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Configuring Interface Router

Configuring an Ethernet Interface

As shown, R1 does not yet have any routes. Let's add a route by configuring an interface and explore exactly what happens when that interface is activated. By default, all router interfaces are shutdown, or turned off. To enable this interface, use the no shutdown command, which changes the interface from administratively down to up.

- R1(config)#interface fastethernet 0/0
- R1(config-if)#ip address 192.169.17.1 255.255.255.0
- R1(config-if)#no shutdown
Note: Although enabled with no shutdown, an Ethernet interface will not be active, or up, unless it is receiving a carrier signal from another device (switch, hub, PC, or another router).

Configuring Serial Interface

- R1(config)#interface serial 0/0/0
- R1(config-if)#ip address 172.168.18.1 255.255.255.0
- R1(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.

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.

Commands to Verify Interface Configuration:

1. Click show ip interface brief in the figure.
The show ip interface brief command also shows verifies this same information. Under the status and protocol, you should see "up".

2. The show running-config command shows the current configuration of this interface. When the interface is disabled, the running-config command displays shutdown; however, when the interface is enabled, no shutdown is not displayed.