In a lab environment when you create a serial connection between two routers, keep the following in mind. Modern Cisco ISR routers can autodetect this and set the clock rate to 2000000.
Determine what connection a router has by issuing sh controllers s0/0/0
You may see a DCE cable connection:
Router#sh controllers s0/0/1
Interface Serial0/0/1
Hardware is PowerQUICC MPC860
DCE V.35, clock rate 1000000
Of you may see a DTE cable connection:
Corp#show controllers s0/0/1
Interface Serial0/0/1
Hardware is PowerQUICC MPC860
DTE V.35 TX and RX clocks detected
Remember, a serial connection with a DCE interface requires that a clock rate be set as the other interface will have a DTE connection that receives the clock rate you set.
You can set the clock rate like so:
Router#conf t
Router(config)#int serial s0/0/0
Router(config-if)#clock rate 1000000