What causes Switch Loop

If I understand when all switches are configured with Spanning tree, which is on by default. So how do switching loops find a way to happen ?


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A few things can cause loops:
1) Not defining a primary spanning tree root
2) Not defining a secondary root.
3) Not selecting a certain spanning tree protocol
4) Different makes and models of switches may have different implementations of  spanning tree and not agree or understand the messages they receive from each other (usually due not doing #3)
5) Not using the spanning tree priority command to set root switches with a low number, distribution switches with a mid range number, and access layer switches with high numbers
jskfanAuthor Commented:
1 and 2, I believe you if you do not configure Primary/Secondary, you still do not get a loop....you might experience topology change if  a link goes go down.

3- STP is on by default on cisco switches
4- I can agree with you
5- I believe you do not have to

I do not think any of the cases above will cause a loop, but probably topology change if one link goes down.

That's what I think
One of the situations where you can get in trouble with loop even if STP is working is Uni Directional Link (UDL), and UDL Detection is not configured.
It is a classic example.
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jskfanAuthor Commented:
Predrag Jovic

I checked the Cisco link you provided..I do not think Cisco did a descent explanation about it:

Switch A is the Root , C has blocking port on the Link Leading to B
It says if the blocking port does not receive BPDUs from B, then the blocking port will transform to forwarding(Designated) and cause the loop....Mmmm, poorly explained or by design STP is poorly designed.
First if the blocking port does not receive BPDUs from C port, why should it become forwarding at the first place, why to forward traffic to a port that you have not heard from ? it is like wiring money to a person that is not alive...
I think it is good explanation, and STP is good designed, problem is that BPDU is sent  but not received on neighbor port. This mostly happens on fiber links when one side is broken.
Why would port go up, it is simple...
Layer 1 link is up and there is no BPDU entering port, so something is changed in network it could be end device attached to port (or some unamanaged switch? -no BPDU), so port is entering listening and learning states and then entering forward state...
If port is not entering forwarded state when there is no BPDU on port and physical link is up that means that if port is once blocked would never come up.

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jskfanAuthor Commented:
Where it does not make sense is the blocked port when it does not receive BPDUs from B Switch , then it transforms to forwarding the C Switch port ... which in turn will forward the traffic to B switch then to A switch then loop  it back to C switch and so on....
The design should be before it turns the blocked port on C into forwarding , it should first block any traffic going to B port , like Null0 interface....

How can you forward traffic to an interface you do not know if it is up or down...that 's where the design flaw resides.
That's my thought though...
jskfanAuthor Commented:
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