We were talking with Debra Curtis of Gartner yesterday about Network Configuration and Change Management (NCCM) and the topic of Rollback came up. Juniper has supported the ‘commit’ operation since they started, allowing their devices to be rolled back easily. However, Cisco only recently added the ‘commit’ feature and when you examine it carefully, you find that it doesn’t always work the way you want (e.g., rollback of ACLs is problematic).
A basic rollback function re-installs the old configuration and expects the device to use that configuraiton instead of the old one. What most managers expect is that the rollback will restore the device to its prior state without a service interruption. That’s not what Cisco IOS does – the configurations merge (to some degree) when a second configuration is loaded into a running box. To get back to the original configuration, you have to load the configuration and then force a reload of the box, resulting in a service interruption. Yuck.
Loading the inverse (by adding or removing the word ‘no’ on the begining of the command) of the most recent configuration commands will not necessarily restore the desired configuraiton state either. Double-yuck.
In yesterday’s blog post, I mentioned a Network World blog by Jamey Heary in which he describes new features of IOS 12.4(20)T. In it is a description of the Auto-upgrade manager, which includes non-stop upgrades whereby a whole new IOS is loaded and started while the forwarding plane continues to run. Jamey called it a warm start. It’s a good feature.
What if the warm start feature were used in the reload operaton to implement a full rollback (the whole configuration is loaded into the device)? The forwarding plane can continue to run while the current image reloads the configuration, re-establishes neighbor relationships, then updates the forwarding plane as needed. I’ll bet that someone at Cisco has already thought of this, or maybe it is documented elsewhere and I’m not aware of it. If not, please add my name to the patent application. 
-Terry
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Re-posted with Permission
NetCraftsmen would like to acknowledge Infoblox for their permission to re-post this article which originally appeared in the Applied Infrastructure blog under http://www.infoblox.com/en/communities/blogs.html
