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Today cisco router come with clock rate enable as factory default setup.

Once you setup your serial interface, automatically cisco will insert clock rate 2000000 to the interface. And no clock rate 2000000 command will not function or can not remove the clock rate 2000000 line.

Solution for this problem is,

1. Shutdown your router, switch off the power.
2. Unplug your serial DTE cable.
3. Start up your router.
4. Type no clock rate 2000000 at your related interface.
5. Type no shutdown.
6. Save your running config (type copy run start).
7. Shutdown your router, switch off the power.
8. Plug in your DTE cable.
9. Make sure your DSU/CSU modem is ready and connected.
10. Start up your router again.
11. Finish.

Clock rate 2000000 will insert automatically when your interface status as DCE. This is usually caused by the interface (serial) of your router not receive clock rate info from your DSU/CSU modem.

Check your interface status with show controller command and make sure your interface status is DTE. Check out 3th line of show controller command result.

Example for serial 0/0/0 interface:

ROUTER#sho controller s0/0/0
Interface Serial0/0/0
Hardware is GT96K
DTE V.35idb at 0x42E43688, driver data structure at 0x42E4ADA4
wic_info 0x42E4B3A8
Physical Port 1, SCC Num 1
MPSC Registers:
MMCR_L=0x000304C0, MMCR_H=0×00000000, MPCR=0×00000100
CHR1=0x00FE007E, CHR2=0×00000000, CHR3=0x0000064A, CHR4=0×00000000
CHR5=0×00000000, CHR6=0×00000000, CHR7=0×00000000, CHR8=0×00000000
CHR9=0×00000000, CHR10=0×00003008
SDMA Registers:
SDC=0×00002201, SDCM=0×00000080
CRDP=0x0F8DDD40, CTDP=0x0F8DF8B0, FTDB=0x0F8DF8B0
Main Routing Register=0×77777707 BRG Conf Register=0×00480000
Rx Clk Routing Register=0×00000080 Tx Clk Routing Register=0×00000090
GPP Registers:
Conf=0×50055600, Io=0×50055600, Data=0xFFFFFBFF, Level=0×00001800
TDM FPGA Registers:
vmcr[0] = 0×00010040, vmcr[1] = 0×00000000,
vmcr[2] = 0×00000000, vmcr[3] = 0×00000000
ntrcr0 = 0×00000000, ntrcr1 = 0×00000000
tdmcr = 0x0000006A, labcr = 0×00000000, tpllr_cr = 0×00000000
nhr = 0×66666660, isr = 0x0000FFFF, imr = 0×00000000

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