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Dear Community,

The idea of a switch loop is something that is not clear to me. I hope i can get a better understanding of it. This question has been posted in other forums before.  I have gone through the explanations but its not settled for me.I know that its a continuous movement of packets between same switches in a topolgy caused by various reasons.

Attached is a topolgy with one router with a bvi interface i.e two interfaces sharing one ip,connecting to two switches that are connected together to with a trunk link.Does a switch loop exist?(bridge topology)

What if i wanted a switch loop to exist in my topology?will my design be as in the topology(switch loop) A where etherchannels are used? Can we always rely on STP to stop switch loops?

How can you ensure to always avoid it?Etherchannels are often recommended,but why?

Any references,documentation,books to clear this confusion is welcomed.

Thank you all,



3 Replies 3

Georg Pauwen
VIP Master VIP Master
VIP Master

Hello Isaac,

in your topology you can create a loop by disabling spanning tree. Since you have redundant connections, that will immediately drown your network. If you post the Packet Tracer files (zip them first before uploading), we can add the necessary commands.

Etherchannels by itself do not prevent loops, as they basically just increase bandwidth.

Have a look at the document below:

Troubleshooting STP on Catalyst Switches Running Cisco IOS System Software

i had the situation where a semi smart small switch connected to a 3750-48T someone connected it twice to it. and it wrecked havoc on everyone connected to that VLAN. the CPU went to the roof and you couldn't even ping the gateway (data plane overload?)    this type of loop is very hard to contain since the switch does not send tags nor BPDUs thru access ports. it was forwarding any bcast traffic in an endless loop. 


i had to turn on broadcast control and set it to 1% and port security to a MAX of 5 per port. 

Mark Malone


Ether channels are recommended for resiliency so you have multiple links and in newer devices like Nexus and VSS technology portchannels can be split across multiple devices too if supported

in your design there by default STP will block a link to prevent a loop , turn off stp on every port if you want a loop , stp bpdufiliter

never do that in real world only in lab as a test as you will cripple the network , once a loop starts cpu etc all gets driven up and normal users can loose access to the network

use this command to check for loops , it will show you the timer changing constantly as the loops occur

show spanning-tree detail | inc ieee|occurr|from|is exec

How can you ensure to always avoid it?

Learn and plan STP correctly before deploying , always be in control of it rather than letting it automatically do its thing that way when it breaks you know what path traffic willm take a layer 2 , only really required on large layer 2 networks

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