10 megabit half duplex causes lots of collisions?

William P.N. Smith wrote in part:

Excess collisions usually mean something is seriously wrong, like bad cabling (split pairs), overlength or too many stations.

They really should not be, but I don't know the BEFSR41v2. I would hope it's a decent 10/100 switch on one side, and

10baseT half only on the WAN side. If not, replace it with a regular 10/100 switch, and plug the router into one port for uplink.

Dual T1s is a lot of upstream bandwidth. And expensive. Is it being heavily loaded?

-- Robert

What are they running over that connection? (what is at the other end of the T1's?) 3mb is good for interent, but pathetically slow for accessing windoes shared folders, etc.

What is "excess collisions" - you will always see SOME collisions with half duplex ethernet - that is normal and unavoidable.

Not is they are on a switch - each port is it's own collision domain.

More likely each is colliding with the swicth. Remember, a switch is nothing more than a big multiport bridge. Very different animal from a hub. Each port on a switch is it's own collision domain.

Collsions can still occur in half duplex mode if the switch tries to send something to the station when the station decides at the same to try to send something - it's half duplex, so you get a collsion. That is normal operation for half duplex ethernet.

What quantity/time frame collsions are being seen? (10 per second, or

100 per second, or what?)

Running duplex would fix this, since you can't have collisions on full duplex.

Yes, that is what a switch does.

Overlength would be _late_ collisions.

I would expect bad cabling to be CRC/FCS errors.

Just to be paranoid, "excess" collisions, not necessarily "excessive" collisions are "bad." Excess collisions would be (IIRC) > 15 attempts to transmit the same frame getting a collision each time. I am thinking "excessive" might be a synonym for "lots and lots."

Just "lots and lots of collisions" isn't in and of itself a bad thing. That is just how CSMA/CD (true ethernet :) does its thing.

So, are the collisions being reported "excess," "late," or simply large in number?

rick jones

FWIW, with standard CSMA/CD backoff algorithms, there is this thing known as the "capture effect" which can have some bad behaviour. Too much to get into here but searching the archives for "ethernet capture effect" will probably find plenty of writeups.

Rick Jones wrote in part:

Agreed. But late collisions often lead to excessive collisions when the distant stations cannot see each other well.

It does this too. Usually only on one pair/direction. The split pair can often pick up signals that cause "false" collisions. I've seen substandard cables light up the collison LED.

Agreed. Collisions are a way of life with bumper cars (CSMA/CD). This shouldn't be with a switch.

-- Robert

Any half duplex device plugged into a switch is going to make that port on the swicth operate as CSMA/CD.

Since the device and the switch port it is plugged into make up their own little CSMA/CD domain, they CAN collide with each other. The device can in fact have a collision with the switch port itself (or vice versa).

snertking wrote in part:

Agreed. But the other ports on that switch should still work as switches.

I think this is what happens with split pairs.

-- Robert

Dunno for sure, but there were lots of dropped packets, which leads me to believe there's some esoteric configuration problem, but a 10/100 full duplex router in the client space is cheap enough that we're going to try it.

It almost sounded to me like every half-duplex connection was getting dumped into the same collision domain due to the nature of the switch (or the traffic?), but that didn't make a lot of sense either. I'll report back on how upgrading to full duplex helped...

Thanks, all!

Back when HDX ethernet was still common, the rule of thumb I was taught was that under 1% was "normal" and anything over that needed investigation.

A very large number of collisions often indicates a duplex mismatch. Maybe those PCs aren't set to HDX after all?

S

Um, not PC's - it goes from routers to a switch. He might try and turn off auto sense on the switch ports involved, and force it to half duplex

10mbit.

With a switch operating in _full_ duplex. With half-duplex it can most certainly happen with a switch.

rick jones

That sounds a bit drastic. I was routinely seeing collision rates >

1% on perfectly healthy 10 Mbit/s setups with stuff like netperf which would serve as a proxy for FTP or the like.

If there is a duplex mismatch, I will assert my own rule of thumb :) that there will be some that are _late_ collisions, and conversely (?) if there are no late collisions (as seen by the half-duplex side) there is no duplex mismatch.

rick jones

the only problem with rules of thumb is sometimes the thumb is best left firmly lodged someplace other than the rulebook :)

Just a FYI that Linksys is 10/100 full duplex on the WAN and LAN side. It is probably one of the most popular routers of all time, there are probably hundreds of thousands of them in service. There have also been several firmware updates (I think some fixed some DoS attacks as well as various bugs).

The switch side is unmanaged store and forward. There is pretty much no diagnostics and I have seen them go bad with no apparent indication (web server works, status lights look normal, but they are obviously not working right and no amount of factory reset or firmware reflash helped).

Ethernet was designed for collision. One of the features of a switch is that each port can run at different speeds and duplex (although both sides of the switch and PC should match). If your ethernet cards have self diagnostics, see what they say. From past experience, I would say the Linksys is just breaking down (assuming everything else checks out OK).