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

802.1d TCN

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.

2 REPLIES

Re: 802.1d TCN

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

Remember to rate the helpful post

Re: 802.1d TCN

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

503
Views
0
Helpful
2
Replies