Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

TCN generated when new link added?

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.

2 ACCEPTED SOLUTIONS

Accepted Solutions
Hall of Fame Super Blue

Re: TCN generated when new link added?

cwet@covadwireless.net

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

Hall of Fame Super Silver

Re: TCN generated when new link added?

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

3 REPLIES
Hall of Fame Super Blue

Re: TCN generated when new link added?

cwet@covadwireless.net

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

Hall of Fame Super Silver

Re: TCN generated when new link added?

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

New Member

Re: TCN generated when new link added?

Hi Jon and Giuseppe

Thanks for your reply.

Regards

Mohamed.

495
Views
0
Helpful
3
Replies