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The key to configuring a Cisco router safely is knowing which configuration is active and which will survive a reboot. show running-config displays the active configuration; show startup-config displays the saved startup configuration. They can differ until you copy the running configuration to startup configuration.
The commands below show a basic IOS/IOS XE workflow and an illustrative interface setup. Interface names, command support, addressing, and save behavior depend on the router family and software release, so confirm them in the guide for your exact device before making changes.
Running configuration versus startup configuration
Cisco describes the running configuration as the software’s current configuration. It is the configuration currently in effect. The startup configuration is the saved configuration used when the router starts. Cisco notes that the two can differ when changes have been made but not saved. Cisco’s configuration-file guide explains the distinction and the commands for inspecting and saving these files.
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Unsaved changes may not persist after a reboot. Save only after checking that the active configuration is the one you intend the router to use at startup; the exact storage location can vary by platform.
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Basic CLI sequence
IOS and IOS XE commands are entered in modes. enable requests privileged EXEC access, subject to the device’s credentials and access policy. configure terminal enters global configuration mode. From there, select a context—such as an interface—before entering commands specific to it. Cisco’s IOS command-line interface guide describes command modes and EXEC commands.
- At the user EXEC prompt, enter
enable. If prompted, provide the authorized credentials. The prompt commonly changes fromRouter>toRouter#. - Enter
configure terminalto enter global configuration mode, commonly shown asRouter(config)#. - Enter the global setting or select the configuration context you need. For example,
hostname Branch-Routerchanges the system name displayed in the CLI prompt. - When finished configuring, enter
endto return to privileged EXEC mode. - Inspect the active configuration and relevant status, then save with
copy running-config startup-configif the changes are ready to persist.
The prompt labels are illustrative; prompt text can change with the hostname and configuration context.
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Illustrative interface configuration
This example follows Cisco’s ASR 900 Ethernet-interface procedure and its IOS XE guidance for interface IPv4 addressing. It is a teaching example, not a universal configuration. The interface identifier, address, and mask must match the actual router and network plan. Do not paste it into a production device without confirming the change plan and maintaining an appropriate access method.
Router> enable
Router# configure terminal
Router(config)# hostname Branch-Router
Branch-Router(config)# interface gigabitethernet 0/0/1
Branch-Router(config-if)# ip address 192.168.50.1 255.255.255.0
Branch-Router(config-if)# no shutdown
Branch-Router(config-if)# end
Branch-Router# show running-config
Branch-Router# show ip interface
Branch-Router# copy running-config startup-configThe interface context matters: after interface gigabitethernet 0/0/1, the prompt changes to interface configuration mode, where the IP address and no shutdown commands are entered. Cisco’s ASR 900 Ethernet-interface guide documents this type of sequence. It does not establish that the interface name or sample address is right for another router.
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Verify before saving
Use inspection commands after configuration to confirm what is active and, where relevant, what is saved. show ip interface displays interface IP parameters; Cisco documents it in its IOS XE IPv4 addressing guide.
- Run
show running-configto inspect the active configuration, including the changes you made. - Run
show ip interfaceto inspect interface IP parameters. - Run
show startup-configwhen you need to check the configuration saved for startup. - After confirming the intended configuration, run
copy running-config startup-configto save it.
Commands and interface names depend on the platform
“Cisco router” covers multiple hardware families and software releases. The examples in Cisco’s documentation are specific to their stated platforms: the 4000 Series ISR guide and the 8100 Series Secure Routers guide cover different products. Interface naming, supported commands, and startup configuration behavior should therefore be checked against the documentation for the exact model and IOS or IOS XE release.
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An optional command in some basic-router guidance is no ip domain-lookup, which disables attempts to translate unfamiliar words as names. Whether that behavior is appropriate depends on the operating environment; verify command support and suitability for the target release before using it. Cisco discusses hostname and basic system management in its IOS XE 17.x basic system management guide.
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