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Jul 11th, 2007

How does listening and learning states help prevent loops ( after max age for a port has expired)? An example ( topology ) of this network will be helpful.

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srue Wed, 07/11/2007 - 08:54

listening and learning states essentially give a switch time to figure out if whatever port is going through those states, if a physical loop has been created. Instead of going straight to forwarding state, the switch can listen to BPDU's (listening) and determine if there is a loop. If no loop exists the switchport can go into the learning state as it gets ready to forward traffic.

If you bypass the listening state (and learning state) and have a switchport go straight to forwarding mode (as with portfast), the possibility exists that a loop could at least be temporarily created until the switch learns (via BPDU's) that a loop exists at which point the port will go into blocking mode.

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