[c-nsp] Multicast MAC address

[Phil Mayers]

On Fri, 2007-06-29 at 06:06 -0500, Swaroop Potdar wrote:
> Most WAN media types are P2P in nature so there is no Multicast to MAC
> mapping.

Well yes, but since the OP mentioned multicast MAC addresses, it was a
pretty safe bet to assume he was talking about Ethernet

>  
> Since ethernet is a broadcast and multiple access type media in
> nature, the multicast to mac address mapping is desired to avoid
> flooding of traffic to all hosts connected to the media. 
>  
> So when you enable IGMP Snooping on a Layer 2 switch it makes a note
> of all the IGMP requests and converts them to mac equivalent and
> stores in the cam table. and also reports the same to the router, then
> the router forwards the traffic feed to the switch and the traffic
> reaches to the end destinatiosn which had requested the group, but the
> destination address is the multicast-mac address when it is sent to
> the end host, hence not everybody receives the traffic, but only the
> hosts which sent the igmp join will receive the traffic.

Sure - but in an ethernet-as-wan-link setting, IGMP is not likely to be
in use, it's more likely to be PIM - and PIM snooping (or set
multicast=flood) is needed if a layer2 hop exists between the two layer3
endpoints.

>