethernet Destination MAC address that must be used in Layer 2
encapsulation. This is defined in RFC 1112, as follows:
“An IP host group address is mapped to an Ethernet multicast address by placing the low-order
23-bits of the IP address into the low-order 23 bits of the Ethernet multicast address
01-00-5E-00-00-00 (hex). Because there are 28 significant bits in an IP host group address, more
than one host group address may map to the same Ethernet multicast address.”
NOTE
Since there are 5 bits in the IPv4 Group address that are not used in the mapping, there is a
possibility for up to 32 IPv4 Multicast Groups to use the same Ethernet Destination MAC address.
Taking this into account along with the reserved IPv4 Group address range, it is discouraged for
applications to use IPv4 Multicast Group Addresses that may conflict with the reserved addresses
at the Layer 2 level. This is because some devices may use just the Ethernet Destination MAC
address to take actions on the packet.
Supported Layer 3 multicast routing protocols
Layer 3 Switches support two different multicast routing protocols— Protocol-Independent
Multicast (PIM) protocol along with the Internet Group Membership Protocol (IGMP).
PIM is broadcast and pruning multicast protocols that deliver IP multicast datagrams. The
protocols employ reverse path lookup check and pruning to allow source-specific multicast delivery
trees to reach all group members. PIM build a different multicast tree for each source and
destination host group.
NOTE
PIM can concurrently operate on different ports of a Layer 3 Switch.
Multicast terms
The following are commonly used terms in discussing multicast-capable routers. These terms are
used throughout this chapter:
• Node: Refers to a router or Layer 3 Switch.
• Root Node: The node that initiates the tree building process. It is also the router that sends the
multicast packets down the multicast delivery tree.
• Upstream: Represents the direction from which a router receives multicast data packets. An
upstream router is a node that sends multicast packets.
• Downstream: Represents the direction to which a router forwards multicast data packets. A
downstream router is a node that receives multicast packets from upstream transmissions.
• Group Presence: Means that a multicast group has been learned from one of the directly
connected interfaces. Members of the multicast group are present on the router.
• Intermediate nodes: Routers that are in the path between source routers and leaf routers.