cisco router configuration basics
cisco router configuration
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overview
router configuration controls the operation of the
router:
interface address and netmask
routing information (static or dynamic)
booting and startup information
security (passwords)
cisco router configuration
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overview
configuration statements have different contexts:
global:
enable-password mysecret
interface:
interface ethernet0
ip address 195.176.118.254 255.255.255.0
router:
router ospf 1
network 195.176.118.0 0.0.0.255 area 0
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global configuration
global configuration statements are independent of
any particular interface or routing protocol, e.g.:
hostname myrouter
enable-password mysecret
service password-encryption
logging facility local0
logging 195.176.118.42
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global configuration
ip-specific global configuration statements:
ip classless
static route creation:
ip route 195.176.118.0 255.255.248.0 195.176.31.1
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interface configuration
interfaces are named by type and position; e.g.:
ethernet0, ethernet1,... ethernet5
serial0, serial1 ... serial3
and can be abbreviated:
ethernet0 or eth0 or e0
serial0 or ser0 or s0
cisco router configuration
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interface configuration
ip address and netmask configuration, using interface
commands (interactive configuration example, showing
prompts):
router#config terminal
router(config)#interface e0
router(config-if)#ip address 195.176.118.254 255.255.255.0
router(config-if)#no shutdown
router(config-if)#^Z
router#
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There are two modes on a cisco router
 normal mode
router>
enable mode
router> enable
router# config terminal
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interface configuration
administratively enable/disable the interface
router(config-if)#no shutdown
router(config-if)#shutdown
(config-if)# no ip proxy-arp
(config-if)# no ip directed broadcast
(config-if)# no ip redirect
description
#description 128k circuit 123-ABC to hotel
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where is the configuration?
router always has two configurations:
running configuration
in RAM, determines how the router is currently operating
is changed by using the configuration command
to see it: show running
startup configuration
in NVRAM, determines how the router will operate after
next reload
is changed using the copy command
to see it: show startup
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where is the configuration?
can also be stored in more permanent places:
external hosts, using TFTP to move it around
in flash memory in the router
copy command is used to move it around
copy
copy
copy
copy
copy
copy
cisco router configuration
run start
run tftp
start tftp
tftp start
flash start
start flash
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looking at the configuration
use “show running-configuration” to see the
current configuration
use “show startup-configuration” to see the
configuration in NVRAM, that will be loaded the next
time the router is rebooted or reloaded
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changing the configuration
configuration statements can be entered interactively at
the console, or in a telnet session,
or, edited in a text file and uploaded to the router at a
later time;
some configuration statements, especially access lists,
are very difficult to work with interactively, so editing
and uploading the file is the only practical way to work;
also allows version control and auditing changes
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interactive configuration
can use direct serial connection to console port, or
telnet to vty’s (“virtual terminals”), or
modem connection to aux port
in every case, changes are made (almost) immediately,
to the running configuration
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interactive configuration
enter configuration mode, using “configure term”
prompt gives a hint about where you are:
router#configure terminal
router(config)#hostname K-rtr
K-rtr(config)#ip classless
K-rtr(config)#ip subnet-zero
K-rtr(config)#interface ethernet3
K-rtr(config-if)#ip address 195.176.118.253 255.255.255.0
K-rtr(config-if)#no shutdown
use the no command to nullify command given:
(config-if)# no ip addr 195.176.118.253 255.255.255.0
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storing the configuration on a host
 requires: `tftpd’on a unix host; destination file must exist
before the file is written and must be world writable...
copy run tftp
K-rtr#copy run tftp
Remote host []? 195.176.118.42
Name of configuration file to write [K-rtr-confg]?
/usr/local/tftpd/K-rtr-confg
Write file /usr/local/tftpd/K-rtr-confg on...
host 195.176.118.42? [confirm]
Building configuration...
Writing /usr/local/tftpd/K-rtr-confg !![OK]
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restoring the configuration from a host
use ‘tftp’ to pull file from unix host, copying to
running config or startup
K-rtr#copy tftp start
Address of remote host [255.255.255.255]?
195.176.118.42
Name of configuration file [K-rtr-confg]?
Configure using K-rtr-confg from 195.176.118.42?
[confirm]
Loading K-rtr-confg from 195.176.118.42 (via
Ethernet0): !
[OK - 1005/128975 bytes]
[OK]
K-rtr# reload
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getting help
IOS has a built-in help facility; use “?” to get a list of
possible configuration statements
“?” after the prompt lists all possible commands:
router#?
 “<partial command> ?” lists all possible
subcommands, e.g.:
router#show ?
router#show ip ?
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getting help
“<partial command>?” shows all possible command completions
router#con?
configure connect
this is different (space before “?”):
router#conf ?
memory
network
host
overwrite-network
terminal
<cr>
cisco router configuration
Configure from NV memory
Configure from a TFTP network
Overwrite NV memory from TFTP..
network host
Configure from the terminal
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getting help
this also works in configuration mode:
router(config)#ip a?
accounting-list accounting-threshold
accounting-transits address-pool alias
as-path
router(config)#int e0
router(config-if)#ip a?
access-group accounting
cisco router configuration
address
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getting help
can “explore” a command to figure out the syntax:
router(config-if)#ip addr ?
A.B.C.D IP address
router(config-if)#ip addr 169.222.64.1 ?
A.B.C.D IP subnet mask
router(config-if)#ip addr 169.222.64.1 255.255.255.0 ?
secondary Make this IP address a secondary address
<cr>
router(config-if)#ip addr 169.222.64.1 255.255.255.0
router(config-if)#
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getting lazy help
TAB character will complete a partial word
router(config)#int<TAB>
router(config)#interface et<TAB>
router(config)#interface ethernet 0
router(config-if)#ip add<TAB>
router(config-if)#ip address ...
195.176.118.1 255.255.255.0
not really necessary; partial commands can be used:
router#conf t
router(config)#int e0
router(config-if)#ip addr 195.176...
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getting lazy
command history
IOS maintains short list of previously typed commands
up-arrow or ‘^P’ recalls previous command
down-arrow or ‘^N’ recalls next command
line editing
left-arrow, right-arrow moves cursor inside command
 ‘^D’ or backspace will delete character in front of cursor
cisco router configuration
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