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1) What's the minimum possible "hello time" for Rapid Spanning Tree (RSTP)? Is it possible to reduce the hello time to e.g. 0.1 seconds?

2) Is the minimum dictated by the RSTP standard, or would it be switch-dependent? Or would it be a situation-dependent practical minimum due to increased BDPU traffic or some other issue?

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I'm going to answer your questions out of order.

What's the minimum possible "hello time" for Rapid Spanning Tree (RSTP)?

Per standard, the range is 1-10 seconds, with a recommended default of 2 seconds.

Is the minimum dictated by the RSTP standard, or would it be switch-dependent?

Min/Max ranges are dictated per standard, but the default value is recommended (as mentioned above).

Actual implementation would technically be vendor specific, though they'd be unlikely to deviate from something so fundamental. For example, both Juniper and Cisco adhere to the 1-10 second range, and 2 second default values.

Is it possible to reduce the hello time to e.g. 0.1 seconds?

In theory? Sure. In practicality, not really as you'd have to find a vendor that supports it.

Or would it be a situation-dependent practical minimum due to increased BDPU traffic or some other issue?

This would be up to the network's operator and the specific reason for any variance in configuration of the hello timer would be pure speculation.

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I general, almost all switches are usually configured for default hello time of 2 seconds and generate BPDUs every two seconds (hello time), but you can change it. The standard's range is 1 - 10 seconds.

In case, if you reduce the hello time interval, BPDUs shall get generated in very high quantities in short intervals which in turn can cause excessive CPU load as the switch has to process many BPDUs. This load inturn can impact its performance and impact drastically if there are many VLANs and trunks in your network.

It is purely dependent on the number of network entities in your network topology, processing capabilities and also the use case with you.

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