Configuring a Serial Interface


let's configure the Serial 0/0/0 interface on router R1. This interface is on the 172.16.2.0/24 network and is assigned the IP address and subnet mask of 172.16.2.1/24. The process we use for the configuration of the serial interface 0/0/0 is similar to the process we used to configure the FastEthernet 0/0 interface.
R1(config)#interface serial 0/0/0
R1(config-if)#ip address 172.16.2.1 255.255.255.0
R1(config-if)#no shutdown
After entering the commands above, the state of the serial interface may vary depending upon the type of WAN connection. This will be discussed in more detail in a later course. In this course, we will be using dedicated, serial point-to-point connections between two routers. The serial interface will be in the up state only after the other end of the serial link has also been properly configured. We can display the current state of serial 0/0/0 using the show interfaces serial 0/0/0 command, as shown in the figure.
As you can see, the link is still down. The link is down because we have not yet configured and enabled the other end of the serial link.
R1#show interfaces serial 0/0/0
Serial0/0/0 is administratively down, line protocol is down
We will now configure the other end of this link, Serial 0/0/0 link for router R2.
Note: There is no requirement that both ends of the serial link use the same interface, in this case, Serial 0/0/0. However, because both interfaces are members of the same network, they both must have IP addresses that belong to the 172.16.2.0/24 network. (The terms network and subnet can be used interchangeably in this case.) R2's interface Serial 0/0/0 is configured with the IP address and subnet mask 172.16.2.2/24.
R2(config)#interface serial 0/0/0
R2(config-if)#ip address 172.16.2.2 255.255.255.0
R2(config-if)#no shutdown
If we now issue the show interfaces serial 0/0/0 command on either router, we still see that the link is up/down.
R2#show interfaces serial 0/0/0
Serial0/0/0 is up, line protocol is down

The physical link between R1 and R2 is up because both ends of the serial link have been configured correctly with an IP address/mask and enabled with the no shutdown command. However, the line protocol is still down. This is because the interface is not receiving a clock signal. There is still one more command that we need to enter, the clock rate command, on the router with the DCE cable. The clock rate command will set the clock signal for the link. Configuring the clock signal will be discussed in the next section.

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