TCN generated when new link added?

Answered Question

Hi

Suppose if a new link is added in an existing STP switched network.  Does TCN is generated?  If not, then how all switches learn the new link.  Or even the same switch, which might have a different root port, but after adding the new link, it could be the lowest cost to the root.  So I wanted to know how the new link added is known to the same switch and other switches.

Regards

Mohamed.

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Correct Answer by Giuseppe Larosa about 7 years 2 weeks ago

Hello Mohamed,

the TCN is generated and it is indeed the trigger for a new STP calculation that involves all the switches.

so a new better root port can be chosen following your example. in steady state only designated ports transmit BPDUs

STP duty is to handle link redundancy at layer2 by building and mantaining a loop free topology.

it is a dynamic process and in this it is similar to a routing protocol

to be noted that STP is not interested in advertising MAC addresses or links but root bridge reachability

Hope to help

Giuseppe

Correct Answer by Jon Marshall about 7 years 2 weeks ago

[email protected]

Hi

Suppose if a new link is added in an existing STP switched network.  Does TCN is generated?  If not, then how all switches learn the new link.  Or even the same switch, which might have a different root port, but after adding the new link, it could be the lowest cost to the root.  So I wanted to know how the new link added is known to the same switch and other switches.

Regards

Mohamed.

Mohamed

When you say new link do you mean a new switch ? A TCN can be generated simply by a host going up and down on a port if portfast is not enabled.

However if a switch is added that switch will generate BPDUs which will be received by the other switches. This is how the other switches are aware of a new switch being added.

Jon

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Correct Answer
Jon Marshall Fri, 01/08/2010 - 02:56

[email protected]

Hi

Suppose if a new link is added in an existing STP switched network.  Does TCN is generated?  If not, then how all switches learn the new link.  Or even the same switch, which might have a different root port, but after adding the new link, it could be the lowest cost to the root.  So I wanted to know how the new link added is known to the same switch and other switches.

Regards

Mohamed.

Mohamed

When you say new link do you mean a new switch ? A TCN can be generated simply by a host going up and down on a port if portfast is not enabled.

However if a switch is added that switch will generate BPDUs which will be received by the other switches. This is how the other switches are aware of a new switch being added.

Jon

Correct Answer
Giuseppe Larosa Fri, 01/08/2010 - 04:32

Hello Mohamed,

the TCN is generated and it is indeed the trigger for a new STP calculation that involves all the switches.

so a new better root port can be chosen following your example. in steady state only designated ports transmit BPDUs

STP duty is to handle link redundancy at layer2 by building and mantaining a loop free topology.

it is a dynamic process and in this it is similar to a routing protocol

to be noted that STP is not interested in advertising MAC addresses or links but root bridge reachability

Hope to help

Giuseppe

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