spanning tree tcn cam table aging

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Feb 5th, 2010
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Hi


Can anyone explain to me, when a tcn is generated, i know the root switch tells all other switches to change the cam table age timer to 300 secs, why is this? i read up on it along time ago and have forgot the reasons


thanks


Carl

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Ganesh Hariharan Fri, 02/05/2010 - 10:42
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Hi


Can anyone explain to me, when a tcn is generated, i know the root switch tells all other switches to change the cam table age timer to 300 secs, why is this? i read up on it along time ago and have forgot the reasons


thanks


Carl

Hi Carl,


Topology Change Notification is genrated when a bridge discovers a change in topology, usually because of a link failure, bridge failure, or a port transitioning to forwarding state.The TCN BPDU is set to 0x80 in the Type field and is subsequently forwarded on the root port toward the root bridge. The upstream bridge responds back with acknowledgment of the BPDU in the form of Topology Change Acknowledgment (TCA).


When a topology change occurs, the bridge temporarily lowers this timer to the same as the forward delay timer (default: 15 seconds). This allows the STP network to react to changes in topology by having the bridges quickly relearn the MAC address changes that occur when links change state


Hope that help


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Ganesh.H

Giuseppe Larosa Sat, 02/06/2010 - 12:34
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Hello Carl,

cam aging time reduction helps convergence: at the end of STP calculation at least one link will change state.


The effects of this activity from the point of view of MAC address learning is that some MAC addresses that were learned from port X will be re-learned from port Y.

Not all switches will be affected but the action is performed on all of them because the final topology cannot be predicted.


Hope to help

Giuseppe

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