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STP - 50 seconds?

I'm working on labs for my CCNA and I'm curious about the stp update time. All the books that I'm reading tell me that STP will be in blocking mode for 20 seconds, listening for 15, and learning for 15, for a total of 50 seconds before ports were forward. In all of my testing I never see the 20 seconds for blocking. When I type show spanning-tree brief I can clearly see ports change from listening to learning and then forwarding in 30 seconds.

Can anyone tell me where this 20 seconds blocking period is?

Thanks for any help

dk

2 REPLIES

Re: STP - 50 seconds?

The 20s blocking is only applied when the port is in blocking state when the root port fails.

It then blocks for another 20 before going through listening & learning mode.

You are describing a port that was down and comes up while you connect a PC or whatever to it.

Regards,

Leo

Bronze

Re: STP - 50 seconds?

Hello,

the 20 seconds are the default max-age timer: if the switch doesn´t receive a bridge protocol data unit (BPDU) message from the root switch within this interval, it recomputes the Spanning Tree Protocol (STP) topology. Add to that the forward-delay default 15 seconds times two, and you end up with 50 seconds...

Regards,

Nethelper

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