From: pk@pk.invalid   
      
   On Wednesday 13 May 2009 09:31, Mark wrote:   
      
   > "pk"  wrote in message news:gubfr4$28m$1@aioe.org...   
   >> 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.   
   >   
   > I heard such behavior - forming multicast Ethernet addresses - is also   
   > inherent to layer2 devices, supporting IGMP, i.e. aware of IGMP snooping.   
   > Is that correct?   
      
   That is a different matter. Since an ethernet multicast address does not   
   belong to a specific host, what switches used to do (and those that don't   
   support IGMP snoopig still do) is to flood multicast packets they receive   
   out of all ports belonging to the same broadcast domain, which is not   
   efficient (think a broadcast domain with 100 hosts and 2 multicast   
   receiver). So what those switches do is to intercept IGMP messages they   
   receive to try to understand on which ports multicast receivers may be   
   located, and flood multicast frames only out of those ports (simplified).   
      
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