From: Mike Williams (ccie2be@swbell.net)
Date: Thu Feb 05 2004 - 02:18:47 GMT-3
Scott's right.......... With the 568, you're only flipping 1/2 and
3/6........ 4/5 and 7/8 are left alone..... All patch cables were made
for us and pretested........ Both straight thru and crossovers.........
I knew the Intel NICs (some of them) could do that autodetect MDIX.....
However, even that capability can't overcome the fact that a standard
10/100 ethernet crossover cable is the wrong cable for 1000Base-T and
(in theory) cannot work. Unless they're actually detecting down at the
pair level (i.e. the port is smart enough to see that 1/2 and 3/6 are
swapped but that 4/5 and 7/8 aren't)...... But I've never heard of
such......... However, that's all moot since both devices are Cisco Cat
switches running IOS (native on the 6500s) and NOTHING Cisco has says
they can do this auto-MDIX except for one doc saying the 6500s can with
3 specific line cards using 12.1(13)E and up, etc, etc.........
I just can't understand how this kind of stuff happens......... I'm
frustrated with that "the more you know, the less you understand" (or I
prefer to say "the more you know, the more you see how much you don't
know yet") thing that's happening....... Everytime you think you have a
good handle on how things work, crap like this pops up........
Oh well........ C'est la vie! =)
Mike W.
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Scott Morris
Sent: Wednesday, February 04, 2004 9:22 PM
To: 'Michael Snyder'; ccielab@groupstudy.com
Cc: 'Mike Williams'
Subject: RE: 1000Base-T crossover working when it shouldn't?!?!?
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.
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