On Fri, Mar 26, 2004 at 02:25:34PM -0800, Wes Peters wrote:
...
> In the Xylan (now Alcatel) second-generation switches (The "X-Frame"
> backplane) the switching hardward was capable of switching on the MAC
> header *or* other predefined parts of the packet if no MAC header matches
> were found. This feature was used to implement hardware routing (the HRE-X
> module), allowing us to route packets between IP networks at a million
> packets per second.
i think you need to tell the full story, such as what was the
limit on the routing table, and whether switching packets for
which there wasn't a host-specific entry was slower.
Finally, cost is not an inessential detail here... I
pointed to an L2 switch which can switch around 2.5Mpps and
costs Eur 60, retail...
An L2 switch has two big advantages over an L3 switch:
+ only an exact match on the MAC address is necessary, as opposed to
the longest prefix match which is required for a router.
Surely you need more/different hw to do longest prefix match
than the one needed for L2 exact match.
Sure, you can install host-specific entries and then use an
exact match on those, but the 'miss' case is more expensive, and
if you want to do a worst-case rating, then you need to
use that number;
+ in case of a miss, an L2 can flood all ports, a router can't
(well, in principle even a router could do that, but i think the
reviews wouldn't be so nice if a product did this).
So an L2 thing is inherently cheaper as it can play tricks to
cut costs down and still behave within the specs.
cheers
luigi
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