What makes switch shutdown when they found loop

[eeebbunee]

Hi, I'm trying to reduce a gap between real networking management and logical theory about networking.

Logically, STP prevents loop by blocking port.

However in reality, when more than 2 switches makes a loop, all network couldn't communicate until we unplug the issued port.

What makes different results?

It depends on complex network infra? or different kinds of loop issue? 

Are you recommend to use same manufacturer's switch product in order to manage easily STP (or pvst)? 

[Kasun Bandara]

Hi,

normally, loop happens when STP is not in use. when switch comes up, ports are changing to different states and learning the MAC addresses connected to ports and running STP algorithm to find redundant ports and will block additional ports automatically. so there will not be any looping.

if you disable STP on switches which have redundant connections, of course you will get loops. below guide explain clearly how STP works.

https://www.computernetworkingnotes.com/ccna-study-guide/stp-spanning-tree-protocol-explained-with-examples.html

rate this and mark as answer if this solved your concern

[eeebbunee]

Thank you for your replying.

I believe all of our switches turned on pvst+ as a default.

Is this not enough to preventing STP loop issue? I thought pvst+ can also managing STP too. 

However, we have other brand L2 switch too, so obviously this brand L2 switch does not know about pvst protocol. (but RSTP is enabled)

Do you think using multiple protocol (pvst+ and RSTP) could be the reason why it doesn't prevent a loop?

[Joseph W. Doherty · Accepted solution]

"Do you think using multiple protocol (pvst+ and RSTP) could be the reason why it doesn't prevent a loop?"

Possibly.  Many other vendors implementations of STP or rapid-STP use one, or a common, or single, instance of STP for all VLANs.  Cisco's STP and rapid-STP provide an instance per VLAN.  You can run into issues mixing these two implementation types.  Generally the best solution, when dealing with this, is to use MSTP for both Cisco and non-Cisco equipment.