Auto-mdix / cross-over cables

[Cooky]

We've always used cross-over cables in our datacenter when we need to inter-connect switches.

A colleague who started a few months ago suggested to change that policy, and use straight-through cables across the board instead.

His reason was it's easier to maintain only one set of cables instead of two.
From management and cost saving's point of view, it makes sense.

However, I want to make sure in terms of technology, it'll work 100%, w/ no glitch.
I had always thought it's best practice to not having to rely on the auto-mdix feature.

We use Cisco switches, and hard-code speed & duplex the ports.
Thought for auto-mdix to work, you have to use auto-sense instead of hard-code.
We've seen it work even when the port is hard-coded though..so that was weird.

Comments or thoughts?

 

[ScottMac]

IMO, for critical infrastructure, "Auto-{anything}" means "You 'Auto' not use it."

Basically you're putting your important stuff in the hands of a programmer (!!!!) that may or may not have any idea what the code does that he/she is writing.

Some of the convenience stuff that can't really affect operation, OK, sure, let it happen automatically. But for anything that can directly affect operation or performance, my preference is to do it manually to make sure it is operating exactly the way I want it to.

I came from a dip-switch setting generation, so I'm probably just being paranoid.

FWIW

 

[DisgruntledVirus]

How much of a cost savings is it? I doubt it would be that much of a difference to warrant the change. I'm with Scott in that I'd rather as much be hard coded/wired as possible. Using auto-mdix introduces a new layer of possible issues, where using a crossover cable won't (as long as it's correctly wired obviously).

Unless there is a massive cost savings, I would be against that change personally.

 

[spidey07]

I recommend using straight thru. As much as I've been against auto-anything like Jack said, 1000 Base-T seems to be bullet proof for autonegotiation and mdi-x and I've yet to see a problem with it.

I think a lot of the paranoia stems from the 10/100 Base-T days and it's autonegotiation problems.

-edit-
Yes, you're correct. If you hard set speed/duplex then auto-mdi is turned off, at least for 100 Base-T ports, don't know about 1000 Base-T because I've always used autonegotiation (and as such auto-mdi). Since auto mdi is built into 1000 base-t it may still be on even if you hard set speed/duplex to 1000/full.

 

[DisgruntledVirus]

Originally posted by: spidey07
I recommend using straight thru. As much as I've been against auto-anything like Jack said, 1000 Base-T seems to be bullet proof for autonegotiation and mdi-x and I've yet to see a problem with it.

I think a lot of the paranoia stems from the 10/100 Base-T days and it's autonegotiation problems.

OP didn't state if they are using 1000 Base-T or not.

 

[spidey07]

Originally posted by: DisgruntledVirus

Originally posted by: spidey07
I recommend using straight thru. As much as I've been against auto-anything like Jack said, 1000 Base-T seems to be bullet proof for autonegotiation and mdi-x and I've yet to see a problem with it.

I think a lot of the paranoia stems from the 10/100 Base-T days and it's autonegotiation problems.

OP didn't state if they are using 1000 Base-T or not.

The year is 2009. It's easy to assume things if we're talking interlinks between network devices.

 

[DisgruntledVirus]

Originally posted by: spidey07

Originally posted by: DisgruntledVirus

Originally posted by: spidey07
I recommend using straight thru. As much as I've been against auto-anything like Jack said, 1000 Base-T seems to be bullet proof for autonegotiation and mdi-x and I've yet to see a problem with it.

I think a lot of the paranoia stems from the 10/100 Base-T days and it's autonegotiation problems.

OP didn't state if they are using 1000 Base-T or not.

The year is 2009. It's easy to assume things if we're talking interlinks between network devices.

Yeah, but there could be low end switches/older stuff that isn't 1000 Base-T yet.

I work for a fortune 500 company, and we just switched out switches in one of our cabinets to 10/100 models to the desktop using 1000 Base-T uplinks/trunks and fiber backbone about a month ago.

If they have 1000 Base-T for uplink/trunk ports then I would be more inclined to agree with you about the use of straight throughs. I still have preference of hard wiring it personally though

 

[Cooky]

Just to clarify, we do 1G & 10G between switches in our datacenter.
We still have a few switches that do 100M, but only in remote sites.

I guess it's a personal preference at this point?

 

[DisgruntledVirus]

Originally posted by: Cooky
Just to clarify, we do 1G & 10G between switches in our datacenter.
We still have a few switches that do 100M, but only in remote sites.

I guess it's a personal preference at this point?

If it's just for the stuff at the data center (that is 1000 Base-T+) then you can do whichever you choose (technically).

Unless there was a good business case as to why you should do it though, I wouldn't. This question will depend on your company's size. A few thousand difference for my company is nothing and we wouldn't do it (but as I said before we are a large company). If you're a smaller sized shop, where a few grand is a big deal then it's something to look into.

What's the price difference for your company per cable, what quantaties, and how often are your orders?

 

[spidey07]

It's really a personal preference, as long as you're using manufactured crossover cables (I always preferred a standard of RED = xover and it's labeled as such).

It comes down to setting an across the board standard everywhere and sticking to it no matter what, so you'd have to factor in the change itself and the support costs to see if it's worth it. Part of this standard should also include checking auto/hardset on both ends, even pros have been burned by duplex mismatches between router LAN interfaces and switches.

 

[Cooky]

Thanks for the responses guys.
Since we already have a standard (green = x-over, orange = straight-through), I think we should try to stick to it.

I've always wanted to ask about hard coding gig ports though...like spidey said 1000t spec is auto-sense.
However, on Cisco switches (not sure about the other ones) give you an option to hard code speed/duplex even for gig ports, so we always do that as part of best practices.
Anyone know for sure if hard-coding to 1000/full is essentially the same as setting it to auto/auto?

I tried to hard-code a Packeteer/Bluecoat PacketShaper to 1000/full, and it basically gave me a middle finger, and changed itself back to auto/auto.
We noticed during a fail-to-wire scenario, it takes longer for the PS to resume traffic flow if the immediate Cisco devices are hard-coded w/ 1000/full, but would almost immediately resume operation if they're set to auto/auto.

Based on my above experience, it looks like 1000/full is different from auto/auto, but the question why if the 1000T spec doesn't allow it?

 

[spidey07]

Cooky - that is most likely flow control/pause frame stuff. As far as I'm concerned it's best practice to do autonegotiation on 1000 Base-anything. I don't know much about the interaction because frankly I never had a good reason to force anything with 1000-base-anything. Normally if it's a flow control thing you don't get link (or you only get link on one side) and that's where the defaults of different manufacturers can come into play.

I know it's hard to trust auto-anything but with 1000 and 10G stuff I'm fine with it. Never had a problem with it.

 

[Pheran]

Originally posted by: spidey07
I recommend using straight thru. As much as I've been against auto-anything like Jack said, 1000 Base-T seems to be bullet proof for autonegotiation and mdi-x and I've yet to see a problem with it.

I think a lot of the paranoia stems from the 10/100 Base-T days and it's autonegotiation problems.

I'm with Spidey on this one - auto-MDIX is part of the gigabit spec and in my experience it works perfectly. I haven't used a crossover cable in ages - they are pretty much worthless now except in special cases.

Cooky, if I came to work with you and found you were still using crossover cables, I'd be suggesting the same thing as your colleague, as long as your networking hardware supports auto-MDIX (almost all of it does these days).

 

[Cooky]

I did a search on mdix, and couldn't find a matrix that tells you which models of Cisco switches & modules support auto-mdix.
Looks like it's not supported on every model.

Cisco wiki

I wish we had the same model of switches across the board that support auto-mdix, but we have everything from low-end to high-end, and have to cover everything.
(we still have a few 6148 modules in a pair of old 6513's)
So we'll probably keep stocking cross-overs for now.