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

Tom Ammon tom.ammon at utah.edu
Tue Jul 13 01:04:30 EDT 2010


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

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
> 172.16.0.29 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
> 172.16.0.14 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 172.16.0.29 or 172.16.0.14.
>
>    

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Tom Ammon
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Center for High Performance Computing
University of Utah
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