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ccna udemy.However, in some cases, you may want to connect two routers back-to-back using the router's serial interface. For example, if you are building a lab to practice using CCNA commands, it is likely that you will connect the routers back-to-back to reduce equipment costs. In this case, each router defaults to DTE. one of these two routers must act as an external DCE because of the clock frequency that must be provided to boot the interface. To do this, use the interface subconfiguration mode command clockrate on the serial interface.Router(config)# interface serial [s2ot_#/]port_# Router(config-if)# clock rate rate_in_bits_per_secondThe clock rate value cannot be specified arbitrarily; the available values depend on the type of cable used and the type of interface connected. To learn which clock frequencies are supported by the serial interface, use the context-sensitive help. Here are some possible values: 1200, 2400, 4800, 9600, 19200, 38400, 56000, 72000, 125000, 148000, 500000, 800000, 1000000, 1300000, 2000000 and 4000000.Note that for routers connected back-to-back, you cannot just choose one of them to act as the DCE: which router should act as the DCE depends on the connection method. One end of the cable is DTE and the other end is DCE; some cables are marked out and some are not, depending on where the cable was purchased. If you are not sure which router is connected to the DTE end of the cable and which router is connected to the DCE end, you can use the command show controller to make a determination.Router> show controller serial [slot_#/]port_#This is one of the few commands where you cannot put the type and port number together (you must separate them with a space). Here is an example of the use of thiscommand is used as an example.Router> show controller serial 0 HD unit 0, idb = 0X121C04 driver structure at 0x127078buffer size 1524 HD unit 0, DTE V.35 serial cable attachedNote that line 2 of the above command output contains two important pieces of information: the connection type (DTE) and the cable type (V.35). o The following is the output of this command for the interface connected to the DCE side of the cable.