[Olsr-users] OT: Ethernet multicast [was: Route-Flapping...]

Roar Bjørgum Rotvik (spam-protected)
Wed Dec 29 00:42:56 CET 2010

Den 28.12.2010 11:28, skrev Juliusz Chroboczek:
>>> regarding ethernet layer 2 any mac adress having the last bit of the
>>> first byte set to 1 is a multicast,..
>>> this includes broadcasts FF:FF:FF:FF:FF:FF or various types of multicasts,..
>> So the first 3 bytes in ethernet destination must be "01-00-5E", not
>> only last bit of first byte for the packet to be interpreted as mcast.
> Sorry, but no.  Markus is correct.
> At the link layer (layer 2), a MAC-48 destination address is interpreted
> as multicast when the ``g'' (group) bit in the MAC-48 is set to 1.
> While it is true that IPv4 addresses are mapped to 01-00-5E, the link
> layer doesn't care -- only the g bit is significant at layer 2.
> 802.11 drivers are pure layer 2 devices -- they only check the g bit
> when deciding whether to use the multicast rules for sending a frame
> (lower bit rate, no link-layer ARQ, no RTS/CTS exchange).

Ok, I stand corrected then :)

I have worked a lot with mcast traffic, but only for L3 (IPv4 and IPv6) 
so I was not aware of the L2 difference. I see that this is explained here:

Thanks for the clarifying description.

Roar Bjørgum Rotvik

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