Dell S4820T Configuration Manual page 814

The range is from 50 to 950 milliseconds.
Example of Verifying Hello-Time Interval
Dell(conf-rstp)#do show spanning-tree rstp brief
Executing IEEE compatible Spanning Tree Protocol
Root ID Priority 0, Address 0001.e811.2233
Root Bridge hello time 50 ms, max age 20, forward delay 15
Bridge ID Priority 0, Address 0001.e811.2233
We are the root
Configured hello time 50 ms, max age 20, forward delay 15
NOTE: The hello time is encoded in BPDUs in increments of 1/256ths of a second. The standard
minimum hello time in seconds is 1 second, which is encoded as 256. Millisecond. hello times are
encoded using values less than 256; the millisecond hello time equals (x/1000)*256. When you
configure millisecond hellos, the default hello interval of 2 seconds is still used for edge ports; the
millisecond hello interval is not used.
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Rapid Spanning Tree Protocol (RSTP)