Network is unreliable at 100mbit???

[cubby1223]

I have a client with a very strange issue. They originally had a collection of random aging computers in their office, and the ethernet connections to each office worked fine at 100mbit.

They began with replacing two of the machines with new Dell desktops. One of these new machines always had a flakey network connection, dropping randomly. It was very sparse, I never could get a handle of what was happening, I lowered the link speed in the driver properties to 10mbit and things seemed better.

Now another pair of computers were replaced, both brand new Dell desktops, and both of these show immediate and consistent dropping of the network connection, preventing any use of the network, it is like maybe two minutes between the connection dropping. Again, forcing the connection speed to 10mbit provides a reliable connection.


But obviously I think most people can understand why they don't want a 10mbit connection to their server. But that is what is weird, the old computers have no problems with 100mbit, yet 3 of the offices have serious problems with that speed after upgrades.

Linksys 10/100 router and a Cisco small business 10/100 switch. The desktops are not even mistakenly trying 1gbit, I forced the connection speed to 100mbit to no success in connection stability.


Does this make any sense?

I recommended obtaining help from someone I know who is far more experienced with cabling than I am to try to figure out what the problem is.

Thanks

 

[lif_andi]

Make sure autonegotiation is not turned on, or if it's mismatched between network devices and computers. Is the switch managed ? If it is make sure to set values identical to that of your computers. If not, you can try setting the computers to 100Mbit half duplex and see if that works better (at least faster than 10Mbit).

 

[ch33zw1z]

Prefabricated or homemade cables?

I would start at the cabling.

Try a different switch if you have one handy.

 

[imagoon]

Trying 1 gig on 10/100 means the cabling is likely bad and the timing signal is getting lost or damaged. Autonegotiation should work and turning it off is rarely the "correct" solution. Try plugging the computer right to the switch with a new unopened patch cord and see if the issue goes away.

 

[lif_andi]

It depends on how old the network equipment is. Some older switches default to half duplex and auto-negotiation will not work, or work badly. If the switch is on half duplex, and the computer is on full duplex, there will be problems, such as intermittent network connections. Not saying this IS the problem, but it is something to keep in mind. Might even be that the switches have been set to half duplex manually. In any case there seems to be a link problem, most likely on some port settings, since I doubt the cable is at fault for all the new computers, and some looking into port settings seems like a good idea.

 

[imagoon]

You must be talking about really old gear. Stuff that won't auto negotiate and used half duplex by default is mostly late 80's early 90's gear. I would move the computer to a switch and plug it directly in. 1 gig has the same timing signal as 100 and the computers won't attempt it unless the signal is on all 4 pairs typically since 100meg should only be on a single receive pair. Failed auto negotiation is normally 10/half also. If they are trying it indicates the cabling plant is bad or not up to spec or the switch is bad.

If the stuff is really that old, it might be time to run over to the store and pick up new switches.

 

[SecurityTheatre]

Before you go replacing switches, I would think that the cabling is most likely at fault. Poorly insulated/twisted cable can do this.

Are you using good CAT5e? A lot of old buildings ran CAT3 back in the day, which isn't 100MBps full-duplex capable.

Many older devices would negotiate 100MBps half-duplex, which is half the bitrate of full-duplex, possibly explaining the difference.

Still, for a cheap test, try running a brand new Ethernet cable (CAT5e or CAT6) and see if that solves the problem.

 

[drebo]

I'd get the cabling certified and then replace the switch if the cabling checks out.

Just FYI, depending on the number of drops, the certification will likely cost more than the switch that you'd be replacing.

 

[ScottMac]

Before you go replacing switches, I would think that the cabling is most likely at fault. Poorly insulated/twisted cable can do this.

Are you using good CAT5e? A lot of old buildings ran CAT3 back in the day, which isn't 100MBps full-duplex capable.

Many older devices would negotiate 100MBps half-duplex, which is half the bitrate of full-duplex, possibly explaining the difference.

Still, for a cheap test, try running a brand new Ethernet cable (CAT5e or CAT6) and see if that solves the problem.

It'ss not so much the insulation and twisting, it's the pair order (i.e., <pair><par><pair><pair> versus TIA/EIA 568a or b).

by putting all the pairs side-by-side, in any order, you will have, at the least, a split pair on pair 2 (pins 3&6) which blows the crosstalk values right out the window.

The classic split-pair symptoms are poor performance at 100 meg, poor performance at 10Meg full-duplex, but works OK at 10BASE-T (10meg, half duplex). It also might work "OK" at 100 half duplex ... but often sucks because of pair-pair proximity.

GigE is not likely to work at all, or eventually downshifts to 10BASE-T

 

[cubby1223]

Thanks. I haven't had a chance to get over to the client again. I was working late Friday and had to leave before fully testing out various possibilities, I left them all at 10mbit full duplex.

It's cat5 cabling, I know that. The one wall jack I pulled apart and rewired was put together rather haphazardly (I was getting an occasional break in one of the pairs using my cheapo cable testers). The wires are in the proper order, the green pair surrounds the blue pair. The router is old and I know needs to be replaced, but the switch is a brand new Cisco 10/100 switch.

This type of troubleshooting is beyond my expertise, I'll have to call in a local guy I know who is more experienced with cable installs than I am. My guess is all new cabling will need to be installed. I'd like to upgrade the server too, it's an aging Pentium D with server 2003, and add on gigabit so their Outlook profiles can be stored on the server, not locally (server is not an exchange server), so new cabling will be of great benefit all around.

Thanks