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Re: NIC to NIC connection

From: Bruno Wolff III <bruno(at)wolff(dot)to>
To: Gaetano Mendola <mendola(at)bigfoot(dot)com>
Cc: pgsql-admin(at)postgresql(dot)org
Subject: Re: NIC to NIC connection
Date: 2004-10-21 10:19:23
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-admin
On Thu, Oct 21, 2004 at 10:07:33 +0200,
  Gaetano Mendola <mendola(at)bigfoot(dot)com> wrote:
> 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, 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.

Switches don't do arps; they are a layer 2 device (and hence don't know
about IP addresses. They cache mac address port bindings. If their cache
expires before the cached IP mac address bindings in some attached device
then they can get requests to send to mac addresses for which they don't
have a cached port. In that case they are supposed to do a port flood of
the packet.

I actually had a problem with a switch that was misconfigured and didn't
do a port flood in these circumstances. That combined with a linux box
that wasn't chatty enough to stay in the switch's cache and that the
router cached IP address - mac address bindings for hours while the switch
kept mac address - port bindings for minutes caused my machine to be
unreachable to other hosts for periods of time.

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