Mapping IP Multicast to Ethernet

Mapping IP Multicast to Ethernet
Multicast addressing began on MAC addresses. Growth needs required that there be a way
to use multicast across routers instead of limiting it to the physical segment where hosts were
located. In regular unicast, MAC addresses are layer 2 addresses, and in order for the local host
to reach remote hosts, layer 3 logical IP addresses are used to route data to the destination. After
TABLE 1 9 . 1
IP Multicast Reserved Addresses
Address Purpose Reserved Category
224.0.0.0–224.0.0.18 Use by network protocols Local-link
224.0.0.1 All hosts Local-link
224.0.0.2 All routers Local-link
224.0.0.19–224.0.0.255 Unassigned Local-link
224.0.1.0–224.0.1.255 Multicast applications Misc. applications
224.0.1.1 NTP Misc. applications
224.0.1.8 NIS+ Misc. applications
224.0.1.39 Cisco-RP-Announce Misc. applications
224.0.1.40 Cisco-RP-Discovery Misc. applications
224.0.1.80–224.0.1.255 Unassigned Misc. applications
224.0.0.10 EIGRP Local-link
239.0.0.0–239.255.255.255 Private multicast domain Administratively scoped
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Chapter 19 
Understanding and Configuring Multicast Operation
the packet reaches the remote subnet, the ARP is used to find the MAC address of the host. By
using an existing ARP table, or via an ARP request, the MAC address that is associated to the
layer 3 IP address is found and the packet is forwarded to the destination host.
IP multicast generates a MAC address based on the layer 3 IP multicast address. The MAC
frame has a standard prefix of 24 bits. This prefix, 01-00-5e, is used for all Ethernet multicast
addresses. This leaves another 24 bits for use in creating the multicast MAC address. When the
MAC address is generated, the 25th bit (or high order bit) is set to 0 and then the last 23 bits
of the IP address are mapped to the remaining 23 bits of the MAC address. Figure 19.4 depicts
how this looks.
MAC addresses are made up of two sets of addresses, each with 24 bits. The
first set is an address reserved for a particular manufacturer. The second set
identifies a particular device by that manufacturer. This is why Cisco devices
always seem to have one of a small number of “first halves.” Multicast MAC
addresses use 01-00-5E for the vendor code, with the device code based on the
IP address.
Let’s look at some examples of mapping layer 3 multicast addresses to layer 2 multicast
addresses. A local IP multicast address is 224.0.0.1. Refer to Figure 19.5 to see how this is
mapped. The conversion from binary to hexadecimal reveals the MAC multicast address. The
prefix was 01-00-5e. The last 23 bits, including the high order bit, give you 00-00-01. Put them
together and you get 01-00-5e-00-00-01 as the MAC address.
Now let’s try one a little bit harder. Suppose, for example, you have the IP multicast address
of 225.1.25.2 (follow along in Figure 19.6). Part of the 225 octet falls within the Class D mask.
However, there is one bit that is not masked. By looking carefully at the location of the bit, you
see that it is part of five lost bits and is not mapped to the layer 2 MAC multicast address.