Today I was talking to a colleague about an issue at one of our sites, unfortunately a change had been made which brought down the network in that building. Basically some port changes were being made and unfortunately an engineer made the change to a port which was a member of a port channel and not to the port channel itself. It just so happened this was the very port channel which connected that switch to the rest of the network. With the link mismatch the port channel suspended and many a user weapt tears of sorrow.
Anyway all that aside this post covers a quick ‘get out of jail’ technique which you can leverage when modifying switches/routers/firewalls.
In this case I’m going to demo the ‘technique’ on my Cisco SG300 switch – obviously the commands used may vary by supplier and model so do your research.
Let’s see what options I have for the reload command –
SG300-20#reload at Reload at a specific time/date cancel Cancel pending reload in Reload after a time interval <CR> SG300-20#
OK we have a couple of options…
- First off we can specify an exact time to reload, this can be used in many situations. For example you make a change that requires a reload out of hours, schedule it!
- Next is a very important option for this post – cancel! I’m going to assume you know what this does…
- Finally we have the ‘in’ option – basically we will just tell the system reload in x amount of time from now
Let us use the ‘reload in’ as an example –
SG300-20#reload in <1-999> time interval in minutes (mmm format) WORD<4-6> time interval in hours & minutes (hhh:mm format) SG300-20#
As you can see we just need to provide the time interval in either minutes or hours:minutes.
How does this all help?
Let us assume you make some changes which lock out/disconnect you from the system in some way. If you have configured an automatic reload you just have to wait for that to occur and the known good startup configuration to take effect. Then you will be able to connect again and hopefully this time round not make the same mistake!
If your configuration changes go as planned and you are confident all is good you can then save to the startup config file and cancel the reload.
Of course it goes without saying that any changes should have appropriate change control to try and limit the risk of error. Ultimately we all make mistakes and little tricks like this can help save you when things go awry