[Beowulf] Network problem: Why are ARP discovery requests sent to specific addresses instead of a broadcast domain

Tom Ammon tom.ammon at utah.edu
Mon Jul 12 22:04:30 PDT 2010

This is called a gratuitous ARP. Used to update the ARP caches of other 

On 07/12/2010 10:48 PM, Rahul Nabar wrote:
> On Mon, Jul 12, 2010 at 11:25 PM, Patrick Geoffray<patrick at myri.com>  wrote:
>> Rahul,
>> On 7/13/2010 12:04 AM, Rahul Nabar wrote:
>>> I am puzzled by a bunch of ARP requests on my network that I captured
>>> using tcpdump. Shouldn't ARP discovery requests always be sent to a
>>> broadcast address?
>> No, the kernel regularly refreshes the entries in the ARP cache with unicast
>> requests. If that fails, then it sends the expensive broadcasts.
> Thanks Patrick. I wasn't aware of this. I guess it makes sense now
> that I found the correct section of the RFP
> (http://tools.ietf.org/html/rfc1122#page-22).
> I see the converse situation too: Some ARP replies are being sent to a
> broadcast domain instead of a single MAC. Is that normal too?
> 00:26:b9:58:e5:9f>  ff:ff:ff:ff:ff:ff, ARP, length 60: arp reply
> is-at 00:26:b9:58:e5:9f
> 00:26:b9:56:38:71>  ff:ff:ff:ff:ff:ff, ARP, length 60: arp reply
> is-at 00:26:b9:56:38:71
> I'd have (naively) expected these replies to go to the specific MAC
> which had issued an ARP request on or

Tom Ammon
Network Engineer
Office: 801.587.0976
Mobile: 801.674.9273

Center for High Performance Computing
University of Utah

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