Router Interfaces

Routers can have many different types of connectors; from Ethernet, Fast Ethernet, and Token Ring to Serial and ISDN ports.  Some of the available configurable items are logical addresses (IP,IPX), media types, bandwidth, and administrative commands.  Interfaces are configured in interface mode which you get to from global configuration mode after logging in.

Logging in to the Router

Depending on the port you're using, you might have to press enter to get the prompt to appear (console port). The first prompt will look like Routername> the greater than sign at the prompt tell you that you are in user mode. In user mode you can only view limited statistics of the router in this mode. To change configurations you first need to enter privileged EXEC mode. This is done by typing enable at the Routername> prompt, the prompt then changes to Routername#. This mode supports testing commands, debugging commands, and commands to manage the router configuration files. To go back to user mode, type disable at the Routername# prompt. If you want to leave completely, type logout at the user mode prompt. You can also exit from the router while in privileged mode by typing exit or logout at the Routername# prompt.

Global Configuration Mode

Enter this mode from the privileged mode by typing configure terminal or (conf t for short).  The prompt will change to Routername(config)#.  Changes made in this mode change the running-config file in DRAM. Use configure memory to change the startup-config in NVRAM.  Using configure network allows you to change the configuration file on a TFTP server.  If you change the memory or network config files, the router has to put them into memory (DRAM) in order to work with them, so this will change your router's current running-config file.

Interfaces mode

While in global configuration mode you can make changes to individual interfaces with the command Routername(config)#interface ethernet 0 or Routername(config)#int e0 for short, this enters the interface configuration mode for Ethernet port 0 and changes the prompt to look like Routername(config-if)#.

Bringing Up Interfaces

If an interface is shown administratively down when the show interface command is given in privileged EXEC mode, use the command no shutdown to enable the interface while in interface configuration mode.

Setting IP Addresses

In global configuration mode, enter the interface configuration mode (Routername(config)#int e0) and use the command Routername(config-if)#ip address [ip address] [network mask].  If it is the first time using the interface, also use the no shutdown command to enable and bring up the interface.

   Router_2(config)#int e0
   Router_2(config-if)#ip address
   Router_2(config-if)#no shutdown
Secondary IP Addresses

You can add another IP address to an interface with the secondary command. The syntax is the same as setting an IP address except you add secondary to the end of it.  Using secondary interfaces, it allows you to specify 2 IP addresses for 1 interface.  Use subinterfaces instead, since they allow for more than 2 IP addresses on an interface and secondaries will probably be replaced soon.


In global configuration mode you can create virtual interfaces (subinterfaces), so at the prompt Routername(config)# type int e0.1 and the prompt will change to Routername(config-subif)#.  For all practical purposes there isn't a limit to the amount of subinterfaces an interface can have.

Show Interfaces

To view information about an interface, use the command:

   Router_2#show interface e0
   Ethernet0 is up, line protocol is up
   Hardware is Lance, address is 0000.cc34.ec7d (bia 0000.cc34.ec7d)
   Internet address is
     MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, rely 255/255, load 1/255
     Encapsulation ARPA, loopback not set, keepalive set (10 sec)
     ARP type: ARPA, ARP Timeout 04:00:00
     Last input never, output 00:00:07, output hang never
     Last clearing of "show interface" counters never
     Queueing strategy: fifo
     Output queue 0/40, 0 drops; input queue 0/75, 0 drops
     5 minute input rate 0 bits/sec, 0 packets/sec
     5 minute output rate 0 bits/sec, 0 packets/sec
        0 packets input, 0 bytes, 0 no buffer
        Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
        0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
        0 input packets with dribble condition detected
        614 packets output, 58692 bytes, 0 underruns
        0 output errors, 0 collisions, 2 interface resets
        0 babbles, 0 late collision, 0 deferred
        0 lost carrier, 0 no carrier
        0 output buffer failures, 0 output buffers swapped out

Interface Problems

When using the command show interface [type #] interface problems can be seen and appropriate action taken.

Message Solution
Ethernet0 is up, line protocol is up None needed, interface working properly
Ethernet0 is up, line protocol is down Clocking or framing problem, check clock rate and encapsulation type on both routers
Ethernet0 is down, line protocol is down Cable or interface problem, check interfaces on both ends to ensure they aren't shutdown
Ethernet0 is administratively down, line protocol is down The interface has been shutdown, use the no shutdown command in the interface's configuration mode

Serial Interfaces

The serial interface is usually attached to a line that is attached to a CSU/DSU that provides clocking rates for the line.  However, if two routers are connected together, one of the serial interfaces must act as the DCE device and provide clocking.  The DCE end of the cable is the side of the cable that has a female connector where it connects to the other cable.  The clocking rate on the DCE device is set in interface configuration mode with the commands:

   Router3(config)#int s0
   Router3(config-if)#clock rate ?

           Speed (bits per second)

     <300-8000000>    Choose clockrate from list above

   Router3(config-if)#clock rate 56000


Cisco routers ship with T1 (1.544 mbps) bandwidth rates on their serial interfaces.  Some routing protocols use the bandwidth of links to determine the best route.  The bandwidth setting is irrelevant with RIP routing.  Bandwidth is set with the bandwidth command and ranges from 1 - 10000000 kilobits per second.

   Router3(config)#int s0
   Router3(config-if)#bandwidth ?
     <1-10000000>  Bandwidth in kilobits
   Router3(config-if)#bandwidth 10000000

Saving Changes

Any time you make changes and want them saved over the next reboot, you need to copy the running-config to the startup-config in NVRAM.  Use the command:

   Router3#copy run start
You can see either of the files by using the commands:
   Router3#show run
   Router3#show start
To erase the startup file use the command:
   Router3#erase start
Show Controllers

Tells you information about the physical interface itself, it also gives you the cable type and whether it is a DTE or DCE interface.  Syntax is:

   Router_2#show controllers s 1

*Note there is a space between the s and the 1.