network loop with patch cord??

lots of these new switchs / hubs out there are auto sensing, mean whatever cable that you connect to them they will connect.

yes and no,

as peralesa said many new switchs can auto detect the cable in use and cange accourdling. so if you try to connect two switchs together with a stright through (patch) it will still work fine. So in theroy you can create a loop. however not all switchs do this so some switchs will not create a loop if some one dose this.

howeve all new switchs generaly have spanning tree enabled on them. this detects loops and shuts down one of the ports. so even is some one does it, the switch will see it has been connected back in to its self and shut down the link any way.

this prevents braodcast storms and also offers reselence is you make a loop between mutiply switchs on purpose.

so if you have two links between two switchs one is blocked by spaaning tree to prevent a loop. but if the link that is up failes. spanning tree open up the other link so traffic can still flow.

also some switchs wil not allow a loop to be created via a cable. the can tell a lead has been looped in there ports and will block it if both ports are in the same vlan, this is to prevent the case  you have read about ever being able to happen..

but in answer to your question, yes in some cases a patch lead can casue a loop(if the switch supports it). A cross over always will.

thanks for your response DevilWAH and peralesa
if i dont mistake that switch is look like this:http://www.dlink.com/products/?pid=75
can you tell me that with this switch may i have loop (with patch cord cable)??


let me tell you the real story....
in our network we have over 350 clients that connectet to eacher other with foundry,cisco,...swiches.
and in our IT departmant we have 24 port dlink switches for our test,some one connect this switch to one of our cisco switches and after that connect patch cord to two port of that d-link 24 port switch ....in 15 min later all the network went down!after that really i dont know but after some reseting out core switch and other task in the same time that some body see the patch cord and pull the patch cord all the networks came back!! at the same time
again i think that this little d-link switch with this patch cord accourding to that article that say "connecting a cross-connect cable to two ports that belong to the same switch" i think my idea is true....
but again i need to be sure about this
im waiting to your response..........
thanks

that is a non managele switch, meaning a big hub...with autosensing ports.  Yes it could cause problems especially if your other switches do not have span tree enabled.

Late response to this one......just to emphasise what peralesa has said.... If Spanning Tree is not available and enabled on your switches then creating a loop between switches will definitely cause major issues....

See:

http://en.wikipedia.org/wiki/Switching_loop

and

http://en.wikipedia.org/wiki/Spanning_tree_protocol

if it is a non managed switch then it  will not have spanning tree. Spanning tree is a form of managment.

I would suggests that your main hub/switch should always be managed and set up to prevent these kinds of issues affecting the whole network.

there is more to it than simple making a loop though. with a true switch and unicast packets you will not get a problem. It is only a problem with broacast packets that get stuck in the loop and each time through it re broadcast the packet to the whole network.

A good managed switch can sence broadcast storms (basicaly if it sees the same packet get sent many times in a short period it knows some thing is wrong) and will shut down the port it is reciving them on. this will take that part of the network down but will stop the problem affecting any where else on the network.

However the best way to solve the problem is to get spanning tree on to your network.

thnaks mates...
again i think not....let me explain...
in switches pins number one and two is for reciving data and three and six is for sendiing

port 1:  1   2   3    4   5    6    7    8
             |    |
             |    |
port 2   1   2:  3    4   5    6     7    8
so port one send data from pins number onw and two but!:port two only listening one pins number three and six!
i thinks that only problem accour here is we have collision in patch cord
maybe you said the truth but i cant understand becuase the only problem that i think is collision.
any idea?

thanks DevilWAH and others....
please don't be angry i forget auto senceing and decision with out it....sorry