Running and Startup Config

When working with Cisco routers or switches, it’s essential to understand how the device stores and applies its configuration.

Each Cisco device maintains two versions of its configuration:

• The running configuration is stored in RAM. It’s a volatile memory, meaning all changes are lost when the device reboots.

• The startup configuration is stored in NVRAM, a persistent memory that survives restarts and is automatically loaded at boot.

Figure 1 – Save running config to startup config to keep changes after reboot

This distinction is crucial. If you make changes to the configuration during a session, you're actually modifying the running configuration. But unless you save those changes, they'll be lost as soon as the device reboots.

To ensure your changes are kept permanently, you must manually save the running configuration into the startup configuration.

Let’s walk through what happens from the moment the device powers on.

When the router or switch boots up, it begins by loading the startup configuration from NVRAM.
This configuration is copied into RAM and becomes the running configuration, which the system uses to operate in real time.

Figure 2 – At startup, the startup config is loaded into RAM as the running config

So the process works like this:

• The startup configuration defines how the device boots

• The running configuration reflects the current live state

• Any configuration changes apply to the running config

• If you want those changes to persist after a reboot, you must manually save them with: copy running-config startup-config

This flow is key to understanding how configuration behaves on Cisco devices:

• On reboot → the startup config is copied into the running config

• When saving → the running config must be copied into the startup config

To check what configuration is currently active, use:

SW1# show running-config
hostname SW1
!
interface GigabitEthernet0/1
 negotiation auto
!
spanning-tree mode pvst
!
ip ssh version 2
!
line vty 0 4
 login
!
end

This displays everything currently stored in RAM the live configuration used by the device.

Now let’s modify a setting. For example, configure the STP priority:

SW1# conf t
Enter configuration commands, one per line. End with CNTL/Z.
SW1(config)# spanning-tree vlan 1 priority 24576
SW1(config)# end

➔ The change is active immediately in RAM but is not yet saved.