Saturday, 16 January 2010

3750 Stackwise Upgrades

Upgrading a Cisco switch is very easy:
  1. Copy IOS image to the flash drive on the switch.
  2. Point the boot statement at the new image.
  3. Reboot the switch.
The 3750 stack is made up of several physical devices with just a single logical management interface so Cisco has implemented the archive-sw command to make upgrading it easy. It doesn't quite work as well as it should and I wouldn't recommend using it. The method below is much easier and more reliable:

1. Copy IOS image to the flash drive on the switch
Use the same command as usual, but run it once for each switch in the stack. The flash drives are numbered flash1: flash2: flash3: etc.
copy tftp flash1:
copy tftp flash2:

Delete any old images or html files to clear space as necessary.

2. Point the boot statement at the new image
This can be done with a one liner (replace image.bin with whatever file you want to use):
boot system all flash:image.bin

3. Reboot the switch
The "reload" command will restart the entire stack.

So what's wrong with archive-sw?
The upgrade process is just the 3 simple steps above. Last time I used archive-sw I had about 45 minutes of instability while various individual processes restarted on the switch, routing went down but basic IP services stayed up so HSRP couldn't fail over and the network was unreachable until archive-sw had finished doing whatever it was doing. Maybe it was a bug in the archive-sw version I was using, maybe the config on that device triggered it, maybe it's just not very good.

Either way, I'm sticking to the old method of upgrades because it works and only causes an outage for the time it takes the switch to reboot (assuming it comes up again, which doesn't always happen with stacks!).

ISSU - In Service Software Upgrades
ISSU allows devices to be upgraded without outage. Unfortunately it is not available for 3750 stacks.
It may be possible to carry out the instructions above and then just reboot one switch at a time in the stack, if the old and new versions are similar enough then you might get away with it and the stack might re-form, allowing you to reboot the other switch while maintaining connectivity. Personally I wouldn't try it, if you want maximum uptime then you need two separate logical switches or to forget about upgrading them.

This method was worked out with our partners are Cacti Networks Ltd, network and security providers in the Ross-on-Wye area.

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