Router Ethernet Interface Configuration - dummies

Router Ethernet Interface Configuration

By Edward Tetz

You start the process of configuring router Ethernet interfaces by making a physical connection to your router. After you establish the connection, you can proceed with the basic configuration of Ethernet, Fast Ethernet, or Gigabit Ethernet connections.

Connecting to your router

To start your configuration, you need to connect to your router and get into Configuration mode using the following set of commands:

Router2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.

The next option is to choose the port by number (it is also known as an interface because it is a networking port), which is how to correctly identify the specific interface with which you want to work. You can choose from Ethernet, Fast Ethernet, or Gigabit Ethernet interfaces. Then you specify the port number.

All devices in current IOS versions are numbered starting at the motherboard in one of these formats:

  • network-module-slot/port

  • 0/interface-card-slot/port

  • network-module-slot/interface-card-slot/port

All modules are effectively connected to the motherboard slot on the router, which is always slot 0 and is represented by the first 0 in the interface name. After that, the numbers are for an integrated interface, or for another expansion slot in your router.

For example, if you have WIC or HWIC installed, you specify the motherboard slot (0), and then the slot into which the card is installed (0), and then you specify the interface number; 0 is the first interface for routers, and 1 is the first interface for switches (which can be installed into a router HWIC). So the first switch port for an HWIC switch is 0/0/1:

Router2(config)#interface FastEthernet 0/0

Using auto settings

You can set the specifics of the network connection or use the auto settings for duplex and speed settings. Duplex modes include Full Duplex mode or Half Duplex mode, whereas speeds typically are from 10 Mbps up to the speed of the interface.

Router2(config-if)#duplex auto
Router2(config-if)#speed auto

Adding a description

You might not want to use this feature for switches, but on your router, providing a description helps to prevent changing the configuration on the wrong interface. “Oops, was that the corporate WAN interface I just changed the IP address on? Time to polish up that resumé.” A description does not assist with the configuration; it just helps prevent human error.

Router2(config-if)#description Internal Interface

Configuring a VLAN identifier

Because the interface works similarly to a switchport — and if you do not want to use the access settings on the switch to which you have connected the router — you can configure a VLAN identifier for the interface. You do so with the vlan-id or vlan-range command:

Router2(config-if)#vlan-id dot1q 1

Setting a routing interface

Because you plan to route from this interface, you need to give the client devices an IP address to connect to, as follows:

Router2(config-if)#ip address

You do have the option of configuring your router to use DHCP using the command ip address dhcp, but typically this command is not used for static network devices such as routers. Connecting this router as a Network Address Translation (NAT) device to an ISP represents the only case where a DHCP-configured interface is likely to be used.

Enabling your interface

Now, you have done all of this work, and there is a good chance you want to use this interface; however, if you exit the configuration and check the running configuration, you will notice one configuration item that is a little strange.

Router2#show running-config interface FastEthernet 0/0
Building configuration...
Current configuration : 199 bytes
interface FastEthernet0/0
 description Internal LAN Interface
 ip address
 speed auto
 vlan-id dot1q 1
 no mop enabled

One important item is missing — or, one item is present that should not be present. Unlike switches, all interfaces on your router should be, by default, shut down using the shutdown command. At first, this may seem strange, but think about where routers are used; in many cases, they exist as a gateway between you and the unprotected Internet.

Setting the port as disabled gives you some additional security around your router until you are ready to open the flood gates. As with switch interfaces, the command to get rid of the shutdown command is no shutdown. So you need to complete the configuration of your router interface with the following command:

Router2(config-if)#no shutdown

If you are working on the console or have terminal monitor enabled, you should receive a status message telling you that the interface has been enabled. This message will be similar to the following:

%LINK-5-CHANGED: Interface FastEthernet0/0, changed state to up

Double-checking your settings

After you have the interface up and running, if you are using an auto setting for the speed and duplex settings, examine the interface to ensure that it has detected settings you are happy with. Do so with the show interface command, as shown here:

Router2#show interfaces  FastEthernet 0/0
FastEthernet0/0 is up, line protocol is up
  Hardware is AmdFE, address is 000f.8f4b.a600 (bia 000f.8f4b.a600)
  Description: Internal LAN Interface
  Internet address is
  MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 100Mb/s, 100BaseTX/FX
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:07, output 00:00:02, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     417167 packets input, 39317868 bytes
     Received 415431 broadcasts, 0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog
     0 input packets with dribble condition detected
     149499 packets output, 17447327 bytes, 0 underruns
     0 output errors, 0 collisions, 3 interface resets
     0 babbles, 0 late collision, 0 deferred
     4 lost carrier, 0 no carrier
     0 output buffer failures, 0 output buffers swapped out

In the preceding information, notice that the interface and line protocols should both be up. In this case, the interface detected Full-duplex as well as a speed of 100 Mbps. From here, you can verify the IP address of the interface, and you can see if any packet errors are on the interface.

Incorrect duplex settings between ends of a connection can cause packet errors.