From: pk@pk.invalid   
      
   On Tuesday 12 May 2009 11:15, Mark wrote:   
      
   > Hello   
   >   
   > studying multicasting and IGMP specifically, and have a few points I don't   
   > clearly understand. First of all why do we need Ethernet multicast   
   > addresses, wouldn't IP multicasting suffice to distribure traffic among   
   > the groups of hosts? I only guess, there are protocols exploiting   
   > Ethernet-based multicasting?   
      
   Ethernet multicast addresses are needed to deliver multicast packets on   
   ethernet networks. A multicast packet has a multicast IP as destination,   
   which cannot be ARPed for. So the last-hop router just puts the multicast   
   packet into an ethernet frame using as ethernet destination address the   
   ethernet address derived from the multicast IP. Hosts interested in that   
   traffic will be listening on that ethernet address and will pick up the   
   frame. Remember that there can be hundreds of receivers on the same   
   network.   
   Other alternatives are inferior and less efficient because they either   
   involve the router broadcasting the multicast frame at the ethernet level,   
   or sending multiple copies of it.   
      
   As for the protocols, yes ethernet multicasting is used all the time by   
   protocols like OSPF, IS-IS, IPv6 for example.   
      
   > And the second. From RFC1112: "IP host address is mapped to Ethernet   
   > multicast address by placing the low-order 23-bits of the IP address into   
   > the low-order 23 bits of Ethernet multicast address 01-00-5e-00-00-00".   
   >   
   > Does this mean that in order to send a multicast packet, a host must   
   > supply a properly built MAC address in the form defined above,  in its   
   > Ethernet header field (destination address)? So the OUI, purchased by   
   > equipment manufacturers would not suffice?   
      
   Yes. Again, you can have hundreds of receivers, each with its own OUI. The   
   ethernet mapped multicast address will reach all of them and the sender   
   will have to send only one frame. (I'm ignoring the implications of having   
   non-local receivers for simplicity, but you get the idea)   
      
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