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

Juliusz Chroboczek (spam-protected)
Tue Dec 28 23:28:47 CET 2010

>> 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).


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