I have about 15-18 stacks of Cisco 3750 switches and each of the stack have 3-4 switched in it. Now, we need to upgrade the switch to 3850 ( replace all the 3750 switches with the 3850). Did anybody done this kind of projects? Please help me by providing the detail procedure on how to physically replace the switch in the production environment and how to transfer the current configurations in the switches. We need to have all the previous configuration in the new switch. The switch replacement is done in weekends.

I have researched almost every where, but could not find any answers.

Thanks in advance.

  • 2
    It is a simple matter to show the configuration, capture it to a text file, and paste it into the new switch. You can even pre-configure the replacements before actually replacing the existing switches. – Ron Maupin May 31 '16 at 16:06
  • Agree with Ron. Build up the 3850 stack in advance and deploy the configuration of the old stack. Make sure to have the cabling documented. Then, at the weekend, just replace the switches physically. Not sure what advise you need here. – waza-ari May 31 '16 at 22:01
  • @Ron Maupin and waza-ari – user25763 May 31 '16 at 22:21

Make sure your port names and numbers match exactly between the old and new stacks. This means that for example, GigabitEthernet1/0/1 is on the top most switch and is actually named GigabitEthernet1/0/1 on both switches. Since this is 3750 to 3850, that shouldn't be a problem. You just want to make sure the top switch is 1, the second switch is 2, etc. on both stacks. You can use the show switch detail to make sure and the switch number (I think) command to make any changes.

Where you might run into issues is if you use a different SFP port for your fiber uplink trunks. Make sure those are in the same ports to make porting the config easier.

Set up a TFTP server on a PC and use the copy run tftp command to copy the config over to the PC.

Set up an IP address on the 3850 stack (and gateway if the PC will be on a different network) and use the copy command (i.e. copy tftp newconfig) to copy it over to the new stack.

I believe 3850s also have a USB port so you can play with putting the config on a thumb drive and copy from the USB port (disk0: I think) to the flash in the switch.

Do a copy newconfig start command to copy the new config to the startup-config which is what the switch will use when reloaded. newconfig is whatever you named the file when you copied it over. start is short for "startup-config"

Do a show start command to make sure the copy took.

Do a reload command to reload the stack and monitor the reload for errors. There might be commands in the old config that have been changed or deprecated. Cisco usually does a good job of handling these assuming the 3850 is running a newer version of IOS than the 3750.

Make sure you are connected to the switch via a console cable when you do all this. The same PC that the console cable is connected to can also be your TFTP server connected directly to the switch to make it easier.

You may also have license file issues to deal with. At the very least, make sure that if your 3750s were running IP Base, the 3850s are at also running IP Base or IP Services and not LAN Base. The 3750s will probably be running the IOS 12 train which I do not think uses the new license PAK scheme. The 3850s are probably running IOS 15 train which may need license codes to enable certain features.

| improve this answer | |
  • Does the brand new, out of the box switches have ios installed or do we have to install it before we can start copying the configuration. – user25763 Jun 1 '16 at 21:14

RE the question about IOS being installed on out of the box switches. Yes, IOS is installed on out of the box switches. If you buy them all at the same time and the same place (and they're the same model) then they will most likely have the same IOS version.

Having the same (or very close) version helps when you're stacking the switches (using the stacking cables in back to turn 2 to 9 switches into a single unit). If the version numbers are too far apart, the switches will not stack and you may need to upgrade the IOS on the older versioned switches manually. There's an auto upgrade feature but it doesn't always work like I would expect it to. :)

The point here is that even though the switches might be stacked and act as one unit, each switch still has and needs its own version of IOS. Even when upgrading a stacked set of switches, you need to copy the IOS on to each separate switch. The following link is for 3750 switches and there may be a newer one for 3850s but I doubt it will be much different. This contains all sorts of commands and instructions for stacking upgrading and copying IOS files between switches:


| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.