High rate of collisions on 100-megabit networks

[Lequang]

Dear all,

does anybody experience the "High rate of collisions on 100-megabit networks".

Some NIC on the same physical segment resend the packet too quickly (?)

Thanks for any feedback

 

[irweazel]

nope not something you see very often, what hardware are you running with, do you have something that broadcasts frequently?

 

[iolair]

Do you have a "chatty" NIC? Can you put a traffic monitor on the net and see who's talking the most?

Iolair MacWalter

Iolair MacWalter
Director of IT

 

[Jasen]

How many decives on that broadcast domain?
How much bandwidth of that 100Mb is being used?

 

[electronicsfreak]

Why not get a switch?

 

[Papa4]

When I installed a few PC's with 10/100/1000 Nic's on
our very old network there were a few PC's that were
very slow on retrieving data from the server, and surfing
the web. When I looked at the port on the switch for these
sluggish PC's I could see the activity and collision Leds
flickering very rapidly...even when I unplugged the Cat5
cable from the back of the PC.

In my case it turned out to be old Cat3 cable with no
shielding run across electrical cables and flouresent
light fixtures in the ceiling.

I purchased a few thousand feet of Cat5e shielded cable
and ran new runs of cable to most of our PC's, being
careful to route around light fixtures and away from the
electrical wires. This has dramatically improved the
responsiveness of the PC's and the collisions dropped
to a very low rate.

Keep in mind that not all switches will work with the
Cat5e shielded wire, but our old IBM 8275-326 switches
work fine with this new wiring.

You will always see collisions on an Ethernet network.
What is excessive for you on your network may be normal
on mine.

papa4

 

[Lequang]

Dear all,
thanks for your feedback.
Here are some deteils:
1. we use switch on each floor (layer 2) with uplink gigabit to core switch (layer3)where reside the servfarm (all servers)
2. Each floor : only PCs XT. Cabling system is systemix gigabit. (equivalent to Cat5e)
3. The symptoms are as Papa4 has described: some PCs are slow to retrieve date from file servers, etc...
BUT the phenomenon is not permanent and lasr only a few minutes.

We are taking some actions:

1. Sniffing the segment to detect any excessive collisions, and broadcast ...
2. Tracking via the helpdesk users complaints to locate on which segment we have problems.

 

[Jasen]

Good plan so far. As soon as you know the segment, also sniff for excessive bandwidth use. You could just have somebody in there downloading some huge files. And if it's on an already busy segment, it slows everyone else down. Or a PC with a damaged NIC packet flooding the segment.

 

[TheLad]

I have seen this when there is a speed/duplex mismatch, but generally with servers when the switch port has been forced to 100Mb Full Duplex and the server has been left at Auto Negotiate - devices will not auto negotiate to Full Duplex so the server was picking up 100Mb Half Duplex thus causing slow response and network isuses.

Generally, servers should have their switch ports and network card forced to a certain speed/duplex and PC connections can be left as auto negotiate on both switch and network card configuration.

------------------------------------------------------------------------------------
"Life is like a box of chocolates, you never know what you're gonna get!"
------------------------------------------------------------------------------------

 

[Lequang]

Hi TheLad,

Yes I agree with you about "Negociation" problem: indeed if 2 devices are on "negociate" mode, then if the negociation fail then one of the device will drop to the lowest speed (10 Mb).

But the problem I 'm experiencing here is that everything work well until a week ago, and nothing (I hope so) change in the infrastructure.

We are sniffing and hope that the phenomenon happens so we can trace and do some diagnostics.

Anyway, I will come back to give you all the results of my investigation.

Thanks a lot

 

[Sympology]

Could it be someone has induced a loop somewhere in the system and your Layer 3 switch is flapping spanning tree? Seen this happen before. In theroy it should shut down the loop, but we've seen it where it flaps for a while then settles, then flaps etc..

Stu..

Only the truly stupid believe they know everything.
Stu.. 2004