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

Rahul Nabar rpnabar at gmail.com
Tue Jul 13 00:48:47 EDT 2010


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.

-- 
Rahul
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