network loop with patch cord??

Posted on 2008-10-15
Medium Priority
Last Modified: 2011-09-21
can simple patch cord cause network loop?
in my network someone connect simple patch cord cable in two port of d-link swithes,and another people said that this one cause loop!
but i said them that if we connect "cross-over cable" in two port of the same swith we have loop  and with patch cord we dont have any loop in entire of the network
accourding the this article:
A network loop appears when two ports belonging to the same VLAN have a link to each other, or when there are two or more paths between two switches. Malicious users can physically create network loops, and the easiest way to do that is connecting a cross-connect cable to two ports that belong to the same switch and the same VLAN (the same broadcast domain).
i want to ask you that is my idea is true or not?
Question by:chmdznr
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LVL 17

Expert Comment

by:Andres Perales
ID: 22724272
lots of these new switchs / hubs out there are auto sensing, mean whatever cable that you connect to them they will connect.
LVL 16

Expert Comment

by:Aaron Street
ID: 22724466
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.


Author Comment

ID: 22724799
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..........
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LVL 17

Expert Comment

by:Andres Perales
ID: 22724896
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.
LVL 14

Expert Comment

ID: 22725304
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....




LVL 16

Expert Comment

by:Aaron Street
ID: 22728862
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.

Author Comment

ID: 22729412
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?
LVL 16

Accepted Solution

Aaron Street earned 150 total points
ID: 22729542
ahh but on a switch that is auto senceing it can change the pins it sends and recives on.

so if it detects that it is a stright through it alters what pins it uses to send and recive at one end so that data can go accross the link.

so one end of the link will cange so that it recives on pins one and two and transmits on pins 3 and six.

so you can get a loop and not collisions.. In fact if you attach a stright through cable between two ports taht can't autosence. then it will not send any data across at all. as it will nto be able to detect that a cable has been installed and will not enable the ports to send any data.


Author Comment

ID: 22729677
thanks DevilWAH and others....
please don't be angry i forget auto senceing and decision with out it....sorry

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