Multicast Architecture

Multicast-capable routers replicate packets on the multicast network, which has exactly the same topology as the unicast network it is based on. Multicast routers use a multicast routing protocol to build a distribution tree that connects receivers (also called listeners) to sources.

Upstream and Downstream Interfaces

A single upstream interface on the router leads toward the source to receive multicast packets. The downstream interfaces on the router lead toward the receivers to transmit packets. A router can have as many downstream interfaces as it has logical interfaces, minus 1. To prevent looping, the router's upstream interface must never receive copies of its own downstream multicast packets.

Subnetwork Leaves and Branches

On a multicast router, each subnetwork of hosts that includes at least one interested receiver is a leaf on the multicast distribution tree (see Figure 75). The router must send out a copy of the IP multicast packet on each interface with a leaf. When a new leaf subnetwork joins the tree, a new branch is built so that the router can send out replicated packets on the interface. The number of leaves on an interface does not affect the router. The action is the same for one leaf or a hundred.

A branch that no longer has leaves is pruned from the distribution tree. No multicast packets are sent out on a router interface leading to an IP subnetwork with no interested hosts. Because packets are replicated only where the distribution tree branches, no link ever carries a duplicate flow of packets.

In IP multicast networks, traffic is delivered to multicast groups based on an IP multicast group address instead of a unicast destination address. The groups determine the location of the leaves, and the leaves determine the branches on the multicast network.

Multicast IP Address Ranges

Multicast uses the Class D IP address range (224.0.0.0 through 239.255.255.255). Multicast addresses usually have a prefix length of /32, although other prefix lengths are allowed. Multicast addresses represent logical groupings of receivers and not physical collections of devices, and can appear only as the destination in an IP packet, never as the source address.

Notation for Multicast Forwarding States

The multicast forwarding state in a router is usually represented by one of the following notations:

• (S,G) notation—S refers to the unicast IP address of the source for the multicast traffic and G refers to the particular multicast group IP address for which S is the source. All multicast packets sent from this source have S as the source address and G as the destination address.
• (*, G) notation—The asterisk (*) is a wildcard for the address of any multicast application source sending to group G. For example, if two sources are originating exactly the same content for multicast group 224.1.1.2, a router can use (*, 224.1.1.2) to represent the state of a router forwarding traffic from both sources to the group.