[Cialug] Basic Networking ARP and Routers

[Nick Graber]

Oh,  I forgot to ask if it was an issue that the Ethernet frame check
sequence was incorrect?

Sample ARP summery from wireshark:
1 0.000000000 Schniede_10:ea:16 Broadcast ARP 64 Who has 172.20.87.235?
 Tell 10.1.88.250 [ETHERNET FRAME CHECK SEQUENCE INCORRECT]


On Thu, Jul 24, 2014 at 3:38 PM, Nick Graber <nick at graber.me> wrote:

> I checked the settings on the Schneider PLC
> IP: 10.1.88.250
> SNM: 255.255.255.0
> GW : 10.1.88.254
>
>
>
> On Thu, Jul 24, 2014 at 3:16 PM, Zachary Kotlarek <zach at kotlarek.com>
> wrote:
>
>>
>> On Jul 24, 2014, at 1:04 PM, Nick Graber <nick at graber.me> wrote:
>>
>> > The Schneider PLC is using ARP outside of it's subnet and expecting a
>> reply
>> > from the router with it's MAC to make a connection with the AB PLC I
>> think.
>> > Should the router replay to such and inquiry or should the PLC do an ARP
>> > for the Default Gateway then send the packet to the Default Gateway?  It
>> > appears it is using ARP to try and discover the router that controls
>> > 172.20.87.255 and I'm not sure that is possible.
>>
>>
>> It looks like the Schneider thinks that 172.20.87.235 is link-local.
>> Otherwise it would ARP for the router, not the destination.
>>
>> My guess is the Schneider is not configured to use the router, either
>> because the subnet information it has is wrong (maybe set to 0.0.0.0) or
>> because it does not support IP routing (which sometimes happens on dumb
>> equipment even though it pretends to speak IP).
>>
>> It is possible to bridge the two networks in such a way that this works,
>> but I’d start by ensuring that 172.20.87.235 doesn’t match the subnet on
>> the Schneider.
>>
>>         Zach
>>
>>
>> _______________________________________________
>> Cialug mailing list
>> Cialug at cialug.org
>> http://cialug.org/mailman/listinfo/cialug
>>
>>
>