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Is loop created in Stack Switches in this scenario?

Lasandro Lopez
Level 1
Level 1

I've two core switches in stack (as in picture)

And i've some L2 switches that support STP, and i want to connect two interfaces of this layer2 switch with two core switches that are in stack (one interface per core switch). So the idea is...if one of the core switches in stack is failed, the other one to keep online the services required.StackSwitch.PNG
my question is:

will the loop be created here?

how the two switches in stack, interpret, understand these two lines that come from Layer2 switch?

Thank you!

4 Replies 4

David Lucas
Cisco Employee
Cisco Employee

Hello,

If you build an aggregate interface (port-channel) using LACP then a loop will not be created. If you don't do this, then a loop will be created and one interface will be blocking.

The recommendation would be to use a port-channel and use LACP.

Hope this helps.

Dave

Joseph W. Doherty
Hall of Fame
Hall of Fame

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

Confirming what David has already posted, a loop depends on whether the two physical links between the stack and the downstream switch are two logical links or one logical link (port-channel/Etherchannel).  The former will, by default, create a loop, the latter will not.  The former will only transfer traffic on one link (the second is a redundant "spare") the latter can transfer data across both links.  Failure of the fomer will likely cause a longer traffic interruption than the latter.

So, if i configure the two ports as port-channel, there will be no loop, right?

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

Correct (for the topology you've provided).

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