Configuring router interfaces involves accessing their own specific configuration environments. Each interface is configured individually, according to its name and numeric identifier. To begin, you need to enter global configuration mode. Then, you’ll need to access a specific interface using the interface command. The output below demonstrates entering global configuration mode, and then attempting to determine the command to access our router’s Ethernet interface.
toronto-1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
toronto-1(config)#interface ?
Async Async interface
BVI Bridge-Group Virtual Interface
Dialer Dialer interface
Ethernet IEEE 802.3
Group-Async Async Group interface
Lex Lex interface
Loopback Loopback interface
Null Null interface
Port-channel Ethernet Channel of interfaces
Serial Serial
Tunnel Tunnel interface
Virtual-Template Virtual Template interface
Virtual-TokenRing Virtual TokenRing
Notice the option for Ethernet. To continue, enter interface ethernet ?.
toronto-1(config)#interface ethernet ?
<0-0> Ethernet interface number
The command requires that we specify the number of the Ethernet interface that we wish to configure. Remember that some routers will have more than one Ethernet interface. Our Cisco 2501 has only one Ethernet interface, nominally numbered 0. If we issue the command interface ethernet 0 and press Enter, notice how the command prompt again changes.
toronto-1(config)#interface ethernet 0
toronto-1(config-if)#
The prompt now shows toronto-1(config-if)#. This designates that we are in interface configuration mode, although you should note that the prompt doesn’t actually identify the interface – that part you’ll need to remember on your own. Accessing a serial interface would have been just as simple – the command would be interface serial, followed by the nominal interface number.
You can also use shortcuts to access interfaces. For example, we could have accessed interface Ethernet 0 using the command int e0.
toronto-1(config)#int e0
toronto-1(config-if)#
The shorthand version is a great deal quicker – I’m sure that’s ultimately what you will end up using.
Depending upon the router model, accessing interfaces can be slightly different. Some of the routers that we looked at in the Chapter 6 are modular. In these cases, interfaces are numbered according to both their slot and interface number. For example, on a Cisco 2600 router, the first Fast Ethernet interface would be known as FastEthernet0/0. The numbering designates the interface’s slot/port number, and would be accessed as shown below.
Router(config)#int fastethernet 0/0
To exit the configuration of an interface and return to global configuration mode, use the exit command. You can also exit global configuration mode completely by pressing Ctrl+Z.
toronto-1(config-if)#exit
toronto-1(config)#
We’ll explore the actual configuration of router interfaces later in the chapter. For now, we’ll continue with our look at the different configuration modes.
