STP and the max-age timer

Hi,

Im doing up some study on STP, and in that regard I would like some info on the max age timer. I have a hard time figuring out exactly when this timer is used, and when its not. Can someone clarify this for me, thanks!

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Comments

Deltah_

MaxAge timer defines how long a bridge or switch should wait after the last received hello message before believing that the network topology has changed, and it can no longer hear the hello messages sent by the root bridge or switch. The MaxAge timer specifies the lifetime of a stored BPDU received on a designated port. After the timer expires, other ports can become designated ports.

kpjungle

Deltah_ wrote:

MaxAge timer defines how long a bridge or switch should wait after the last received hello message before believing that the network topology has changed, and it can no longer hear the hello messages sent by the root bridge or switch. The MaxAge timer specifies the lifetime of a stored BPDU received on a designated port. After the timer expires, other ports can become designated ports.

Originally i thought that if a blocked port hadnt received any BPDU's for MAX-AGE (20 seconds), it would start the listening and learning states. Now i also did a test, that shows that if a Root port doesnt receive its BPDU within 20 seconds, one of its other blocked ports will go into listening/learning state.

So, to sum it up, so far I have two "rules":

- If a blocked port doesnt receive an inferior BPDU(relayed) within 20 seconds (max-age), that port will go into listening and then learning state.

- If a root port doesnt receive its BPDU within 20 seconds (max-age), it will put one of its other blocked ports into listening and then learning state.

I guess im just confused on what ports need to miss BPDU's for the MAX-AGE timer to be taken into account.

kpjungle

Deltah_ wrote:

MaxAge timer defines how long a bridge or switch should wait after the last received hello message before believing that the network topology has changed, and it can no longer hear the hello messages sent by the root bridge or switch. The MaxAge timer specifies the lifetime of a stored BPDU received on a designated port. After the timer expires, other ports can become designated ports.

Does that mean that the MaxAge timer only takes effect if a bpdu hasnt been heard on a root port (example.. wait MaxAge on root port, and then take a blocked port out of blocking state), or is it a blocked port as well? I dont get why a designated port would "implement" the max-age timer, since it will only send out bpdu's, not receive them.

This just wont stick

CrossTalk

Do a search on Topology Change Notifications (TCNs) and Topology Change Acknowledgements (TCAs). I recommend this because I too am studying STP and found some information about TCNs and TCAs late last night in my CCIE Routing and Switching book.

The information is new to me so I don't want to confuse you by trying to explain it here. But I found it interesting.

Good luck!

uttam.baroi

Hey,

if you guys are still active on this site then please reply if its actually Max_Age which is responsible for a port to be designated ?