802.1d TCN

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May 31st, 2010
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Hi

In 802.1d when the switch detects the topology change (TC) it sends TCN to the root bridge , then root bridge sends BPDU to all other switches .Then other switches set the bridge aging time to forward delay for faster flushing of the mac address table . I would like to  know why ?

I mean How did they come up with forward delay time why not any other timer ?

Thanks.

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Ganesh Hariharan Mon, 05/31/2010 - 23:48
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Hi

In 802.1d when the switch detects the topology change (TC) it sends TCN to the root bridge , then root bridge sends BPDU to all other switches .Then other switches set the bridge aging time to forward delay for faster flushing of the mac address table . I would like to  know why ?

I mean How did they come up with forward delay time why not any other timer ?

Thanks.


Hi,


It is designed as per the protocol that following stages should come to decalre a port in block stage that why STP stages along with Timers are designed to avoid loop in network so if you see when ever there is change in the network The root bridge will inform the nonroot switches of the change via BPDUs.


Hello Time is the interval between BPDUs, two seconds by default.

Forward Delay is the length of both the listening and learning STP stages, with a default value of 15 seconds.

Maximum Age, referred to by the switch as MaxAge, is the amount of time a switch will retain a BPDU's contents before discarding it. The default is 20 seconds.


Hope to Help !!


Ganesh.H


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milan.kulik Tue, 06/01/2010 - 00:04
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Hi,


as http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094797.shtml

says: "It is more beneficial to reduce the aging time instead of clearing the table because currently active hosts, that effectively transmit traffic, are not cleared from the table."

I'd add: After forward delay expired, there's a good chance a new correct entry in the forwarding table occurs.


BR,

Milan

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