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Spanning Tree Protocol

nnn_sss22
Level 1
Level 1

[[{"type":"media","fid":"1464256","view_mode":"default","attributes":{"alt":"atp","title":"stp","class":"media-element file-default","data-delta":"1"},"fields":{}}]]

I know on blocking state it will take 20 sec, listening state 15 sec and learning state 15 sec.

Please define me. when first path goes down then how Switch A Fa 1/0/2 port will know. it should convert now blocking to listening state. how it will know first path has down and from where it will receive BPDUs?
2 Replies 2

Philip D'Ath
VIP Alumni
VIP Alumni

That is if you are using "old school" spanning tree.  Hopefully people aren't doing that anymore.  Rapid Spanning Tree replaced this old archaic system some time ago.

To answer your question, read this:

http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/12013-17.html

Predrag Jovic
Level 3
Level 3

If it is directly connected port like in your topology, there is no waiting for dead timer to expire. Carrier loss on root port will move other port from blocking to listening state right away. So, connectivity will be restored in 30 seconds (2 x Forwarding time).

And generally, for the cases where there is no carrier loss:

Both ports on Switch A are receiving BPDUs all the time. Reconvergence will happen because active port missed 10 BPDUs (default 20 seconds - can be tuned to 6 - 40 seconds). That's how Switch A would "know" the first path is down. Also, this, in cases of unidirectional traffic can lead to network loop.