Date: Tue, 12 Dec 2006 10:20:55 -0800 From: Chuck Swiger <cswiger@mac.com> To: Javier Henderson <javier@kjsl.com> Cc: freeBSD List <freebsd-questions@freebsd.org> Subject: Re: How does my computer work with an empty arp table? Message-ID: <D0751C40-E58A-4D1F-A6DA-7C29ECD9F6C5@mac.com> In-Reply-To: <F03B85A1-8CBE-4691-B43B-3632361CBC25@kjsl.com> References: <20061203174849.GA4561@host.my.domain> <20061204154222.GA636@host.my.domain> <200612120014.22107.mapsware@prodigy.net.mx> <4448FE68-D588-4ABA-84E2-504582EFD80F@mac.com> <F03B85A1-8CBE-4691-B43B-3632361CBC25@kjsl.com>
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On Dec 12, 2006, at 10:08 AM, Javier Henderson wrote: >> The ARP table only contains information about machines on the >> directly connected collision domain(s). > > Are you sure it's not the same broadcast domain? Yes. The term "collision domain" predates the wide deployment of switches, and switches have to treat ARPs in a special fashion: > A computer on port A on a switch would be on a different collision > domain than a computer on port B on the same switch, yet as long as > they're on the same VLAN (ie, broadcast domain), both would have > each other in their resepctive ARP tables if they were exchanging > Ethernet traffic. ...in particular, ARPOP_REQUEST traffic will be propagated to every port on the switch which is configured to be a part of that VLAN, or, quite possibly, other ports including "trunk ports" or sometimes even ports configured on other VLANs. [1] Many switches will do this for all ethernet packets with an ether_dhost (ie, destination MAC) of all-ones. -- -Chuck [1]: And yes, Virginia, this has negatory implications if your security relies on VLANs to actually be completely hidden from each other.
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