How to prevent Loops in the Network

How to prevent Loops in the Network

I would like to know if a root bridge is configured manually would that prevent a loop in the network ? I mean if someone else plugs a device in the network that would still not cause a loop ?

Thanks
jskfanAsked:
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AkinsdNetwork AdministratorCommented:
For the "most part", yes.
Generally, manually configuring root bridges would ensure that only that switch is the root bridge and may help prevent loops. You may want to ensure that all ports aside of your uplink are hardcoded as access ports
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jskfanAuthor Commented:
STP prevents Loops.

Sometime we need to prevent Topology change...

I believe it is the same we may need to manual configure the Root Bridge and Switch ports as access ports ,

that way if someone accidentally plugs in a switch to any port it would not send BPDUs, not trunk will be formed and no topology will be changed...

Is that correct ?
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jskfanAuthor Commented:
I also am assuming STP is in every cisco switch by default. Does that mean Loops do not happen. Hopefully they do not....

So now , what to do t prevent change topology when we plug a new switch to another port switch....Nowadays even when you plug a Blade or ESX host can negotiate a trunk...that might cause topology change too
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AkinsdNetwork AdministratorCommented:
The reason negotiation occurs is because the switchport is set to "auto" by default. To avoid auto negotiation, hardcode the switchport to access port.
Assuming you have a 48  fastethernet port switch with an uplink port 1 and 2. You can then hardcode ports 3 to 48 as access ports
eg
conf t
int range fa0/3 - 48
switchport mode access


It is also recommended to shutdown unused ports and only turn them on when commissioned
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jskfanAuthor Commented:
Thanks...
Want to know since STP is by default on...how would the Loop be able to happen ?
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AkinsdNetwork AdministratorCommented:
Want to know since STP is by default on...how would the Loop be able to happen ?
Loops would still be prevented as redundant ports would be blocked still. It's just better to not leave your switchports as AUTO or use default STP settings.
You may get along with STP basic default settings depending on your environment. Nevertheless, I wouldn't use default settings for STP on my network. One example, STP uses the oldest MAC address to determine root bridge. This means a switch made in 1985 would be the root bridge even if it is not in a strategic position and not robust enough to handle network requests.
This link may help
http://www.networkworld.com/article/2223757/cisco-subnet/9-common-spanning-tree-mistakes.html
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jskfanAuthor Commented:
Thanks
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