Automatically reverting configuration mistakes in JunOS and IOS


The old joke goes: I don’t make mistakes. I thought I did once, but I was wrong.

But here’s how to save yourself from mistakes when configuring a JunOS or IOS device:

JUNOS is easy, “commit check”, “commit confirmed” and the handy “rollback”. Frankly, if you’re on a JUNOS┬ádevice, you should already know about these.

But how to do the same on IOS? Cisco’s configurations happen as soon as you hit enter on the config, right?

Here’s how:

  • The brute force method: “reload in 5”. Write your proposed changes in notepad++ (or notepad.exe if you insist). Login to the device that you’re going to change, and before you do any of your changes, run “reload in 5”. Then, make your changes. If you did something boneheaded that will disable the device or your connection to it, the device will reload from it’s start-config in 5 minutes, effectively erasing your running-config.

Of course, if this is a shared device, you’ve taken everyone offline for the time it takes the device to reboot. Or maybe you don’t want your config saved to start-config as a security measure*. So what do you do in order to prevent a reboot and still maintain the ability to revert a recently made change? Ah, you’ll be wanting the somewhat more sophisticated method 2:

  • First, turn on archive:

cisco-router#configure terminal

then configure a path for where it’ll be stored and save that:

cisco-router(config-archive)#path usbflash0:backup-config
cisco-router#write mem

Now you should be able to use “configure terminal revert time 5” which is IOS’ functional equivalent of JUNOS’ “commit confirmed 5”:

cisco-router#conf t revert time 5

Here it will tell you that your config is being backed up to where you told it to and that the config will be rolled back in 5 minutes. So make your changes:

cisco-router(config)#int gig 0/0

cisco-router(config-if)# shutdown

If everything is good, “configure confirm” to keep the change:

cisco-router(config-if)#configure confirm

For the truly worried, use both methods: “reload in 10” and “revert in 5”. If the revert doesn’t work, the reload will (this will still take down the router, but hey…)

Categories : HowTo  Networking  Security