Adding a Cisco 3750 Switch to an Existing Stack

Another simple task that Cisco doesn't quite make 100% clear.

Let's say you have an existing Cisco 3750 Switch Stack and you have had a bit of an office redesign and you now need to add another member to the stack to add additional port capacity for the new users moving in.

This is mostly simple to do, but there are several checks to make along your journey, and hopefully I can point them out clearly.

1. First, check the stacking status of your existing stack using the below command. It should say that the stack ring speed is "full". If it doesn't you need to ensure you have a complete stack "ring" thereby having redundant stacking paths on each switch.
   
   

   ZZStack#show switch stack-ring speed
   
   Stack Ring Speed        : 32G
   Stack Ring Configuration: Full
   Stack Ring Protocol     : StackWise

   
   
   See this publication for stack-wiring help:
   http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/hardware/quick/guide/3750GSG3.html#wp49279
   
   
2. Once you know your stack is a complete ring("full") you can safely break this ring to insert your new switch. ENSURE YOUR NEW SWITCH IS POWERED OFF. Rack this new switch adjacent to your existing switches. "Break" the ring in one place so you can wire your new switch so it fits nice and neat in this new stack, following the wiring schema linked above. Once everything is wiring in place, you may NOW power on your new switch.
   
   
3. After a few minutes your switch will have booted and the existing stack should recognize it as joining the stack. You can verify its status with:
   
   

   ZZStack#show switch stack-ring speed
   
   Stack Ring Speed        : 32G
   Stack Ring Configuration: Full
   Stack Ring Protocol     : StackWise

   
   
   

   ZZStack#show switch
   Switch/Stack Mac Address : 0024.9803.8e80
                                              H/W   Current
   Switch#  Role   Mac Address     Priority Version  State
   ----------------------------------------------------------
   *1       Master 0024.9803.8e80     15     0       Ready
    2       Member 0023.ac0f.7880     1      0       Ready
    3       Member 001a.e267.0080     1      0       Version Mismatch

   
   
   In this example switch 3 was added to the stack, the stack ring looks good, but on the switch status output, instead of saying "Ready" it says "Version Mismatch." You now need to ensure your switch gets the same IOS version as its stack-mates and reboots for this to take effect(see below).
   
   If your stack says your new switch is in the "Ready" state, you are in luck and you are done! (except for configuring your new user ports)
   
   
4. Automatic Upgrade is a great thing in theory, but I've not had the best of luck with it. From what I can tell if your existing stack and this new member you are adding are running different IOS featuresets, the automatic upgrade will NOT WORK.
   
   To check to see if its working or not, check the log. If you see log entries resembling any of the below, it appears automatic upgrade is working as it should.
   

   ZZStack#show log
   ---snip---
   Nov 24 16:58:36.388 EST: %IMAGEMGR-6-AUTO_COPY_SW_INITIATED: Auto-copy-software process initiated for switch number(s) 3

   
   This shows that it has started the auto upgrade process. You can check the status with the "show archive status" command or by continually checking the log.
   
   If you see the below it was successful and is now rebooting this member switch so the correct IOS loads and it can finally join the stack as it should(and you should be done):
   

   ---snip---
   Nov 24 17:06:04.453 EST: %IMAGEMGR-6-AUTO_COPY_SW: Software successfully copied to
   Nov 24 17:06:04.453 EST: %IMAGEMGR-6-AUTO_COPY_SW: system(s) 3
   Nov 24 17:06:04.453 EST: %IMAGEMGR-6-AUTO_COPY_SW: Done copying software
   Nov 24 17:06:04.453 EST: %IMAGEMGR-6-AUTO_COPY_SW: Reloading system(s) 3

   
   
   If you don't see any of the above log entries, go on to the next step.
   
   
5. This is the step they don't really explain well. Thankfully, the "Version Mismatch" state, while not activating any of its ports, does allow you to manipulate the flash filesystem of the inconsistent member so you can stage it from the main stack interface.
   
   If you made it to this step, it probably means there is a featureset mismatch(or some other problem) and you need to force this new member to take the IOS version that the stack is currently running. In this scenario, even the "archive copy-sw" command does not work, so you must either load the IOS bin file manually or use use the appropriate "archive download-sw" command with the "/allow-feature-upgrade" switch to load the IOS to the ENTIRE STACK again, including this new member(but you only need to reboot the new member). I prefer to use the archive command, its slow but its so easy!
   
   Here's the link to the software upgrade caveats/howtos for stack configurations(if it seems like I breezed through this last step):
   http://www.cisco.com/en/US/products/hw/switches/ps5023/products_configuration_example09186a00804799d7.shtml

Renumbering a Cisco 3750 Stack

For those of you familiar with the Cisco 3750 switches, their killer feature is their ability to "stack". Stacking sets one switch as the "master" and utilizing stack cables out of the back of the switch, each switch in the stack appears as one. This allows for expandability, and ease of management.

Anywho, one annoyance of stacking 3750s is that an engineer building or adding to a stack can often install switches "out of order" meaning their stack ID does not match their order as-racked. Basically, most English(or any other Western language) speaking peoples. would assume that a group of 5 switches in a rack would be numbered 1-5, top to bottom. If this isnt the case, ASSumptions can lead to some mistakes and mistakes lead to fussy customers. Yes, this can be checked by pressing the mode button on the front of any stack member, but isnt it better to not worry with that and have it right from the beginning?

Anyway, as a best practice, I always set the order as racked to match the stack order. Its not as simple as issuing the renumber subcommand as it won't move your switch port configurations with the move, you have to do some trickery to move the configurations(it even warns you of this when you hit the commands). If you want to renumber switches in a stack, here is a little HOWTO, since Cisco really doesn't go over this scenario, and I've done it a few times.

*************
Example:
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Stack of 2 switches in a rack, Switch 2 is on top, switch 1 is on the bottom.

Step 1: Prime the switches by setting their new stack numbers
We want 2 to become 1 and 2 to become 2, right?

In config mode:

switch 1 renumber 2
switch 2 renumber 1

Then write your config...simple enough.

Step 2:
Copy the startup config to your favorite text editor via screen scrape or file transfer. Do a find replace on all 2/0/ and replace with X/0/ and then find replace 1/0/ with 2/0/, and finally X/0/ with 1/0/. Basically you have flip-flopped all the port configs between the two switches using the power of find/replace.

Step 3:
Upload the file you edited above and have it overwrite the startup config of the stack. Reload the stack

Step 4:
Test. (of course).