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 192.168.1.1 255.255.255.0
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.
Subinterfaces
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 192.168.1.1/24
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)
1200
2400
4800
9600
19200
38400
56000
64000
72000
125000
148000
250000
500000
800000
1000000
1300000
2000000
4000000
<300-8000000> Choose clockrate from list above
Router3(config-if)#clock rate 56000
Bandwidth
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.
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