Re: Multicast MAC and Unicast IP Address

In article <4305f422$1_2@xxxxxxxxxxxxxxxxx>,
Matthias Schaerer <matthias.schaerer@xxxxxxxxx> wrote:

>
> How if not by looking at the multicast bit does a switch decide to
> multicast/broadcast a frame? I doubt that many implementors make a
> distinction between broadcast and multicast by looking at more bits than
> that one.
>

The default action of a switch is the same for both multicasts and
broadcasts; the frame is forwarded onto all ports except the port on
which the frame arrived. Thus, for this simple case, the switch need
only look at the unicast/multicast bit, as you note.

The standards allow for more complex handling of multicast than this
simple default behavior, however. Automatic multicast pruning (e.g.,
using GMRP) can restrict the propagation of specific multicast addresses
onto specific ports. Manual configuration of the switch's forwarding
tables (by a network administrator) can also be used to control the
handling of specific multicast frames. In both of these cases, all 48
bits of the multicast must be examined in order to make the appropriate
forwarding decision. From the switch designer's perspective, it may be
easier to think of a multicast not as a "limited broadcast," but as a
unicast that may be forwarded onto more than one port. The entire
address is compared to the entries in a lookup table, which outputs the
set of ports onto which to forward the frame. The output for a unicast
will generally be only a single port; the output for a multicast will
generally be one or more ports. Thus, unicasts and multicasts are
processed identically.


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Rich Seifert Networks and Communications Consulting
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