Cisco Systems RJ-45-to-AUX X bits are not used. The remaining bits, Special Multicast Addresses

The X bits are not used. The remaining bits

000 1010 0000 1000 0000 0101

give you 0a:08:05. Put that result together with 01:00:5e: and you get 01:00:5e:0a:08:05.

Here is another example, using the multicast address 224.127.15.10:

1110 0000 0111 1111 0000 1111 0000 1010 XXXX XXXX X111 1111 0000 1111 0000 1010

After removing the X portion, the section

111 1111 0000 1111 0000 1010

yields 7f:0f:0a. Again, you put this result together with 01:00:5e to get 01:00:5e:7f:0f:0a.

This process creates an ambiguity when a Layer 3 IP address is mapped to the Layer 2 MAC address. A 32−to−1 relationship (2^5) exists, meaning that for every multicast MAC address, there are 32 corresponding IP addresses. Figure 7.5 shows the MAC address ambiguities.

Figure 7.5: The 32−to−1 relationship of IP addresses to MAC address.

A host that is set up to receive multicast group 224.127.15.10 will have the network interface card interrupt the CPU when a frame with the destination MAC address 01:00:5e:7f:0f:0a arrives. This MAC address is also used by 31 other multicast groups. If any of these groups are active on the LAN, the CPU will have to examine the frames for both multicast groups to decide if the frame that interrupted the CPU belongs to the desired multicast group. This process could degrade the performance of the host PC if many multicast groups are on the network. You must consider this fact when assigning multicast groups to applications.

Special Multicast Addresses

The range from 224.0.0.0 to 224.0.0.255 is reserved for use on the network by network protocols. Cisco routers by default do not forward IP multicast frames that arrive for this range. Routers that forward these packets are known as broken routers. Routing protocols use this range to communicate with each other (see Table 7.1).

Table 7.1: Partial list of non−routed local multicast addresses.

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