The STP path cost automatically changes when a port status such as speed/duplex is changed

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Wed, 07/22/2009 - 19:27
Jun 18th, 2009

Core issue

If the port costs of the switch ports are calculated and assigned, it is possible to ensure that the shortest   lowest cost   distance to the root switch is used to transmit data. In order to calculate and assign lower path cost values, or port costs, to higher bandwidth ports, use either the short method or the long method. The short method, the default, uses a 16-bit format that yields values from 1 to 65535. The long method uses a 32-bit format that yields values in the range of 1 to 200,000,000.

Resolution

The spanning tree algorithm always chooses the path with the greatest bandwidth cost in order to determine the spanning tree path to keep unblocked. When a port is lowered or raised in speed/duplex status, the spanning tree associated values for path cost must also change. This happens automatically and does not have to be manually adjusted.

Refer to the Calculating and Assigning Port Costs section of Configuring Spanning Tree for more information on the spanning tree port costs.


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