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New Member

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

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Accepted Solutions

Re: STP - 50 seconds?

The 20 seconds correspond to max-age, the time STP takes to age out some information it received on a port. It's not obvious to see because if a port goes down, the information is aged out immediately, without waiting for max-age.

If you want to see max-age in action, the simplest is to configure bpdufilter on a designated port. The remote port, that was receiving the BPDU periodically, will transition from a root/alternate/backup role to a designated role in max age. Once in designated role, if it was blocking, it will take an additional 30 seconds (2xfwd_delay) to go forwarding, as you already observed.

regards,

Francois

3 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

Re: STP - 50 seconds?

The 20 seconds correspond to max-age, the time STP takes to age out some information it received on a port. It's not obvious to see because if a port goes down, the information is aged out immediately, without waiting for max-age.

If you want to see max-age in action, the simplest is to configure bpdufilter on a designated port. The remote port, that was receiving the BPDU periodically, will transition from a root/alternate/backup role to a designated role in max age. Once in designated role, if it was blocking, it will take an additional 30 seconds (2xfwd_delay) to go forwarding, as you already observed.

regards,

Francois

New Member

Re: STP - 50 seconds?

Thank you,

Your explaination was super clear.

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