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Link Layer Multicast

Link Layer Multicast

Posted Feb 20, 2013 5:54 UTC (Wed) by dlang (subscriber, #313)
In reply to: Link Layer Multicast by butlerm
Parent article: Opera moves to WebKit and V8

There are two layers of multicast.

There is IP based multicast (224.x IP addresses). This layer is managed by the routers knowing what downstream routers need copies of the traffic and sends it to all of them

There is ethernet link-layer multicast (a '1' in the least significant bit of the first byte of the MAC address, i.e. 01:00:00:00:00:00), traffic to these MAC addresses get handled by the ethernet switch.

These two can be used in combination with each other, but you can use the link-layer multicast with any IP address (and with no modification to the sending or recieving software), and I assume that you could use the IP based multicast without using the link-layer multicast, but I also wouldn't be surprised to learn that most of the time things default to using link-layer multicast if you are using the IP multicast range.

I also think that you will find that even the rather cheap switches handle link-layer multicast nowdays.

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Link Layer Multicast

Posted Feb 20, 2013 17:59 UTC (Wed) by butlerm (guest, #13312) [Link]

IP multicast (224.x.x.x) addresses have been hardwired to a range of Ethernet multicast addresses (01:5e:00:xx:xx:xx) for a very long time, and unfortunately overriding that mapping doesn't seem like it would accomplish anything, because any Ethernet switch is just going to broadcast anything it doesn't recognize anyway.

A practical example of this is where you have multicast IPTV traffic arriving from a ISP network into a home/office network. You generally have to filter that out and direct it over a separate network of some kind to every IPTV device, because if you just forward it directly onto the local subnet, inexpensive switches will broadcast everything to every port, which is a problem.

Does anyone really want to run a separate network to their set top boxes simply because link layer multicast is synonymous with link layer broadcast? It makes it difficult to watch television on ordinary desktops because they are connected to the wrong network, for example. Perhaps inexpensive Ethernet switches will implement IGMP snooping in the future for this reason. It isn't common yet though.

Link Layer Multicast

Posted Feb 21, 2013 4:03 UTC (Thu) by foom (subscriber, #14868) [Link]

Well, at least it's possible to get IGMP snooping these days without paying a hugely excessive amount of money.

E.g. Netgear GS108 ($53) has no IGMP snooping, while Netgear GS108T does ($80).

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