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Posted by Will on July 11, 2007, 2:24 am
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This was a good guess, but it didn't solve the problem. I can configure
the ethernet adapter on the host computer as any of:
10BT Half
10BT Full
100BT Half
100BT Full
Autosense
and I get more or less equally unreliable effects, packets getting dropped.
As soon as I connect the host to a hub the problem goes away.
I tried to use an aggregating tap to look at the traffic, but sure enough
you connect the host to the aggregating tap and the problem goes away again.
So definitely it looks like some kind of compatibility issue between the
adapter and the switch, but very strange that I cannot get it to work at any
fixed configuration.
--
Will
>
>> I have a strange situation that I think others in this group could
>> probably
>> clarify for me. I have two old Bay Networks / Nortel Accelar 1200
>> switches, A and B. Switch B uplinks to A, which uplinks in turn to a
>> Netgear switch which then connects to a firewall. When I plug
>> Computer
>> One to switch B or switch A directly, the computer works but unreliably.
>> Occasionally incoming packets are getting dropped (maybe one in six).
>> Computer One has a single Netgear 10/100 ethernet card in it.
>>
>> If I take the same Computer One and plug it to a Netgear hub, and then
>> uplink that hub to either switch A or switch B, now the Computer One
>> works
>> very reliably, both fast and no dropped packets at all.
>>
>> Does anyone have an explanation as to why such a behavior would happen?
>> The sniffer trace when run from the Computer One clearly shows missing
>> packets, as compared to the sniffer trace taken from the firewall on the
>> subnet outbound from the firewall to the switch A / B complex. I assume
>> the problem is at the ethernet level, which unfortunately the trace
>> doesn't
>> show, except for layer 2 / ARP. I suppose there could be another
>> computer
>> doing man-in-the-middle attacks and stealing packets using Computer One's
>> ethernet address, but I'm hoping to find a simpler explanation before
>> going
>> off on a wild goose chase for a hacker.
>
> There are a few possibilities, but I'm going to go with
> autonegotiation failure. The device is probably choosing half-duplex
> even though the switch is choosing full-duplex.
>
> If the switches are configurable:
>
> 1) If they have an 'auto' setting, try that first.
>
> 2) DO NOT set the switches duplex (and especially do not set full-
> duplex) unless you can also set the devices the same! It is a serious
> mistake to hard-set the duplex on only one end of an Ethernet link
> (except in *very* limited circumstances).
>
> 3) If the switches have no 'auto' setting, either set them to 'half-
> duplex' or set both the switch and the device to 'full-duplex'. (Most
> likely, your device will choose half-duplex if it cannot negotiation.)
>
> 4) If possible, manually confirm that both ends have the same duplex
> setting.
>
> DS
>
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