Configuring A Router

CHAPTER 3Configuring a RouterThis chapter provides information and commands concerning the following topics: Configuring a router, specifically:— Names— Passwords— MOTD banners— IP host tables— Saving and erasing your configurations show commands to verify the router configurationsRouter ModesRouter User modeRouter#Privileged modeRouter(config)#Global configuration modeRouter(config-if)#Interface modeRouter(config-subif)#Subinterface modeRouter(config-line)#Line modeRouter(config-router)#Router configuration modeTIP: There are other modes than these. Not all commands work in allmodes. Be careful. If you type in a command that you know is correct—showrun, for example—and you get an error, make sure that you are in the correctmode.PART II CCNA 2— Interfaces

18Configuring PasswordsGlobal Configuration ModeRouter Can see config, but not changeRouter#Can see config and move to makechangesRouter#config tMoves to global config modeRouter(config)#This prompt indicates that you canstart making changesConfiguring a Router NameThis command works on both routers and switches.hostname CiscoRouter(config)#hName can be any word you chooseCisco(config)#Configuring PasswordsWorks on both routers and switches.enable password ciscoRouter(config)#eSets enable passwordenable secret classRouter(config)#eSets enable secret passwordline con 0Router(config)#lEnters console-line modepassword consoleRouter(config-line)#pSets console-line mode password toconsoleloginRouter(config-line)#lEnables password checking at loginline vty 0 4Router(config)#lEnters vty line mode for all 5 vtylinespassword telnetRouter(config-line)#pSets vty password to telnetloginRouter(config-line)#lEnables password checking at login

show Commands19line aux 0Router(config)#lEnters auxiliary line modepassword backdoorRouter(config-line)#pSets auxiliary line mode password tobackdoorloginRouter(config-line)#lEnables password checking at loginCAUTION: Enable secret password is encrypted by default. Enable password isnot. For this reason, recommended practice is that you never use the enablepassword. Use only the enable secret password in a router configuration.CAUTION: You cannot set both enable secret and enable password to the samepassword. Doing so defeats the use of encryption.Password Encryptionservice passwordRouter(config)#sencryptionApplies a weak encryption topasswordsenable password ciscoRouter(config)#eSets enable password to ciscoline con 0Router(config)#l password CiscoRouter(config-line)#pContinue setting passwords as above no service passwordRouter(config)#nencryptionTurns off password encryptionCAUTION: If you have turned on service password encryption, used it, and thenturned it off, any passwords that you have encrypted will stay encrypted. Newpasswords will remain unencryptedshow Commandsshow ?Router#sLists all show commands availableshow interfacesRouter#sDisplays statistics for all interfacesshow interface serial 0Router#sDisplays statistics for a specificinterface, in this case Serial 0show ip interface briefRouter#sDisplays a summary of allinterfaces, including status and IPaddress assigned

20Interface Namesshow controllers serial 0Router#sDisplays statistics for interfacehardware. Statistics display if theclock rate is set and if the cable isDCE, DTE, or not attachedshow clockRouter#sDisplays time set on deviceshow hostsRouter#sDisplays local host-to-IP addresscache. These are the names andaddresses of hosts on the network towhich you can connectshow usersRouter#sDisplays all users connected todeviceshow historyRouter#sDisplays history of commands usedshow flashRouter#sDisplays info about Flash memoryshow versionRouter#sDisplays info about loadedsoftware versionshow arpRouter#sDisplays the ARP tableshow protocolsRouter#sDisplays status of configured Layer3 protocolsshow startup-configRouter#sDisplays configuration saved inNVRAMshow running-configRouter#sDisplays configuration currentlyrunning in RAMInterface NamesOne of the biggest problems that new administrators face is the names of the interfaces onthe different models of routers. The following chart lists the names of the Ethernet, FastEthernet, and Serial interfaces on the 2500, 1700, and 2600 series of routers.Fixed Interfaces (2500Series)Modular (Removable)Interfaces (1700 Series)Modular (Removable)Interfaces (2600 Series)intRouter(config)#ierface type portinterfRouter(config)#iace type portinterfaceRouter(config)#itype slot/portintRouter(config)#iserial0 (s0)interfRouter(config)#iace serial 0int serialRouter(config)#i0/0 (s0/0)intRouter(config)#iethernet 0 (e0)interfRouter(config)#iace fastethernet 0intRouter(config)#ifastethernet 0/0 (fa0/0)

Configuring an Ethernet/Fast Ethernet Interface21Moving Between InterfacesWhat happens in Column 1 is the same thing as is occurring in Column 2.int s0Router(config)#iint s0Router(config)#iMoves to interface S0modeexitRouter(config-if)#eint e0Router(config-if)#iIn int S0, move to E0int e0Router(config)#iRouter(config-if)#In E0 mode nowRouter(config-if)#Prompt does notchange; be carefulConfiguring a Serial Interfaceint s0/0Router(config)#iMoves to interface Serial 0/0modedescription Link to ISPRouter(config-if)#dOptional descriptor of the link islocally significantip address s address and subnetmask to interfaceclock rate 56000Router(config-if)#cAssigns a clock rate for theinterfaceno shutRouter(config-if)#nTurns interface onTIP: The clock rate command is used only on a serial interface that has a DCEcable plugged into it. There must be a clock rate set on every serial link betweenrouters. It does not matter which router has the DCE cable plugged into it, or whichinterface the cable is plugged into. Serial 0 on one router can be plugged intoSerial 1 on another router.Configuring an Ethernet/Fast Ethernet Interfaceint fa0/0Router(config)#iMoves to Fast Ethernet 0/0interface modedescription Accounting LANRouter(config-if)#dOptional descriptor of thelink is locally significant

22Assigning a Local Host Name to an IP Addressip address s address and subnetmask to interfaceno shutRouter(config-if)#nTurns interface onCreating a MOTD Bannerbanner motd # This is aRouter(config)#bsecure system. Authorized Personnel Only!Router(config)### is known as a delimitingcharacter. The delimitingcharacter must surround thebanner message and can beany character so long as it isnot a character used withinthe body of the messageSetting the Clock Time Zoneclock timezone EST –5Router(config)#cSets the time zone fordisplay purposes. Based oncoordinated universal time(Eastern Standard Time is 5hours behind UTC)Assigning a Local Host Name to an IP Addressip host london 172.16.1.3Router(config)#iAssigns a host name to theIP address. After thisassignment, you can use thehost name instead of an IPaddress when trying toTelnet or ping to thataddressping londonRouter#p ping 172.16.1.3Router#pTIP: The default port number in the ip host command is 23, or Telnet. If you wantto Telnet to a device, just enter the IP host name itself:london Router#ttelnet london Router#ttelnet 172.16.1.3Router#l

exec-timeout Command23no ip domain-lookup Commandno ip domain-lookupRouter(config)#nRouter(config)#Turns off trying toautomatically resolve anunrecognized command to alocal host nameTIP: Ever type in a command incorrectly and left having to wait for a minute ortwo as the router tries to translate your command to a domain server of255.255.255.255? The router is set by default to try to resolve any word that is nota command to a DNS server at address 255.255.255.255. If you are not going to setup DNS, turn this feature off to save you time as you type, especially if you are apoor typist.logging synchronous Commandline con 0Router(config)#llogging synchronousRouter(config-line)#lTurns on synchronouslogging. Information itemssent to console will notinterrupt the command youare typing. The commandwill be moved to a new lineTIP: Ever try to type in a command and an informational line appears in themiddle of what you were typing? Lose your place? Do not know where you are inthe command, so you just press and start all over? The loggingsynchronous command will tell the router that if any informational items getdisplayed on the screen, your prompt and command line should be moved to anew line, so as not to confuse you.The informational line does not get inserted into the middle of the command youare trying to type. If you were to continue typing, the command would executeproperly, even though it looks wrong on the screenexec-timeout Commandline con 0Router(config)#lexec-timeout 0 0Router(config-line)#eRouter(config-line)#Sets time limit when consoleautomatically logs off. Set to0 0 (minutes seconds) meansconsole never logs off

24Configuration Example: Basic Router ConfigurationTIP: exec-timeout 0 0 is great for a lab because the console never logs out. Thisis very dangerous in the real world (bad security).Saving Configurationscopy run startRouter#cSaves the running-config to local NVRAMcopy run tftpRouter#cSaves the running-config remotely to TFTP serverErasing Configurationserase startRouter#eDeletes the startup-config file from NVRAMTIP: Running-config is still in dynamic memory. Reload the router to clear therunning-config.Configuration Example: Basic Router ConfigurationFigure 3-1 shows the network topology for the configuration that follows, which shows abasic router configuration using the commands covered in this chapter.Figure 3-1Network Topology for Basic Router Configuration172.16.10.10172.16.10.1fa0/0Network 0.2 BuffaloNetwork 172.16.20.0/24172.16.30.30172.16.30.1fa0/0Network 172.16.30.0/24Boston RouterenRouter eEnters privileged modeclock set 18:30:00 15 Nov 2004Router#cSets local time on routerconfig tRouter#cEnters global config mode

Configuration Example: Basic Router Configurationhostname BostonRouter(config)#hSets router name to Bostonno ip domain-lookupBoston(config)#nTurns off name resolution onunrecog-nized commands(spelling mistakes)banner motd #Boston(config)#bCreates an MOTD bannerThis is25the Boston Router.Authorized Access Only#clock timezone EST –5Boston(config)#cSets time zone to EasternStandard Time (–5 from UTC)enable secret ciscoBoston(config)#eEnable secret password set tociscoservice password-encryptionBoston(config)#sPasswords will be given weakencryptionline con 0Boston(config)#lEnters line console modelogging syncBoston(config-line)#lCommands will not beinterrupted by unsolicitedmessagespassword classBoston(config-line)#pSets password to classloginBoston(config-line)#lEnables password checking atloginline vty 0 4Boston(config-line)#lMoves to virtual Telnet lines 0through 4password classBoston(config-line)#pSets password to classloginBoston(config-line)#lEnables password checking atloginline aux 0Boston(config-line)#lMoves to line auxiliary modepassword classBoston(config-line)#pSets password to classloginBoston(config-line)#lEnables password checking atloginexitBoston(config-line)#eMoves back to global configmode