提高Xmodem的传输速率

 通过修改switch 波特率，提高Xmodem的传输速率，使通过Xmodem灌IOS速度大为提升

How to Speed Up Xmodem Recovery When a user tries to recover the switch from a corrupted or missing IOS, the 
copy xmodem: flash:[IOS filename] command might not be displayed under the switch: prompt. The copy 
command might be displayed under the switch: prompt, but not the copy xmodem: command.
Complete these steps in order to speed up the Xmodem recovery:
Set the baud rate to 115200 on the switch: prompt.
switch: set BAUD 115200
!--- The screen goes blank after you enter this command.
Restart HyperTerminal.
Under COM PORT properties, select the bits per second as 115200. The switch: prompt is displayed.
Start the Xmodem recovery.
After the Xmodem recovery, set the BAUD rate back to 9600. If the set BAUD 9600 command does not bring the 
baud rate to 9600, issue the unset BAUD command in order to bring the baud rate to a default value of 9600 bps
可在switch:下用 unset BAUD 恢复默认波特率