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Convert Cisco 3750 to Stack - No downtime?

Posted on 2015-01-12
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Last Modified: 2015-01-13
I want to convert from a single switch to a stack.  Easy enough to do but I was curious if I could do it without any downtime.  Here is what I have.  I have a single Cisco 3750 (Switch 1) that a bunch of servers are hooked up to. I have another Cisco 3750 (Switch 2) that I just added to the rack which doesn't have anything plugged into yet.  I have both ready to configure into a stack.  Both are essentially running as Stand Alone Masters right now.

So given that all the servers are connected to switch 1, can I hook up Switch 2 with a Stack cable, boot it up, and allow it become part of a stack?  And would Switch 1 remain up without dropping any connections or require a reload?
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Question by:CWadmins
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by:pjam
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I believe they need to be powered off before connecting with your cable.
If you want the top switch to be stack master, power it on first and wait 10 seconds or more before powering on the second switch.
You probably want to connect Stack 1 port to Stack 2 port.  However there are numerous configurations possible so i would suggest downloading the Catalyst 3750 Switch Hardware Installation Guide:
http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750/hardware/installation/guide/3750hig.html
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Robert_Turner earned 500 total points
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No downtime needed, having it powered/un-powered is a matter of what you want the end result to be.  In This case, before connecting......

Power the new switch unconnected, allocate the appropriate switch ID and election priority.

Original Switch - Conf T > Switch 1 Priority 15
Additional switch - Conf T > Switch 2 Priority 13

This will ensure the original switch is elected master and provides the config to the second switch, nothing else needs to be done.

Important - turn off the new switch, then connect the stackwise ports , A to B, B to A
Power on the new switch stack, show switch to verify stack membership

I would expect to see.

3750-Stk>show switch
                                               Current
Switch#  Role      Mac Address     Priority     State
--------------------------------------------------------
*1       Master     0000:1111:2222     15         Ready
 2       Slave      3333:4444:5555     13         Ready

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by:Craig Beck
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I would advise that you make sure that the new switch is running the same version of code as the existing switch.  If you don't the switch will boot and give a version mismatch error.  Depending on the version of code on the new switch you may be able to auto-upgrade from the existing switch (or downgrade as the case may likely be) but that can fail and may cause your stack to reload in come cases.

What Robert said about turning off the new switch is important, as he said.  If you add the switch to the stack while it's powered it may cause the existing switch to reload and then elect itself as master.  That could cause your port configuration to become incorrect.
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by:CWadmins
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So, to clarify

Both switches have the exact same system image loaded.  I upgraded both to the most recent version to prepare them for Stacking already. Since every server connected to the switch uses network teaming, I was able to get switch B running with a stand alone config, move all the 2nd network cables for every server to it, wait for the team to come back up, then disconnect switch A from each server.  That allowed me to load the new image on each switch.

So right now, each server is actually connected to each switch, one port on each server to each switch and the network team works.  I can turn off either switch and all the connections still work just fine for each server.  So knowing that, I can just power off the switch that I don't want to be the master, connect my stack cables, and then power up the switch.  Sounds like that switch will see it has a Master and become part of the stack but I will adjust the priority before shutdown and stack connection just to be sure.
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