cisco router configuration basics

overview

• router configuration controls the operation of the router:

• interface address and netmask• routing information (static or dynamic)• booting and startup information• security (passwords)

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

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

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

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

interface configuration

• ip address and netmask configuration, using interface commands (interactive configuration example, showing prompts):router#config terminalrouter(config)#interface e0router(config-if)#ip address 195.176.118.254 255.255.255.0router(config-if)#no shutdownrouter(config-if)#^Zrouter#

There are two modes on a cisco router

• normal mode• router>

• enable mode • router> enable• router# config terminal

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

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

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 run start– copy run tftp– copy start tftp– copy tftp start– copy flash start– copy start flash

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

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

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

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

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]

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

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 ?

getting help• “<partial command>?” shows all possible command completions

– router#con?– configure connect

• this is different (space before “?”):

– router#conf ?– memory Configure from NV memory– network Configure from a TFTP network host– overwrite-network Overwrite NV memory from TFTP... network host– terminal Configure from the terminal– <cr>

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 address

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)#

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...

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