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Irratic behaviour of the switch network

We have more then 50 switches with more then 200 computers. suddenly the network has started behaving iratically. all the network gets jammed and stops accessing to the central switch.When one switch is removed from the network it starts working fine. but swich may be any of the 50 switch.wat may be reason.  1) I could not renew the antivirus, is it possible due to virus. 2)My switches are more then 4 years old .Is the problem related to the old switches. 3) the correct method of detecting the problem?

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I'm assuming that the network topology didn't suddenly change so the number of switches probably isn't an issue. I've been a net tech for (unnamed organization) at the enterprise level (Thousands of switches, hundreds of sites, each with dedicated routers, and too many network clients to count). I have seen problems like this numerous times and it is ALMOST ALWAYS caused by a 'loop', or a broadcasting virus (like sasser, netsky, and numerous others). A 'loop' will cause what I like to call 'Digital Feedback'. If your organization has a way to detect broadcast storms (packet sniffer, or a hardware-based network analyzer), you should be able to ID the affected/infected switches/computers. If the problem came about suddenly, I would look for a loop first and, since you mentioned that you can't update your AV definitions, a virus is also something to watch for (an infection is usually traceable with a packet sniffer such as WireShark (previously named Ethereal)). If you could give a general topology layout of your network, I may be able to be more helpful. As far as the 7 switch limit... I think this limit is based on each switch's maximum limit on the number of MAC address (per port or per switch) though, 7 is a VERY high number of 'inline' switches (if they are indeed 'chained' together or connected 'in series').

Posted on Dec 29, 2009

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Well 1 can tell you that if I remember right with cisco networking the maximum number of swiches possible between two hosts in a LAN network is 7.
Try to imagine a line with 2 pc at the end points. You can put maximum 7 swiches between them or expect problems.
I suggest you to try to make a current network map if you don't have one already.
Than do some counting between points just like the way your routing protocol is managing the packets route and see how many swiches you hit.

You can read about a very similar kind of incident with brand new CISCO equipement over here.

Maybe the problem's root is something else, but what you wrote was giving me this idea first to think of.
Hope I gave you an idea.

Regards
Imre

Posted on Nov 26, 2008

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