[Snort-users] Cisco Catalyst - SNORT
flynngn at ...6811...
Fri Jun 27 19:55:07 EDT 2003
Jeff Nathan wrote:
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>- --On Friday, June 27, 2003 7:34 -0400 Gary Flynn <flynngn at ...6811...> wrote:
>>I'm not saying the switch works this way but if the packets are on a bus
>>and configuring a span port just means telling the port to grab anything
>>on the bus, it would seem there would be no performance hit.
>Grabbing those frames and sending them out a different interface still
>requires I/O operations (interrupt request processing and the like).
>Hidden as it may be, there's always a price.
If the bus is synchronous, the clock could be used to gate the bits into
and ASICs dedicated to the port. No impact on central processing. No
The port hardware is told to accept all data on every clock pulse by a
simple logic level
on a gate. It may be more complicated than that if there is data on the
than the packet stream but you get the idea. The data on the bus may
identify itself as
packet data. There may be codes that delimit packet data. Lots of
can be handled by hardware with a couple of logic level changes that doesn't
require a processor.
I don't know if it works that way but Cisco switch/routers process the
of a flow in software and then claim to switch the rest of the flow in
hardware. I can
see where the processor could set up some registers and flip-flops and
cascade through discrete logic gates as long as the address/port/ID
inputs match the preset
values. To do it promiscuously would seem to be trivial in comparison.
Sort of goes back to the old computers that were set up by an operator
cords and then let everything fly through it for a fixed set of
operations. Once set up,
the data itself drove everything through as it was presented.
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