Introducing IP Multicast

With IP Multicast, an application source (S) sends traffic to a group (G) of receivers across your network. You assign the source with a unicast IP address and each group with a single multicast IP address within the registered multicast class D IP address range. When receiving hosts want to receive traffic from a multicast flow G, it is their responsibility to join the group themselves by using Internet Group Management Protocol (IGMP). Multicast routers do not track the IP addresses of the receivers, only whether or not there are any on the local LAN that want to receive the multicast traffic. That is, only those receivers that explicitly join a group receive traffic for that group.

You can configure your network for multicast using one of the following technologies.

• Internet Standard Multicast (ISM) Any number of sources deliver ISM Multicast streams to a group of receivers. The receivers subscribe to groups, and the routers track the sources sending the multicast traffic in (S, G) state table entries. The receivers do not need to be source-aware and use IGMPv1 or IGMPv2 to join (*, G) host groups. Bidirectional streams are also available with ISM.

  Note

  The notation (S, G) indicates that the individual source S is actively sending traffic to group G. In contrast, the (*, G) notation refers to any source * sending to group G.

  
  

• Source Specific Multicast (SSM) With SSM Multicast, the network delivers a stream from one sender to a group of receivers. The receivers are source-aware and explicitly request multicast traffic from individual senders of a desired group in (S, G) IGMPv3 Include/ Exclude Join messages. The routers do not need to track the sources of the multicast groups and store only (*, G) state in their multicast routing tables. Therefore, SSM greatly reduces the complexity of the networks that are required to deliver multicast streams by offloading the source state to the receivers.

Note

The sources do not signal any information to the network in either ISM or SSM; they simply start sending their traffic to the network when you schedule them to send.

Multicast IP addresses fall within the registered class D multicast range 224.0.0.0239.255.255.255. This range conveyed in binary, with the first half octet (4 bits) set to 1110, is

11100000.00000000.00000000.00000000 to

11101111.11111111.11111111.11111111

This range gives you 28 total bits of available multicast IP addresses. Table 5-1 gives the uses for the reserved multicast address ranges that pertain to multicast content delivery. Note that many network protocols, such as Open Shortest Path First (OSPF), also rely on multicast addresses to function (for example, 224.0.0.5 and 224.0.0.6) but are not included in Table 5-1.

Table 5-1. IP Multicast Address Usage

Address(es)	Usage
Link Local Scope
224.0.0.1	All systems on this subnet
224.0.0.2	All routers on this subnet
224.0.0.13	All PIM routers address group
Global Scope
224.0.1.0 to 238.255.255.255	Allocated for multicasting traffic across the Internet. SSM reserves the range 232.0.0.0/8. The addresses 224.0.1.39 and 224.0.1.40 are used for Auto-RP negotiation. You can assign the remaining IP address to your ISM applications.
Administrative Scope (AS)
239.0.0.0/8	Allocated for organizations that own an AS number to multicast across the Internet. The AS number of the organization is embedded in the 2nd and 3rd octets of the multicast IP address. For example, the AS 64501 is FBF5 in hexadecimal, with FB and F5 (or 251 and 245 in decimal) representing the 2nd and 3rd octets of the IP address, respectively. The resulting subnet 233.251.245.0 is globally reserved for AS 64201 to use. These addresses are called GLOP addresses.

The multicast IP addresses in Table 5-1 map to the Ethernet MAC addresses that range from 0100.5e00.0000 to 0100.5e7f.ffff. These addresses are owned by the Internet Assigned Numbers Authority (IANA). This range allocates 23 bits of the MAC for a direct copy of the last 23 bits of multicast IP address. For example, in Figure 5-1, the multicast IP 224.1.16.33 maps to 0x0100.5e01.1021.

Figure 5-1. A Sample Multicast MAC to Multicast IP Address Mapping

Recall that multicast IP addresses use 28 bits of IP address, resulting in the first 5 bits of multicast IP address being lost during the address copy. As a result, you can see in Figure 5-2 that 224.129.16.33 also maps to the MAC address 0x0100.5e01.1021.

Figure 5-2. An Illustration of MAC-to-IP Address Overlapping

Figure 5-3 shows the 32 multicast IP addresses that map to the MAC address 0x0100.5e01.1020.

Figure 5-3. Each Multicast MAC Has 32 Corresponding Multicast IP Addresses