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Switching Loop


Hi Everyone,

How do switch determine that loop is created in the network & it is required to run STP in order to recover from loop ? 


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 - STP should be standard in place and usually is , on default configurations. A loop can be observed by looking at logs and mac addresses being reported on different ports at  very short intervals.


Joseph W. Doherty
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Ideally, STP isn't used to "recover" from a loop, it avoids creating one (which if configured to fully do so, is, I believe, generally, 100% effective.)

If a loop is created (possible using something like portfast, which short cuts some STP processing), Cisco STP might determine there's a loop and then "break" the loop, but this isn't guaranteed.

Thanks Joseph.

Can we say that switch / stp determines there is a loop when it receives same mac address from more than one ports/interface? 


MAC, no.

If you're unfamiliar with STP, I'm sure you can find multiple primers if you search the Internet, or perhaps even Cisco's main site.  (They would likely be better than I in trying to provide a broad explanation of STP.  These forums are best for specific questions.)

Thanks Joseph. I've read about STP and it cleared my doubt. Thanks again.


What you are referring to is MAC FLAPPING which happens due to loop.
Since at layer 2 there is no TTL value a switch never knows there is a loop. 

STP helps by making sure that there is only one path to destination logically. If you disable STP the switch won't ever know that it's in loop and won't block anything and you will see same MAC on different ports and MACFLAP logs.


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