Figure 35 Overlap of Multicast MAC Addresses

32 IP Multicast Addresses

Multicast MAC Addresses


An Ethernet multicast MAC address has some overlap—the same MAC address is assigned to 32 different multicast groups. If one user on an Ethernet segment subscribes to multicast group, and a nother user subscribe s to 225 .w29. 1.1, both users receive b ooh multicast streams. I n en gineering multicast networks on LAN segments, this overlap needs to be specifically watched for and engineered to avoid the problem.

With Token Ring networks, the overlap is even greater. As mentioned earlier, Token Ring uses the concept o" functional addfesses. Al so. remember t hct Token Ring uses noncanonical addressing, so the bits are swapped on a byte level. Layer 3 IP multicast addresses are mapped to a single Cun ctional address, lea vicg j ust a Mttle bit of overlag. Subtracting the fitst 4 bits th at Cll IP mu lticast ad dresses have in comm on leave s 28 tltt oU ou5cIip, oo aperoxima"e ly 268,435^00 mult icast addtesses mapped into a single MAC a ddresc.

Needless to say , the be st way to engineed mu Itica st o n Layet 2 is to noo use To ken Ri ng. Within Cisco coufiguration, the derauit mechanism is to map mult icadt packets into btoadcast frames (FFFF.FFFF.FFFF).

It yod want to use the Token Ring functio nal address, use ohe ip multicast use- functio nal command on the Token Rino intertace. This uses C00y.0000.0000 to map the multieast IP packets.

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