Re: NIC to NIC connection

On Thu, Oct 21, 2004 at 10:07:33AM +0200, Gaetano Mendola scratched on the wall:
> Bruno Wolff III wrote:
> >Also I believe that if
> >a switch doesn't remember where a particular mac address is it will send
> >the packet to all of the attached ports.
>
> I don't think so, I guess the switch perform a sort of arpping in order to
> detect who have a macaddress assigned,

No, he's right. If the MAC to port mapping has not been learned by
the switch, the packet is flooded to all ports or (for really bad
switches) dropped. A switch is a pure layer-two device and ARP
involves layer-three addresses and concepts.

The only part of a switch that will ARP is the management system,
assuming it has an IP address of its own.

This is actually a standard test we run when evaluating switches for
deployment on campus. We hook about ten devices up to a switch and
send them tons of traffic. The systems count incoming packets but
they do not generate any traffic, so the switch is forced to flood
all the traffic. The vast majority of low-cost switches roll over and
die at that point.

> even the multicast is not sent
> to all ports but only to that ports where "someone" sent an arp packet
> saying the he was registered to a multicast address.

Not ARP, IGMP (Internet Group Management Protocol). It's part of the
IP suite, and is much higher up in the protocol stack.

And again, this isn't true unless the switch does IGMP snooping or
has multicast assist from the router. IGMP snooping is becoming more
common on higher-end switches, but there are some companies that
prefer the router assist design. The problem with that is that the
switches and routers need to be from the same company.

IGMP snooping is also really tricky to do right, and there are still
some situations where you are forced to flood traffic.

-j

--
Jay A. Kreibich | Comm. Technologies, R&D
jak(at)uiuc(dot)edu | Campus IT & Edu. Svcs.
<http://www.uiuc.edu/~jak> | University of Illinois at U/C