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STP Optimise BPDU Transmission?

Answered Question
Mar 5th, 2006
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Can anyone tell me what this command does. The default for our switches is for it to be disabled, ie. "no spanning-tree optimise bpdu transmission". I have checked the documentation for the relevant switch models and can't find it in the command references or configuration guides.


thanks

Correct Answer by Francois Tallet about 11 years 5 months ago

That's really a subtle IOS architectural optimization so I don't want to enter into the details, but basically:

-Without optimized transmission, BPDUs are sent independently one by one from the STP process.

-When optimized BPDU transmission is used, the BPDUs for a given vlan are prepared first (they all look the same in fact) and then sent in a row at interrupt level (with no further CPU interruption). The main advantage is that the same packet buffer is allocated once and recycled for all the BPDUs, instead of being allocated and freed for each and every BPDU.


Regards,

Francois

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Francois Tallet Sun, 03/05/2006 - 23:13
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The optimize bpdu transmission is a mechanism that allows BPDUs to be sent in one shot at interrupt level. It is supposed to relieve the STP process in PVST mode. It's not a fundamental feature and should not have been exposed to the user. The fact that this is disabled (and thus that it shows up in the configuration) is a bug. So basically, don't worry too much about that. When you have a release that allows to enable it again, you can do it and forget about it.

Regards,

Francois

pgalligan Mon, 03/06/2006 - 14:38
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When you say "sent in one shot" what exactly do you mean?


Thanks!

Correct Answer
Francois Tallet Mon, 03/06/2006 - 16:49
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  • Gold, 750 points or more

That's really a subtle IOS architectural optimization so I don't want to enter into the details, but basically:

-Without optimized transmission, BPDUs are sent independently one by one from the STP process.

-When optimized BPDU transmission is used, the BPDUs for a given vlan are prepared first (they all look the same in fact) and then sent in a row at interrupt level (with no further CPU interruption). The main advantage is that the same packet buffer is allocated once and recycled for all the BPDUs, instead of being allocated and freed for each and every BPDU.


Regards,

Francois

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