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A link is changed on a network with STP. All ports change to the blocking state to prevent a loop. How long will the ports stay in a blocking state?
Indefinite
20 seconds
50 seconds
15 seconds
Blocking—When a switch boots up, all ports start in the blocking state. This is to prevent loops during the time that the STP topology is converging. A port that is a link between switches will stay blocked unless it becomes a Root or Designated port. Blocked ports send no data at all (not even BPDUs), but they do listen for (receive) BPDUs from other switches. All ports will also go to Blocking mode if a Topology Change Notification (TCN) BPDU is received. TCNs are issued when a new link is added or removed—the topology of the switched system is altered. When this happens, STP reacts by blocking all ports until loop-free convergence is achieved.
If a switch dies or a link between switches fails, the other switches connected to it wait for a specific time until they begin the STP convergence process. This interval is called the Max Age Timer, and by default it is 20 seconds. Effectively, it means that a switch will wait until it has missed 10 BPDUs (which are sent every 2 seconds) from a connected switch before it kicks in the STP recalculation.
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