[c-nsp] Multicast MAC address

[Swaroop Potdar]

Most WAN media types are P2P in nature so there is no Multicast to MAC mapping.
 
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.
 
HTH-Cheers,
Swaroop

________________________________

From: cisco-nsp-bounces at puck.nether.net on behalf of Phil Mayers
Sent: Fri 6/29/2007 5:29 AM
To: Vikas Sharma
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] Multicast MAC address



On Fri, 2007-06-29 at 10:43 +0530, Vikas Sharma wrote:
> Hi,
>
> Need help to understand the concept of multicast?
>
> My question is what is the mac address, when a packet is forwarded across
> the WAN? Is it the mac address of connected router (unicast mac address) or
> multicast mac-address? When L2 and L3 mac address are in use in LAN and WAN
> scenarios?

It's the multicast MAC. GigE router-router p2p links are "just another
ethernet" as far as the packet formats go; they just happen to have
routes pointing down them.

(This means if your p2p actually goes via a L2 switch, you will likely
need PIM snooping on that switch)



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