Please excuse the duplicate post (If there is one)
Hello all,
After being tasked to diagnose why a node is reporting
a Half duplex connection I ran across an unusual observation.
I've pretty much always assumed that what a switch
reported as a status regarding the link of a node was the
actual status of the line to be the case. However, when I
checked the actual line state using a Fluke Net tool box
I was surprised to discover a discrepancy. The scenario is
this, the unit in question is a Cisco 4006 Catalyst. The port
being connected to is set to Auto negotiate. Once the unit
in question connects over a fairly poor RJ-45 connection
using auto negotiate (no not beyond 100 meters, actually
280 feet) the switch reports:
Port Name Status Vlan Level Duplex Speed Type
----- ------------------ ---------- ---------- ------ ------ ----- ------------
3/2 connect 106 normal a-full a-100 10/100BaseTX
However the line analyzer says duplex is half, but advertised full.
When I set the node in question to Half Duplex, the switch reports half duplex as well.
Port Name Status Vlan Level Duplex Speed Type
----- ------------------ ---------- ---------- ------ ------ ----- ------------
3/2 connect 106 normal a-half a-100 10/100BaseTX
Is this a common situation or just a mis-understanding of the column that makes up
Duplex for the Cisco IOS? Or is it that The Cisco Switch truly is trying to do
full duplex but the end node is only doing half. If so would/should I see line errors
at some point under heavy load/traffic? And if not why would the line analyzer see the
connection as half when the switch is reporting full?
And btw, Firmware is at 5.5(11)
Any thoughts appreciated.
TIA
-Joe
Hello,
I've pretty much always assumed that what a switch
reported as a status regarding the link of a node was the
actual status of the line to be the case. However, when I
I think that there is no such thing as "the actual status of the line".
Each end of the cable has a status, either full or half. That is, they
don't use or do use CSMA/CD.
Is this a common situation or just a mis-understanding of the column
that makes up Duplex for the Cisco IOS? Or is it that The Cisco Switch
truly is trying to do full duplex but the end node is only doing half.
It's quite common, that one end or the cable works in full duplex mode,
while the other works in half duplex. There can be various reasons for
this. Improper configuration (setting on end to auto-negotiate while
setting the other to fix full-10 for example), software/firmware bugs,
etc.
If so would/should I see line errors at some point under heavy
You should see late collisions on the half duplex end, since the full
duplex end will transmit frames whenever it has them, and this happens too
between the transmission of the 64th octet and the CRC of frames sent out
on the half duplex end, which is a violation of CSMA/CD (late collision).
On the full duplex end, you'll find CRC errors, runts, alignment errors,
because the half duplex end stops transmitting frames in the middle, when
it senses late collisions.
Andras
I've seen the same behavior with a Catalyst 2948G. My servers weren't
experiencing any problems, and appeared to be connecting at 100/Full -
despite what the Fluke reports. If I recall correctly, the 2948s use the
same software base as the 4000s - I've written it off as a software
incompatibility between the 2948/4000 code and the Fluke.
I did a cursory search of Cisco's bug reports, but found nothing related.
My advice is to ignore the auto-negotiate between the 4000 and the Fluke.
J.J.
From: Joes Jobs <joej@mail.rocknyou.com>
Date: Tue, 16 Jul 2002 03:56:22 -0700
To: nanog@merit.edu
Subject: Cisco 4000 series switches
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