Network Crawl, then sudden speed up...

[markwilliamson2001]

We have had a strange issue over the last couple of weeks, which has only reared its head the Friday just gone, and the previous Thursday and Friday. The network would slow to a crawl, then suddenly speed up again, then again to run slowly repeatedly. This caused strangle logons, machines to fail at running startup scripts, and mapped drives to fail...I think these problems were caused by broadcast storms which were flooding the network.

 

I eventually traced (what we think of the source of the problem) to what we think was a dodgy nic in a HP colour printer/copier. Once the printer was removed, the problem then seemed to disappear.

 

I managed to find the problem by tracing the issue through the various managed switches we have. The nearest switch to the dodgy printer was showing up repeated CRC/Alignment errors on the port which corresponded to the printer. All errors were from the date of which was since I had installed the printer. According to the logs, CRC errors were mainly caused by loopbacks, mixmatch of link speed types.

 

I then (once disconnected the printer from the network) checked the config on the NIC on the printer. It was defaulted to 10T Half (AHA I thought!). So set it to be 100TX Full, and then reconnected the printer. The problem then seemed to re-appear, so I just disconnected the printer and left it be until now.

 

Has anyone had a similiar problem like this? We have had these problems before (very rare) due to kids looping cables in sockets. I also thought it maybe something to do with duplicate IP's (since we set all network printers to be manual IP's).

 

While we are on the subject of broadcast storms, I thought it would be worthwhile to discuss VLANS, since it would have prevented the 'entire' network going down. The switches also support STP/RSTP, but I have very little knowledge of this as well. We have around 50 switches in school (with 700 odd machines), so I think VLANS are the way to go.

 

Can anyone help out on setting up VLANS from scratch?

 

Thanks

Mark

[danrhodes]

Hi,

 

No help on VLANS but I do have a software recomendation - Agilent Standard Network Analyzer. Try it, it will help you to resolve problems like this if you ever get them again.

 

Dan

[markwilliamson2001]

Thanks for posting back...

 

The problem had all the symptoms of a loop in our topology, but also disappeared of its own right a little while after...

 

Strangely, it ony affected our curriculum machines, so we had a look at our domain controller for our curriculum domain, and it was full of warnings in event viewer in system category.

 

Once rebooted, we havent had a problem all afternoon, but will find out tommorow what happens. The error hasnt occured either in the eventviewer, so I am unsure as to what the problem is..

 

Regards

M.