Cisco switches cable question

It appears from the 2960 guide (link below) that MDIX is supported and is enabled by default. I have always elected to use crossovers just because it is good to have some laying around, and its a safe way to mitigate risks (I have had many auto-mdix issues related to IBM Regattas in past lives). I'm sure those issues are fixed now, be we had to manually disable mdix on all switchports to those servers for quite sometime due to nobody from either vendor (ibm, cisco) being able to figure out exactly what was happening on boot that would make the switchports change polarity.

Anyway, I'd test first, but yes your solution should work.

Sorry, forgot to paste link:

Gig uses all four pairs, so a standard Ethernet crossover won't work. I don't know if its part of the standard or not (but I think it is) gig ports will automatically detect when crossover is needed. Do yourself a favor and use a straight-thru cable and only use a crossover if that doesn't work.

If you do need a crossover, this is always pretty.

MikeG

Thanks for the responses to this guys....I really appreciate it.

One other (probably dumb) question:

I am going to have to make a couple runs with fiber to our offices that are on different floors of the building. The 2960 has 4 SFP fiber optic modules. Is there either: A cable that is SFP on one end and SC on the other, OR, is there an adapter that will make that conversion? I am trying to go from the 2960 SFP to a 3548 with a WS-G5484 module.

Fiber is really not my thing so I'm learning as I go here....

Thanks again,

Mike

SFP is the slot that the fibre transceiver goes into, not a type of connector. The type of connector you use is determined by what type of connector the SFP module has on it. What SFP module will you be using?

Sorry, my bad. I'm reading up on it more now.

What I meant to ask (I think), was a cable that is LC - SC

I think I found one that would work:

? Maybe ?

The SFP module would be whatever can go into the 2960's mini-GBIC slots.

Thanks

Yes they make LC to SC cables as you found, and they work fine when connecting switches, etc. Just ensure that your other fiber stuff matches (micron, etc). If you can, I would avoid running new fiber and just jumped SC to LC on one end and always have a few spare runs around in case. You always should avoid running a single long cable that is different than the rest to avoid backing yourself into a corner....although sometimes you don't quite have a choice.

But, as someone said earlier, pointless. 1000base-T uses all four pairs in both directions simultaneously so a straight-through cable is the correct thing to use. There are systems that can adapt to crossover cables, but it just makes things more complicated.

Sam

Not so pointless, especially with Cisco gear.

Doing a Gigabit between Cisco GBIC switchport to Cisco GBIC switchport

*requires* a true (ie. all 4 pair) ethernet cross-over cable. Cisco did not support auto MDI/MDI-X on their old-gear GBIC copper switchports. You generally had to make them yourself, since vendors are so lazy and made only half-correct cables in general.

Thankfully, they do on modern gear, and it shouldn't matter to the OP, since he's using one modern to one old-school.

1000Base-T GBICs absolutely support auto MDIX. I have about 60 3550's in closets that are connected together via GBIC's and connected to 6500's all using straight-through cables

Belkin makes a whole litter of fiber cables with different kinds of connectors. But I think CablesToGo have patch cables that may be more handy if you've already got a run in place. If you're replacing your current cable run and depending on the length, you may just want to have a new one made by a professional -- unlike ethernet cables, I wouldn't recommend investing in the hardware and testing equipment to make them yourself unless you plan on doing this on a regular basis.

-Gary

Okay, should have quantified, some gear taking GBIC copper will do it. I wasn't trying to imply it was the GBIC limitation, but rather the age of the gear. Some of the old Cisco gear with GBIC slots do not do auto MDI/MDI-X on the GBIC slots.

*IF* he needs or wants one, the referenced URL has the information (both 10/100base-T and 1000base-T) he is looking for. Or did you not actually read what I wrote?

MikeG

I don't doubt your experience, but WHY? Have a look at the 1000base-T description (figure 7.23 at

for instance) and wonder why on earth anyone would ever think of using a crossover. Any kind of auto-MDI behaviour should only be to correct the erroneous use of a crossover cable!

Shouldn't matter.

Sam

Yeah - we have a variety of 3Coms and Cisco switches with 10/100/1000 interfaces and GBICs/SFPs. I've never knowingly used a crossover cable to connect them together.

Sam

At the risk of repeating myself, what on earth does the MDI/MDI-X distinction mean in the context of a connection that is inherently straight-through in both directions?

Sam

I read exactly what you wrote. I'm just finding it hard to conceive any circumstance (and of course I bow to experience) where 1000base-T would require a crossover cable.

Sam

1000Base-T uses all four pairs, two-pairs for transmit, and two-pairs for receive. At no time is any pair used for both transmit AND receive. Since certain pairs are transmit and others receive, they need to be the opposite on the device at the end. The Medium Device Interface (MDI) specification says which pairs are transmit and which are receive. MDI-X reverses this. All NIC cards (or end devices) are wired for MDI-X, while all switch and hub ports are wired for MDI. For two devices to talk, one side must be MDI, the other MDI-X so that the transmit pairs on one side connect to the receive pairs on the other. Most modern day Ethernet chipsets support auto MDI/MDI-X, but not all. Yes there are 1000-BaseT interfaces that do not support auto MDI/MDI-X. The gig ports on Cisco's 2950 LRE switches do not support this functionality.