From: Scott Morris (swm@emanon.com)
Date: Thu Feb 05 2004 - 00:22:15 GMT-3
If you are doing a 568A to 568B flop, then only the 1-2/3-6 pairs get
flipped. The 4-5/7-8 pairs are in the same color scheme regardless of which
way you go.
Now, of course, if you MESS UP one side or the other AND change from A to B,
then that will certainly yield your gig crossover. :)
Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713, CISSP,
JNCIS, et al.
IPExpert CCIE Program Manager
IPExpert Sr. Technical Instructor
swm@emanon.com/smorris@ipexpert.net
http://www.ipexpert.net
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Michael Snyder
Sent: Wednesday, February 04, 2004 9:26 PM
To: ccielab@groupstudy.com
Cc: 'Mike Williams'
Subject: RE: 1000Base-T crossover working when it shouldn't?!?!?
My (gig) intel pro nic's will auto crossover from to pc to pc. The gig
cross over cable is a straight thru cable in my case.
"you cross 1/2 with 3/6, but you also have to cross 4/5 with 7/8."
If you use a 568A jack on one side and 568b on the other, maybe you are
straightening your crossover cable.
If everything is correct on each end. And the wrong cable is working, it
has be at the chip level. Remember the pins don't watch the voltage levels,
but the change in voltage levels for one's and zero's.
My some smart engineer figured someone would be trying the wrong cable, and
built the mode into the chip. Either on the switch side or the nic side.
In other words, I don't know.
-----Original Message-----
From: Mike Williams [mailto:ccie2be@swbell.net]
Sent: Wednesday, February 04, 2004 8:03 PM
To: CCIELab@Groupstudy.com
Cc: goldchain@stlo.smhs.com
Subject: OT: 1000Base-T crossover working when it shouldn't?!?!?
Okay............. bear with me here, but this is strange, and Cisco doesn't
even have an answer for this........
The other day, one of our server people was setting up a server and asked me
to make sure his port was patched and active (we have patch panels in the
bottom of each server rack that runs back to the patch
panels by the switches). I checked and the port was already patched
over to our of our Cat4507s (with dual SupIVs with 48-port 10/100/1000
RJ-45 blades). I went and set the port for auto-speed/duplex (since the
server folks can't force their Gig NICs to 1000Mbps, we have to use auto on
both ends to get a 1000Mbps connection). He called about an hour later
asking me to check the port saying he didn't have a connection.
Sure enough it was up/down. He tried another patch cable (we're using
Cat6 patches cables with Cat6 runs). Again, nothing........ So he tried a
3rd cable (which he didn't recognize as a crossover, but it was a different
color than our straight thru cables). Lo and behold....
BAM! The connection came up......... communications work fine, no errors,
etc......... Mind you, this run was previously used by another server, and
patched into our switch with a straight thru cable, so I know the cable in
the floor and the patch cable are good and straight as they're supposed to
be.
Then I started thinking...... 1000Base-T uses all 8 wires, not 4 like
10/100Mbps ethernet........ Having never made a 1000Base-T crossover by
hand, I started checking around, and finally found docs (even one on Cisco's
website) showing that for a 1000Base-T crossover, not only do you cross 1/2
with 3/6, but you also have to cross 4/5 with 7/8. Of course all of the
Cat6 crossovers we ordered are "standard" crossovers with only 1/2 and 3/6
crossed........ Then that made me realize something major. All 4 of our
4507s connect to our 2 core 6509s with these Cat6 crossovers!!! All of the
ports (on both the 4507s and 6500s) show a 1000Mbps connection full
duplex.... no errors, or anything. And according to our monitoring
(Compuware) we have been pushing up to
600+Mbps across these gig links, so they seem they're working normally.
HOW IN THE WORLD could this be working?
So we had a theory that somehow these switchports were "autodetecting"
the crossed nature of the cables........ I checked Cisco's website, and
according to them, starting in IOS 12.1(13)E (native IOS on the 6500s) there
is a "mdix auto" command that only works on 3 particular blades, none of
which we're actually using. We do indeed have 12.1(13)E9, and the command
isn't there! (the Cisco document shows it being used in enable mode, but I
tried enable mode, global config mode, and even interface config mode).
Also, there is NO indication that the 4500s support such a thing and given
that that 6500s IOS only supports this feature with 3 line cards, I doubt it
was a high enough priority to put in the 4500s IOS.
Which THEN brought me to this train of thought: EVEN IF...... EVEN IF there
were some magical way that the ports on both ends of these connections
(4500->6500s) detected that there was a crossed cable, how could this
succeed and work with the "standard" 10/100 Ethernet crossover?!?! I mean,
we're not even using true 1000Base-T crossover cables, just ones with 1/2
and 3/6 crossed. We even began to think that this magic ability was down to
the pairs themselves, so that it could actually use the 1/2 and 3/6 pairs
but not the others, giving 500Mbps......... t'was an idea, but as I
mentioned, we verified with our monitoring software that (at night during
backups in the datacenter) the gig links are actually pushing 600+Mbps, so
that blows the 500Mbps theory..........
This is driving me crazy!!!!! Am I missing something here?......... I
can't complain too much, as all of the links seem to be working..........
but this WILL drive me crazy if I can't find the "logical" explanation for
this..........
Any comments or information on this would be appreciated.
Mike W.
This archive was generated by hypermail 2.1.4 : Fri Mar 05 2004 - 07:13:46 GMT-3