[ELDK] ARP Messages

Stefano Babic sbabic at denx.de
Wed Nov 17 14:54:57 CET 2010

On 11/17/2010 12:57 PM, Luigi Rizzi wrote:
> Hello,
> I'm facing a strange behave on the ARP answers from  my TQM885D module (U-boot 1.3.0-rc2 and kernel 2.4.25).
> When I connect both Ethernet interfaces, for example configured as follow:
> Eth1:    IP           MAC 00:23:DE:CC:12:01
> Eth2:    IP         MAC 00:23:DE:CC:12:02
> If an ARP request arrives from the net the module sends two answers (from the two interfaces)  with the same IP and different MACs.

I hope you connect the two interfaces to two different physical networks
and the netmask is not set to

> Here the packets I can see on the Ethernet sniffer:
> Source                         Destination       Protocol    Info
> ------------------------------------------------------------------------------------------------------------------------------------------------
> CompaqHp                   Broadcast         ARP         Who has
> 00:23:DE:CC:12:01       CompaqHp        ARP is at 00:23:DE:CC:12:01
> 00:23:DE:CC:12:02       CompaqHp        ARP is at 00:23:DE:CC:12:02

If both ethernets are connected to the same segment (that is forbidden,
even if in many cases works), both ethernet controller gets the ARP
broadcast request and pass the request to the upper layer. I do not know
if then there is still a further check (consider that the request is
broadcast, so not bound to a specific MAC), but anyway it seems to me
your test is broken. However, if the two networks are not connected
together, I agree there is no reason to get two answers.

> Anyone can tell me the reason why I'm getting also the second answer (of course a wrong answer) ?

Maybe wrong test case...

Best regards,
Stefano Babic

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