In the networking world you have a lot of opportunity to break things and take down entire buildings, WAN links, etc. Even less risky changes can result in a short break in connectivity while routing protocols re converge and so on. Because we often don’t have physical access to both sites while we are making WAN changes there has to be some type of worst case scenario prevention method. This is it:
When making changes to WAN links in particular I always like to make the change to the far side first and obviously have to if there will be a connectivity loss. The issue is, however, that sometimes through mistakes or overlooked steps the remote site remains unavailable even after changes are made on the closest side of the link. A simple but very effective trick I learned from a CCIE (Who found out the hard way years ago) is to schedule a reboot on the remote device, make changes, and cancel the reboot if the changes work successfully. Let me explain with a simple example:
The steps are as follows:
Save the running configurations of each device to start up before making changes.
On R2 view the current time and schedule a reboot in advance based on the present time. Don’t assume NTP is configured or the time is correct on a device or you could be waiting a while for the reboot should you need it to happen… Be sure to give yourself enough time to change the other side of the link as well as to get logged back in to cancel the reboot while not having to wait forever should you have to wait for the remote router to restart.
Make your changes. If you lose connectivity thats fine just be mindful of how long until the reboot occurs.
Make the WAN interface changes on R1.
Assuming all is working you should now be able to reconnect to R2 with its new IP address. If you cannot connect something was not correct in your configuration changes. You now have to wait for the remote router to restart. It will come back to the startup config you saved prior to the WAN interface changes.
You will want to be familiar with the following commands before you attempt this:
# show clock: This shows the systems present date and time. # reload at
: Use this command to actually schedule the reload. # reload cancel: This will cancel the reload.