Configuring an Ethernet Interface


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 172.16.3.1 255.255.255.0
R1(config-if)#no shutdown
The following message is returned from the IOS:
*Mar 1 01:16:08.212: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
*Mar 1 01:16:09.214: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up
Both of these messages are important. The first changed state to up message indicates that, physically, the connection is good. If you do not get this first message, be sure that the interface is properly connected to a switch or a hub.
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).
The second changed state to up message indicates that the Data Link layer is operational. On LAN interfaces, we do not normally change the Data Link layer parameters. However, WAN interfaces in a lab environment require clocking on one side of the link as discussed in Lab 1.5.1, "Cabling a Network and Basic Router Configuration," as well as later in the section, "Configuring a Serial Interface." If you do correctly set the clock rate, then line protocol (the Data Link layer) will not change to up.
The IOS often sends unsolicited messages similar to the changed state to up messages just discussed. As you can see in the figure, sometimes these messages will occur when you are in the middle of typing a command, such as configuring a description for the interface. The IOS message does not affect the command, but it can cause you to lose your place when typing.
In order to keep the unsolicited output separate from your input, enter line configuration mode for the consoled port and add the logging synchronous command, as shown. You will see that messages returned by IOS no longer interfere with your typing.

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